BULLETIN

OF THE

Illinois Qtate I aboratory

OF

NATURAL HISTORY.

Urbana, Illinois.

VOLUME Y

ARTICLE VJII.—A STA7YSTICAL STUDY OF THE PARA-
SITES OF THE UXIONIDM.

BY H. M. KELLY, A. M.

Illinois State Laboratory of Natural History,

rUI'.ANA, ILLINOIS.

March, J 899.



State Laboratory of Natural History.

LABORATORY STAFF.

Professor Stephen Alkked Fokhics, Ph. D.,

Director of State Laboratory and State Entomologist.

Charles Authuu Hakt,

Systematic Entomologist and Curator of Collections.

FRANif Smith, A. M.,

Assistant Zoologist.

Charles Atwood Kofohj, Ph. D.,

Zoologist, and Sxiperintendent of Biological Station.

AValLACE CRAKi, B. S.,

Zoological Assistant.

Mary Jane Snydkr.

Secretary.

Henry Clinton Forbes,

Business Agent and Librarian.

Lydia Moork Hart,

Artist.



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 YIII.—.1 Statistical Studij of the I'drasitcH of the

Unioniihe. By H. M. Kelly.

The studies on the quantitative occurrence of parasites in

different species of the Unlonirke which form the basis of the

present paper were made at the Illinois Biological Station, at

Havana, early in the summers of 1896 and 1897 and con-

tinued both seasons at Mt. Vernon, la., and at Lewisburg and

Phoenixville, Pa., the material used being sec.ured in the

above mentioned localities and examined while fresh. It

was the purpose of the investigation to determine, if ])ossible,

under what conditions infestation would be found to vary in

a series of closely related species of a given locality or of

several localities, and to what extent this variation could be

regarded as a specific characteristic.

The Illinois, Iowa, and Pennsylvania waters from which

collections were made are indicated in Table I., and for the

purposes of this paper may be briefly characterized as fol-

lows : (1) The Illinois Eiver at Havana is a large sluggish

stream with alluvial banks and largely muddy bottom, full

of organic matter of various origin. (2) Spoon Biver, a

tributary of the Illinois, is at Bernadotte and Duncan's

Mills, where collections were made, small, with rapid current

and rocky bottom. It drains a purely agricultural region,

and is subject to frequent and sudden Hoods. (3) Quiver

Creek, near Havana, is a smaller tributary to the Illinois,

draining a sandy plain. (4) Thompson's Lake is a shallow

body of water about six miles long and one mile wide, lying

in the low alluvial flood plain of the Illinois Eiver, with very

muddy bottom and little change of water except as affected

by change of level in the river, with which it is connected.

(5) The Cedar Eiver, near Mt. Vernon, la., is a rapid shallow

stream with shifting sandy bottom for the most part, draining



400 Illinois State Luhoratori/ of N((ti(r(il Histori/.

ji rolling prairie region. (0) Abbey Creek is a small, slug-

gish, and muddy tributary of the Cedar, flowing through a

wooded region of rich black soil. (7) The Schuylkill River

at Phtenixville, Pa., is rapid and shallow, with a rocky bed.

Its waters are largely contaminated with the sewage of the

populous region through which it flows, with manufacturers'

wastes, and with the acid pumpings of anthracite coal mines.

(8) Pickering Creek, a tributary of the Schuylkill, is an

uncontaminated, swift-flowing stream, constant in volume,

and draining a restricted rocky area. (9) French Creek, a

near-by branch of the Schuylkill, is of similar size, but

traverses a country of clay soils, and is subject to frequent

inundations. (10) The West Branch of the Susquehanna

River is, upon most of its course, a large but shallow mountain

stream with rapid current. It is quiet for some miles at

Lewisburg, Pa., where it crosses one of the fertile valleys of

the Appalachian system.

Fifteen hundred and thirty-seven individual unios from the

above localities, belonging to forty-four species, were person-

ally examined by me with reference to their parasites ; and

with the data thus obtained I have incorporated the results

of an examination of seventy-seven individuals, belonging to

eighteen species, made by Dr. C. A. Kofoid in 1895 and

1896, which were placed at my disposal. It is unfortunate

that my material did not furnish equal representation for

each host species, and that the examination extended over

such a long period of time. For control on both these points

an effort was made to examine, if possible, about twenty indi-

viduals of each species which were all taken at the same time

from the same locality and under the same conditions.

Whenever it was practicable individuals of or above the

adult average in size and age were chosen for examination.

Only ninety-seven of the sixteen hundred and fourteen indi-

viduals examined could be regarded as immature, and these

were well distributed among the several species. The inclu-

sion of records for these younger and smaller individuals

would presumably reduce somewhat both the probability of

and capacity for infestation. However, in the case of



A SttiiUj of the Parasites of the Unionidte. 401

Table I. Geooraphicai. Distribution of Unioniu-e Examined.

NAMEJ? OF SPECIES.



402 Illinois State Laboratory of Natural History.

Qiuidrida pustidosa,* where the highest proportion of under-

sized individuals is inchided in the tabulation,—twenty out

of eighty-seven,—it appears that the infestation of these

younger hosts did not differ materially in kind or degree

from that of the larger individuals.

The sex of fourteen hundred and eighty-three individuals

of the sixteen hundred and fourteen examined was deter-

mined by microscopical examination, seven hundred and

eighty-two being males and seven hundred and one females.

In one hundred cases the determination of sex was not

attempted. In the thirty-one remaining, it was indetermin-

able by microