BULLETIN

OF THE

S'^^'TE LLLiNOis State I aboratory

OP

NATURAL HISTORY

Urbana, Illinois.

VOLUME V.

ARTICLE IX. PLANKTON STUDIES. III. ON PLATY-
DORINA, A NEW GENUS OF THE FAMILY VOLVOC-
IDAi, FROM THE PLANKTON OF THE ILLINOIS RIVER.

By C. a. KOFOID, Ph. D.

Illinois State Laboratory of Natural History,

URBANA, ILLINOIS.

December 28, 1899.



ERRATA.

Page 136, line 2, and jiage 182, line 17 from bottom, f(jr 'g^a read 'gj.

Page 226, line 2, page 263, line 17 from bottom, and page 267, lines

2 and 15, for 'g8, read 'g6.

Page 233, line 15 from bottom, for '82 read '82a.

Page 355, line 2 from bottom, for C. F. Hudson read C. T. Hudson.

Page 389, foot-note, for Vol. V. read Vol. IV.

Page 457, line 5, for Genera read Genus.

{



Article IX.

—

Plankton Studies. III. On Plaiydur'uui, a

new Genus of the Family Volvoc idee, from the Plankton of

the Illinois River. By C. A. Kofoid.

The family Volvocidce is well represented in the plankton

of fresh-water ponds and streams. Indeed, with the possible

exception of Steplianosphera, all of the colonial forms in-

cluded in the subfamily Volvocince—Spondylomoruni , Gonium,

Stephanosphcera , Pandorina, Pleodorina, and Volvox — are

pelagic in habit and are found only in the fresh-water

environment. For the past four years, during the summer
and autumn months, a colonial form belonging to this sub-

family has occured in plankton collections from the Illinois

River and its adjacent waters, to which I have given the

name of Platydorina caudata. It appears as early as June 15,

and becomes abundant in the months of August and Septem-

ber, diminishing in numbers in October, and disappearing

in November. It thus attains its greatest development

toward the close of the maximum period of summer heat,

when the temperature of the water in which it is found often

reaches 36° C. This species has occurred in all the waters

examined in the course of the operations of the Illinois

Biological Station; viz. in the Illinois Ptiver, in Thompson's,

Quiver, Flag, Mantanzas and Phelps Lakes, at Havana, 111.,

and in the Illinois River and Meredosia Lake at Meredosia,

111. During the summer and fall of 1899 it was also found

in abundance near Urbana, 111., in Salt Fork, a small stream

tributary to the Embarras River—a confluent of the Wabash.

It was not equally plentiful in all these localities, but showed

a decided preference for shallow water free from vegetation,

reaching its maximum development when the turbid water

was but a few feet, or even less than a foot, deep. In such

situations the shallowness of the water and the absence of

vegetation conduce to a maintenance of the high temperatures

which seem to favor its multiplication. The bottom of the lakes

in question is usually composed of soft mud, rich in decay-

ing organic matter and often covered by a mat of Oscillaria,

but otherwise quite free from vegetation. At Havana
419



420 Ilthiois State Laboratory of Natural History.

we have found Platydorina in greatest numbers in Phelps

Lake, which in 1896, '97, and '98 afforded the conditions

above described. It was likewise abundant in Thompson's
Lake in the late summer and early fall of 1897 and '98, when
the lake was at a low level and contained little vegetation.

In the shallow open waters of Matanzas Lake it was much
more abundant than in Quiver Lake, where there was usually

a large amount of vegetation. At the time of the maximum
abundance of Platydorina in Salt Fork in September the

stream was reduced by drouth to a series of stagnant pools

with no vegetation. In the early part of August it was full

of algae and other aquatic vegetation, and Platydorina was
then present in considerable numbers, although not so abund-

ant as it was in the following month.

On August 2, 1888, Professor H. Garman, while conduct-

ing a biological survey of the aquatic life, in the vicinity of

Quincy, 111., in the bottoms of the Mississippi River (see

Garman '90), found a specimen of this interesting species in

the waters of Libby Lake. He records and sketches it in

notes now on file at this Laboratory, but published nothing

concerning it.

The occurrence of this new genus in the waters of the

Wabash, Illinois, and Mississippi river systems and its

recurrence in our collections for several successive years

indicate its wide distribution and firm establishment in the

Mississippi Valley in waters of some permanency. It has

not yet been noted in temporary pools.

The associates of Platydorina in the plankton have varied

with the season, the locality, and the year. It may be said, in

a general way, that the plankton in which it occurs is charac-

terized by an abundance of flagellates, of rotifers,—especially

Brachionidce

,

—and of immature Copepoda. A water-bloom

composed largely of Euglena, Trachelomonas, Carteria, and
other green flagellates, often appears at the surface of waters

where Platydorina is abundant. Gonium is frequently

associated with it in large numbers, as are also Pandorina,

Eudorina, and Pleodorina, though these three genera may also

be plentiful in the early summer, when Platydorina may



On Platiidorina. 421

be absent or rare. Pleodorina caUfornica was extremely

abundant in Salt Fork in August, but had almost entirely

disappeared by the time that Platydorina had reached its

maximum. A few specimens of Volvox, which, in this locality,

is common in the spring months, were also noted, while

perhaps the most interesting associate in Salt Fork was
Ceratiiim humaoncnse, discovered by Carter ('71) in Hindostan.

Other chlorophyll-bearing associates frequently seen are

Pediastriim, Scenedesiniis, Actinastrum, and Closterium.

Among the diatoms, Melosira, Fragilhiria , and Snrirelht,

were to be seen; and among the Peridinid(e, Peridinium

tahulatum was almost always represented.

The zooplankton associated with Platydorina is not less

varied than the phytoplankton. The Protozoa were usually

represented by Arcella, Dijflugia, and occasionally by pelagic

Amo'ha; by Synura, Mallomonas, Dinobryon, and Uroglena ;

and by Codonella and Coleps. Among the Rotifera the order

Ploima was well represented ; Triarthra and the Brachionidd-

—notably Brachionns mUitaris, B. angidaris, B. j^unctatus, and
B. hdkeri, and its varieties—were most common during the

summer months, while the Syncluetidcf; increase in numbers in

the early autumn. Polyarthra was frequently abundant, and
Rotifer, Philodina, Asjjlanchna, Euchlanis, Cathypna, Distyhi,

Monostyla, and Pterodina, were often represented by one or

more species. The paucity of Entomostraca stands in sharp

contrast with the abundance of rotifers, the former group
being represented by relatively few species and few adult

individuals. The nauplii of Cyclops were, however, as a rule

abundant, but only occasional specimens of adult Cyclops,

Diaptomus, Bosmina, Chydonis, Ccriodaphnia , T)aphnia, and
Cypridopsis were to be seen.

Platydorina candata n. g., n. sp.

The species here described consists of a horse-shoe shaped
ccenobium or colony (PI. XXXVIIL, Fig. 1) of 18 or 32
bitlagellate cells, the anterior end corresponding to the toe of

the horse- shoe and the truncate posterior end to the heel, the



422 Illinois St<ftc Tjahorator;/ of Natural History.

latter carrying 3 or 5 prolongations or tails formed by the

gelatinous substance of the coenobium. The colony is plate-

like, and flat except that the plate is slightly twisted in a left

spiral. This spiral is scarcely noticeable in a face view

(Fig. 1) except by focusing with a high-power objective, but

it can be easily detected in a side view (Fig. 3). It varies

from one eighth to one thirty-second of a turn of the spiral,

and in twenty-five colonies especially examined on this point

it was invariably a left spiral, with the location of the twisting

always in a definite relation to the colony, the right anterior

and the left posterior regions of the colony in face view being

high, while the left anterior and the right posterior are low.

Repeated examinations of specimens, both living and pre-

served, indicate that this spiral form is a constant feature of

structure ; that it is not reversed in direction ; and that it is

subject to but slight variation in the degree of the torsion.

No movement within the colony which would produce or vary

the spiral was noted in living individuals. The form of

Platydorina seems to be as constant and as characteristic as

that of other genera of the family.

The size of the colony varies with the age, with the number
of cells present, and also, perhaps, with the locality and the

season. Colonies of 32 cells in which the first division lead-

ing to the formation of daughter colonies is taking place,

average about 150 m in length, 130 m in width, and 20 M in

thickness. The largest colonies are about 165x145x25 m,

and the smallest free-swimming ones about 25x21x4 m.

Colonies of 16 cells are smaller than those of 32 cells, and

are also narrower in proportion to their length, measuring

about 70x43x16 M-

The colors of Platydorina are quite as striking as those of

related forms. The cells, which are imbedded in the trans-

parent gelatinous matrix of the colony, are a bright chloro-

phyll-green, and each has, as a rule, a red stigma, or eyespot,

of unusual brilliancy.

The substance in which the cells of the colony are imbedded

is similar in appearance to that in Eudorina. It is a trans-

parent colorless substance of considerable consistency, show-



On Platydorina. 42S

ing in tlie living condition, as a rule, no trace of differentia-

tion. The gelatinous nature of the substance is shown by the

great numbers of bacteria which swarm within it in moribund

specimens. Colonies killed in formalin and stained in Dela-

field's hjematoxj'lin exhibit a difference in the intensity of

coloration, indicating the presence of a denser peripheral

layer or sheath 3-4 m in thickness (PL XXXVIII., Fig. 1, 4,

J}. sJi.). This is apparent along the edges of the colony, and

presumably extends over its faces. In several living colonies

a granular differentiation of this outer layer was noted about

the margin. This sheath is similar to that of Eudorina and

Pleodorina, but shows no trace of the concentric layers so

prominent in Pandorina.

One of the most characteristic features of the colony is the

presence, upon the posterior border, of 3 or 5 projections or

tails, which are merely extensions of the sheath. Colonies of

16 cells have but three tails, while those of 32 cells have

uniformly five. These projections are bluntish finger-like

processes without structural differentiation, tapering some-

what to a rounded or pointed end. Occasionally the outer-

most pair, and more rarely the inner one, are slightly diverg-

ent. In the 16- cell colony there are two latero-posterior

tails and one median one (Fig. 2), the former being better

developed, and measuring 15 to 20 /^ in length. The median

tail is variable in length, being sometimes a mere rudiment

appearing on a slight elevation on the margin. Its average

length is about one third that of the adjacent pair, though it

occasionally attains two thirds their length. The latero-

posterior tails are upon each side of the colony directly

behind the marginal row of cells, while the median tail is

midway between the central rows. In the 32-cell colony

(Fig. 1 ) there is, in addition to the three tails above noted,

another pair w^hich may be designated as the lateral pair.

These tails are slightly divergent, arising at the outer pos-

terior angles of the marginal row of cells, between the last

transverse quartet and the last sextet of cells. They are

from 10 to 15 yu in length, and are often of the same size as

the median tail of the colony. The other three tails occupy



424 Illinois State Lahorntory of Natural History.

the same position with respect to the posterior quartet of cells

that they do in the 16 -cell colony, but are as a rule much
larger, the postero-lateral pair measuring from 20 to 30 yw in

length, while the median one reaches only a length of 15 to

18 /<. The five tails do not lie in one plane, but share in the

spiral of the colony, at times, indeed, exceeding it in the

degree of the twisting. These structures are all subject to

considerable variations and irregularities of development

(Fig. 5), such as suppression, inequality of members of pairs,

differences in size and relative development, in attenuation,

and in degree of divergence. These irregularities are often

correlated with the loss of cells in the colony due to parasites

or other causes. The tails, nevertheless, exhibit such a con-

stancy of position and so much of symmetry and regularity

of development that they cannot for a moment be considered

as ephemeral features of little structural importance. In

their position they recall the protuberances noted by me
('98) at the posterior end of Pleodorina illinoisensis . In

Pleodorina, however, these structures are apparent only in

disintegrating maternal colonies, and it may be that they

also indicate the point at which the embryonic cup closes.

On the other hand, in Platyclorina caudata these tails are

present upon the colonies at the time of their escape from the

maternal matrix and persist throughout the life of the adult,

being permanent structures, characteristic of the species.

Within the outer sheath is a homogeneous gelatinous matrix

(Fig. 1, m.) which in Delafield's haematoxylin stains less

readily than the sheath. In the living colony no differentia-

tion of this matrix is to be seen, but, after staining, a delicate

sheath showing deeper color is demonstrated about each of

the cells. In most places a considerable space intervenes

between this secondary sheath (Fig. 4, s. sh.) and the inclosed

cell, so that the sheaths crowd upon each other and appear

to divide the field of the matrix into irregular polygonal areas.

These areas, as a rule, fill the greater part of the plate, leav-

ing unoccupied only a few corners, principally about the

second transverse row. The two poles of this swollen second-

ary envelope are not of equal size, the inner being somewhat



On Platydorina. 4'25

the smaller, and slightly overlapped by those of the contiguous

cells. This is due to the intercalation of the cells of the two

sides of the plate, and to the fact that the outer ends of the

cells are slightly nearer the surface of the plate than are the

inner ones. The gelatinous substance within the secondary

sheaths does not differ in structure or stainibility from that

of the surrounding matrix. As a result of the form of the

colony, the amount of the matrix substance is much less in

Platydorina than in related forms such as Eudorina.

The cells of the colony are all of one type, alike in structure,

and approximately similar in size. Each is biflagellate and

has a central body of protoplasm with a nucleus, two contrac-

tile vacuoles, one stigma, and one chromatophore with a single

pyrenoid.

The number of cells in the colony is either 16 or 32; at

least no normal colony with cells of any other number has

been detected among the hundreds, if not thousands, of

colonies examined. Colonies are frequently seen which, by

reason of parasites or from other causes, have lost one or more

cells, indeed in some cases all but one or two ; but the form

of these colonies is usually preserved, and the empty second-

ary sheaths frequently remain as evidence of the original

complement of cells. The 16-cell colonies are not mere

stages in the development of the 32-cell form, for division of

the cells of this type in observed cases leads to the develop-

ment of new colonies and jiot to the formation of the 32-cell

stage. As in other nearly related genera of the family—for

example, Eudorina, Pandorina, and Pleodorina—the number
of cells in the colony varies, within narrow limits, in the ratio

of geometrical progression. In Platydorina, however, this

pleomorphism is manifested not only by this difference in the

number of cells in the colony, but also by a structural dis-

tinction—the presence of three tails in the 16-cell, and five

tails in the 32-cell, colony. Inasmuch as the two types

always occur together, and since this pleomorphism is in some

respects similar to that of related genera, it does not seem

justifiable to regard the two as distinct species of the genus.

They are, I believe, two forms of one species.



426 Illinois State Laboratory of Natural History.

The arrangement of the cells is characteristic, and is

strikingly different from that of any other genus of the family.

The gelatinous matrix and sheath conform to the horse-shoe-

shaped plate of cells, and even the caudal appendages bear a

fixed relation to the plan of cell arrangement. The 32-cell

colony is composed of a marginal U-shaped row of 12 cells

about three sides of a 20-celled somewhat jectangular plate,

which, in turn, consists of an outer row of 12 cells on three

sides of a row of four pairs of cells. The colony might also

be regarded as made up of three U-shaped rows of 12, 12, and

8 cells respectively, nested in such a fashion that the inner

two project one cell beyond the outermost. The cells also

fall into six quite irregular transverse rows of 4, 6, 6, 6, 6,

and 4 cells respectively, and into the same number of corre-

sponding longitudinal ones. As before stated, the lateral

tails are posterior to the marginal row, while the postero-

laterals are behind the first row within the marginal, and the

median one midway between the innermost rows. In the

colony of 16 cells (Fig. 2) the marginal row has but 10 cells

and the central plate but six. The cells fall into four some-

what u'regular transverse rows, and there are the same
number of longitudinal ones of 4 cells each. The horse-shoe

shape, however, masks somewhat this simple Gonium-like

arrangement. The plate-like form of the colony and the

arrangement of the cells, especially in the 16-cell form, give

this new genus a superficial resemblance to Gonium. It is,

however, fundamentally different, for in Platydorina the two

faces of the plate are exactly alike, while in Gonium the face

anterior in locomotion bears all the flagella, ^nd the other

face presents only the bases of the cells. This similarity of

the two faces in Platydorina, neither of which is anterior or

posterior, is brought about by the fact that every other cell

upon either face presents to that face the pole which bears

the stigma and the flagella, while the intervening cells present

the opposite pole, with its pyrenoid. This alternation of stigma

and pyrenoid is constant, and can be followed in any row of

cells except the diagonal ones (c/., Fig. 1). The cells of the

marginal row, in both the 16- and 32-cell colonies, point



On Platydorina. 427

obliquely outward, the direction alternating, however, in

conformity with the arrangement of cells in the central area,

as may be seen in a view of the edge of the colony (Fig. 3).

The alternation of the cells in the colony as a whole is the

same whichever face is presented, the right-hand cell of the

posterior row of four cells always presenting the basal end

uppermost. From this as a starting point the regular alter-

nation of stigma and pyrenoid can be traced from cell to cell

throughout the whole colony. An examination of twenty-five

colonies showed that all conformed to the same plan of alter-

nation, there being no case of reversal. In the arrangement

of the cells in the colony, Platydorina is thus more like

Eudorina than like Goniuni, being, not a simple plate like the

latter genus but, in reality, a flattened ellipsoid so much
compressed that the cells of the two faces intercalate

regularly, and thus give to the colony its superficial resem-

blance to Gonium.

The individual cells are all substantially alike in size and
structure. They have the form of an oblate spheroid, slightly

larger in the outer hemisphere.* Some cells, especially the

marginal ones, often exhibit a slight flattening or even a

depression at the outer pole. In the full-grown colony the

cells have an equatorial diameter of 15-20 /< and a polar one

of 15-18M- The cells of young colonies still within the

maternal matrix do not exceed 4-6 /^ in diameter. I do not

find that the cells of the 16-cell colonies are appreciably

larger than those of the 32-cell.

The protoplasm is small in amount, consisting of a very

thin pellicle (Fig. 4, j^.) on the surface of the cell on the out-

side of the chromatophore, and an axially-placed knob-

shaped mass (pr.) located somewhat nearer the outer pole

than the inner one. Near the center of this mass lies the

spherical nucleus (Fig. 4, n,), containing a single spherical

nucleolus (ncL). Within tliis protoplasmic mass lie the two

contractile vacuoles {c. v.) and the stigma (st.), while from

the outer end of the cell arise the two flagella (/.).

* As in the case of Eudorina and Pleodorina, the terms "outer" and "inner" are

used to designate respectively the ends which bear the stigma and the pyrenoid.



428 Illinois State Laboratory of Natural History.

There is but a single, cup-shaped chromatophore (Fig. 4,

chr.), which is inclosed within the pellicle above noted and
itself contains the knob-shaped protoplasmic mass. It is of a

brilliant chlorophyll-green color, and contains numerous
small granules of irregular and somewhat angular outline.

Towards the inner end of the cell, imbedded in the thickest

part of the chromatophore, there is a single spherical pyrenoid,

having a diameter of 4-6 /v.

The stigma, or eye-spot, seen from above is circular in

outline, but in lateral view has the form shown in Fig. 4, s.

The slightly convex outer surface appears to project some-

what beyond the rounded contour of the cell. The color is

usually a bright reddish brown, often brightest in the anterior

and marginal cells and rarely entirely faded in the posterior

ones. The stigma is a homogeneous body, showing no trace

of structure beyond the well-defined contour line, which is

best seen in fading and moribund cells. It is normally

present in all cells of the colony, and may readily be demon-
strated by full illumination. The position of the stigmata in

the cells is somewhat unusual, and is significant of the pro-

nounced polarity of the organism. The customary position

in other genera is adjacent to the bases of the flagella. In

Platydorina, however, the location of the stigma is not con-

stant with respect to the liagella, but seems rather to bear a

definite relation to the form of the colony, since it lies towards

the peripheral and posterior region of the cell (Fig. 1), while

the flagella are centrally located and project outward in the

usual manner. This relation appears not only in the mar-
ginal regions but also in the central. The physiological

significance of this arrangement is- not apparent, but it seems
to be correlated with the pronounced polarity of the organism.

Platydorina is positively phototactic. A miscellaneous plank-

ton collection placed in a window with southern exposure in

an aquarium six inches in diameter was, after ten minutes,

quite barren of Platydorina except along the margin towards
the window. On the other hand, this species avoids bright

light. This was very evident in collections fresh from the

field when examined under low power (75 diameters), a very



On Platijdorlna. 429

slight increase above a moderate illumination causing them
to leave the field of view with considerable rapidity. In one

case, where twenty-eight colonies were in the field when placed

under the microscope, only one of them remained after an
exposure of t'wenty-five seconds. A very slight decrease in

the amount of light would invariably insure their return to

the field with almost equal rapidity, the number increasing

as the intensity of the illumination was decreased. It may
be that the asymmetrical position and the somewhat unusual

arrangement of the stigmata are connected with the pro-

nounced phototaxis of this organism. At least, the asym-
metrical position has a tendency to place the long axis of the

stigma parallel to the main axis of the colony, with the outer

end directed towards the source of light in negative, and away
from it in positive, phototaxis.

The flagella are uniformly two in number for each cell, are

similar in the same cell and in different parts of the colony,

and are in the adult colony 20-25 i-i in length. From the

outer pole of the cell they pass through the matrix, leaving

the appearance of a tube-like structure in the gelatinous

substance (Fig. 4). When not in activity the flagella project

beyond the sheath in a position perpendicular to the surface

of the colony at the place of exit. As in other genera of this

family, the flagella persist after the division of the cell to

form the daughter colony, and even after the divisions are

completed still provide locomotion for the maternal organ-

ism. In some instances the flagella could be seen passing

through the matrix toward that cell of the daughter colony

which bears the largest eye-spot.

The contractile vacuoles (Fig. 4, c. v.) are two in number,

and are located in the peripheral layer of protoplasm, near

the outer end of the cell. They lie in the outer part of the

knob-shaped mass of protoplasm, upon either side of the

place of origin of the flagella, being somewhat widely sep-

arated. At diastole the vacuoles of an adult colony have a

diameter of 1.5-2 /^. The contraction is rhythmical, and the

two vacuoles usually alternate at regular and equal intervals.

At a temperature in the laboratory of 20° C. each vacuole



430 Illinois State Laboratory of Natural History.

contracted at intervals of forty-five to fifty seconds. In rare

instances the contractions of the two vacuoles were separated

by unequal intervals, being almost coincident in one case

observed.

The method of locomotion in Platydorina is similar in

many respects to that of other genera of the family. The
lashing of the flagella produces a forward movement of the

colony and causes its rotation about the major axis, either

from left over to right or from right over to left. The
rounded end of the colony is uniformly directed forward in

locomotion ; at least no instance in which the caudal end

led was noticed. The forward movement is, as a rule,

accompanied by the rotation of the colony, though the amount
of rotation varies somewhat with the individual, the freedom

of movement, and the speed of locomotion. When locomo-

tion is blocked by obstructions the rotation continues, as in

Pleodorina, with frequent reversals in direction. In fact,

obstruction to progress seems frequently, though not uni-

formly, to act as a stimulus to the reversal of the direction

of rotation.

The two directions of rotation are not equally prevalent,

that from right over to left having a marked predominance.

Thus, of twenty-five colonies observed in motion twenty were

rotating from right over to left and but five from left over to

right. In another twenty-five the corresponding numbers
were nineteen and six respectively. Keeping a single colony

under observation for some time, it is found to rotate from

right over to left about four fifths of the period and to turn

in the opposite direction the balance of the time, this pro-

portion representing the totals of the periods of rotation,

while the individual periods vary greatly in length, that from

left over to right lasting at times but a few seconds.

This predominance of one direction in locomotion is doubt-

less correlated with the torsion of the colony, whose shape is

such that the rotation would necessarily be from right over

to left in forward locomotion, as a result of the resistance of

the water, unless, of course, there should be some disturbing

factor. The immediate and most potent cause of the direc-



On Platydorina. 431

tion of rotation is doubtless the coordinated action of the

flagella, since reversal of direction does not seem to be

accompanied by any change in the direction of the torsion of

the colony. The evidence upon this point is not conclusive,

but repeated efforts have failed to detect any change in the

form of the plate when the direction of rotation is reversed

in the living and moving colony ; and, again, colonies when
killed suddenly have always the usual form of spiral, though

some of them were moving in the reverse direction. When
the usual direction of rotation is reversed, the forward motion

still continues in spite of the fact that then the form of the

plate favors a backward movement ; the form of the colony,

therefore, does not control the direction of rotation though it

is correlated with the direction which predominates. The
fact that the rotation from right over to left predominates

also in Pleodorina illinoisensis and Eudorina elegans, ivhere

there are no structur(d features favoring such a predominance,

suggests the possibility that the form of the colony in

Platydorina is the result and not the cause of this predom-

inance, and that the function of turning from right over to

left predominantly preceded the structure which favors it.

The organization of Platydorina suggests a descent from a

Eudorina-\\^& form, in which event the systepiatic series and
the phylogenetic series alike afford evidence of a function

arising in an organism before the structure with which it is

correlated appears.

In another connection ('98) the subject of locomotion and

polarity in the different genera of the Volvocime was reviewed

and discussed. It will suffice, therefore, for the present to

give a brief resume of the facts. In the lower genera of the

family, Stephanosphcera and Gonium, as also in Pandorina,

the rotation seems to be indifferently to the right or left, while

in Eudorina and especially in Pleodorina illinoisensis it is

oftenest to the left, rotations to the right in observed cases of

the latter species being to those to the left as 100 to 117-138.

With respect to Volvox there are no data at hand. In Platy-

dorina, we find by far the most pronounced predominance of

one direction of rotation, the ratio in observed cases being



432 Illinois State Laboratory of Natural History.

100 rotations from left over to right to 355 from right over to

left. In this respect, then, so far as there is evidence,

Platydorina is the most highly differentiated genus of the

family.

The polarity of the lower genera, Stephanosphcera and
Gonium, is likewise of the simplest form, being merely physio-

logical, the same pole or face of the colony always leading in

locomotion. In Pandorina, Eudorina, and Volvox, however,

there is the added feature of the greater brightness of the

anterior stigmata, and in Pleodorina the two poles are differ-

entiated by the two types of cells as well as by the characters

found in the genera just mentioned; but Platydorina is the

only genus of the family in which polarity is expressed by the

arrangement of the cells and by structural features of the

envelope.. In regard to polarity, also, the new genus is thus

the most highly specialized member of the Volvocince.

The reproduction of Platydorina has been observed by me
repeatedly in the past five years, but only the asexual phase
has thus far been discovered. All of the cells of the organism

are gonidial, each dividing to form a daughter colony. The
sequence of the divisions arid the position of the successive

planes are of the type found in Eudorina and Pleodorina,

the resemblance being so close that the figures illustrating the

asexual development of Pleodorina illinoisensis ('98,

PI. XXXVII.) might almost be used for cleavage in Platy-

dorina. There is one difference, however, for in Platydorina

the curved plate of cells, which becomes first cup-shaped and
then ellipsoidal, subsequently flattens, the cells of the two
faces intercalating during the process. The daughter colony

acquires the adult form, including the tails and the torsion of

the plate, before it escapes from the maternal matrix, the

young colonies moving about for some time in the disinte-

grating matrix before making their escape through the ruptured

outer sheath. The secondary sheath surrounding the gonidial

cell becomes the outer or primary sheath of the new colony.

No stages of sexual reproduction have been seen, though the

collections examined represent a considerable range of season

and locality. It may be that these are to be sought upon



On Platydorina. 433

the bottom rather than in the superjacent strata of water

where plankton collections are usually made. Aquaria about

to dry up were also searched in vain for sexual stages of

Platydorina.

The mode of development of Platydorina is significant of

its systematic position and its relationships. The number
and the original arrangement of the cells, the type of develop-

ment, and the character of the envelope, all indicate that

Platydorina is a more highly specialized form descended from

some Eudorina-like ancestor, and that it is more closely

allied to Eudorina than to any other existing genus.

Throughout this paper the customary term " colony " has

been used to designate the organism herein described and

others related to it. The wide use of the term in the literature

of the subject is doubtless due to the fact that, as a rule, the

organisms are composed of similar cells arranged in symmet-

rical form with no pronounced axial differentiation, without

contact or protoplasmic connection, separated from each

other by a non-living gelatinous matrix, and each capal)le of

performing all the functions necessary for its own life and the

continuance of the species. Furthermore, the destruction of

individual cells does not impair the life of the other cells of

the organism, for so long as a single cell remains it continues

its customary activity. The use of the term colony is, however,

objectionable. A number of facts militate against this con-

ception of the organism, and the discovery of the new genus

here described adds to the array. (1) The cells are not

always similar, for in all forms with poles physiologically or

otherwise differentiated the anterior stigmata are brighter

than the posterior, and in Pleodorina there are two kinds of

cells, the vegetative and the gonidial, the former distinctly

smaller than the latter. (2) There is in all of the higher

genera a well-defined physiological polarity accompanied by

the difference in the anterior and posterior stigmata, and also,

in Platydorina, by a differentiation of the poles by the

arrangement of the cells and the structure of the envelope,

and by the further differentiation of a transverse axis.

(3) In Pandorina the cells are almost in contact, and in



434 Illinois State Laboratory of Natural History.

Volvox they are actually connected by protoplasmic processes.

(4) The beginnings of histological differentiation are also

evident in the cells composing the so-called colony. In

Eudorina, according to Carter ('58), the cells are differen-

tiated into male and female in definite regions, the male cells

developing from the anterior quartet and the remainder

becoming female ; in Volvox sexual and asexual reproduction

alike are limited to a few of the cells ; and in Pleodorina the

asexual process is confined to the posterior hemisphere. The
cells of the organism are thus histologically and functionally

differentiated in this particular in these higher genera.

Although the degree of differentiation is slight, it is neverthe-

less appreciable. (5) In the matter of locomotion the

activities of the individual cells of the organism are not

independent of each other but are correlated, the flagella

acting together to produce rotation, its reversal, or its cessa-

tion. The predominance of the direction in the higher genera

plainly exhibits the phenomenon of correlated locomotor

activities of the constituent cells.

The facts above cited emphasize the desirability of regard-

ing each of these so-called colonial flagellates of the sub-

family Volvocince as a unit, with an organization of its own,

and not as a colony, that is, an aggregation of independent

and similar cells associated merely as a result of descent from
a common parental cell, the form being a matter of chance
or circumstance. The group of cells as a whole, and not

each of the constituent cells, is the unit of descent, of form,

and of function, and the word colony can be applied to it

only by the license of usage and as a matter of convenience.

Keference has been made frequently in the preceding pages

to the prevalence of colonies whose symmetry has been dis-

turbed by loss of cells. In most instances only the empty
secondary sheath remains, giving no clue to the cause of the

loss of its contents. In collections made in Phelps Lake,

Havana, 111., in August, 1896, however, colonies were often

found which were parasitized by one of the Sporochytriacece,

which upon examination proves to be Dangeardia mamillata,

described by Schroder ('98) as a parasite of Pandorina



0)1 Platydorina. 435

morum. As these infested colonies frequently shoAved a loss

of one cell or more and exhibited all stages in the destruction

of the cell, it seems probable that the loss was due to the

parasite. Eudorina elegans and Pandorina morum occurred

in the same collection and were similarly infested. Two
additional genera, Platydorina and Eudorina, are thus to be

added to the list of hosts of Dangeardta.

For the convenience of systematists a brief statement of

the generic and specific characters of the form herein described

is now given, fallowed by a key to the genera and species of

the Volvocince for the assistance of students of this interest-

ing and not uncommon group of fresh-water organisms.

Species not as yet reported, to my knowledge, from Illinois

are indicated by an asterisk when found elsewhere in this

continent, and by a dagger when not as yet reported from it.

It is not at all improbable that all the species here listed will

yet be found within this State.

Platydorina n. g.

Colony flattened, the two faces compressed so that the cells

of the two sides intercalate ; llagella upon both faces on
alternate cells. Anterior and posterior poles of major axis

differentiated by the arrangement of the cells and by the

structure of the envelope. Long and short transverse axes

differentiated by the flattening of the colony. Cells similar,

biilagellate, each with stigma, chromatophore, and pyrenoid.

Asexual reproduction by repeated divisions of all of the cells,

each forming a daughter colony.

P. caudata n. sp.

Colony flattened, horse-shoe shaped, twisted about one

eighth of a turn from right over to left; cells 16 or 32,

arranged in a marginal row of 10 or 12 and a central area

of 6 or 20 ;
posterior end with 3 or 5 prolongations or tails

formed by extension of the common outer sheath.

Known habitat, lakes and streams in central Illinois.

Types in the collections of the Illinois State Laboratory of

Natural History and deposited in the United States National

Museum.



436 Illinois State Laboratory of Natural History.

KEYS TO THE GENERA AND SPECIES OF THE SUBFAMILY
VOLVOCIN^.

Cells arranged in ccenobia of definite forms varying with

the species, biflagellate, with stigma and one or more chro-

matophores ; surrounded by a gelatinous envelope whose
development separates them to a greater or less degree;

number not uniform, varying in the different species, often,

but not always, definite. Asexual reproduction by repeated

divisions of gonidial cells, which constitute the whole or only

a part of the parental organism, to form daughter organisms
;

sexual reproduction in some species (in otheirs unknown) by

the conjugation of male and female gametes, resulting in the

formation of a resting stage which later develops into a new
organism.

Genera.

Cells arranged in form of plate with flagella upon
one face only, 2.

Cells arranged in spherical, ellipsoidal, or flattened

colonies, flagella not confined to one face. 3.

Cells in a squarish plate, envelope closely adherent.
Gonium.

Cells in a rounded plate, envelope swollen, oval, or

spherical. Stephanasplicera.

Colony ellipsoidal or spherical, cells crowded to-

gether, conical, reaching towards center, outer

\ membrane of concentric layers. Pandorina.

Cells not crowded together, nor reaching towards

center of colony. 4.

Colonies ellipsoidal or flattened, cells uniform in

size. 5.

Colonies spherical or spheroidal, or, if ellipsoidal,

with small vegetative and large gonidial cells. 6.

f Colony ellipsoidal or spherical, poles not differen-

I

tiated by arrangement or size of cells, or by struc-

ture of envelope. Eudorina.

Colony flattened, horse-shoe-shaped, with poles dif-

ferentiated by arrangement of cells, posterior end

with tails. Platydorina.

2 <^

4 <^

5 ^



On Platydorina. 437

Cells not connected by protoplasmic processes, of

two sizes, smaller vegetative at anterior pole and
larger gonidial at posterior. Pleodorina.

Cells connected by protoplasmic processes, not mark-
edly different in size. Volvox.

Species.

Gonium.
Cells, 4. socialc (Duj.).+

Cells, 16. pectorale Miill.

Stephanosphaera.

Represented by a single species, characterized as follows :

Cells 4 or 8, ovoid or spindle-shaped, with numerous pro-

cesses, jduvidlis Cohn.*
Pandorina.

Represented by a single species, characterized as follows

:

Cells 16 or 32, crowded, each with a single chromatophore

and pyrenoid. morinn Bory.

Eudorina.

Represented by a single species, characterized as follows

:

Cells 32, 16, or 64, similar, not crowded together, common
outer membrane without marked concentric structure.

elegans Ehrb.

Platydorina.

Represented by a single species, characterized as follows

:

Cells 16 or 32, arranged in a horse-shoe-shaped plate,

those of the two faces intercalated. Posterior end with 3

or 5 tails. caudata Kofoid.

Pleodorina.

Cells 64 or 128; gonidial cells about 2-3 times the diam-

eter of vegetative cells, which constitute about one half

the total number and lie in anterior hemisphere.

californica Shaw.

Cells 32, rarely 16 or 64; gonidial cells not more than

twice the diameter of the vegetative cells, which con-

stitute the anterior quartet. illinoisensis Kofoid.



438 Illinois State Laboratory of Natural History.

Volvox.

Cells about 10,000 (minimum 1,500, maximum 22,000),
angular, with stout connecting protoplasmic processes

into which the chromatophore may enter. Diameter of

colony about 700 yu (minimum 400, maximum 1,200);
diameter of cell body 3-5 yw. glohator L»

Cells 500-1,000 (minimum 200, maximum 4,400)„
rounded, with slender connecting protoplasmic processes

into which the chromatophore does not enter. Diameter-

of colony 170-850 yu; diameter of cell body 5-8 yw.

aureus Ehrb.

Urbana, 111., Dec. 5, 1899.



On Pliitifdor'nui. 439

LITERATURE CITED.

Carter, H. J.

'58. On Fecundation in Eudorina elegans and Cryptoglena. Ann.

Masr. Nat. Hist, Ser. 3, Vol. 11
, pp. 237-253, PI. VIII.

'71. Note on a Fresh-water Species of Ceratiuni from the Lake of

Nynee (Naini) Tal in Kumaon. Ann. Mag. Nat. Hist., Ser. 4, Vol.

VII., pp. 229,230.

Garman, H.

'90. A preliminary Keport on the Animals of the ^lississippi

Bottoms near Quincy, Illinois, in August, 1888, Part I. Bull. 111.

State Lab. Nat. Hist., Vol HI., pp. 123-184.

Kofoid, C. A.

'98. Plankton Studies, II. On Pleodorina illinoisensis. a new Spe-

cies from the Plankton of the Illinois Kiver. Bull. 111. State Lab.

Xat. Hist., Vol. v., pp. 273-293, PI. XXXVI., XXXVII.

Schroder, B.

'98. Dangeardia.ein neues Chytridineen genus auf Pandorina morum
Bory. Ber. d. deutsch. Bot. Gesellsch., Jahrg. 1898, Bd. XVI.,

pp. 314-321. Taf. XX.



440 Illinois State Laboratory of Naturat History.

EXPLANATION OF PLATE.

A., anterior pole.

r. v., contractile vacuole.

chr.. chromatophore.
/'.. flagellum.

/»., matrix.

n , nucleus.

nrl., nucleolus.

ABBREVIATIONS.

;). .s/i., outer or primary sheath

P. posterior pole.

p., outer pellicle of protoplasm.

pr., knob-shaped mass of protoplasm

pyr., pyrenoid.

s. sh., secondary sheath.

St., stigma.

Plate XXXVIIL*

Fig. 1. Platydorina caudcUa, face view of 32-cel] colony. X ooO.

Fig. 2. Face view of 16-cell colony. X 628.

Fig. 3. Edge view of 32-cell colony. X 350.

Fig. 4. Lateral view of one of the marginal cells. X 1400.

Fig. 5, a—e. Outline of the posterior ends of several deformed colonies.

X 280.

Figures drawn by C. A. Kofoid and inked by Miss L. M. Hart.



Pl.vtk XXXVIII.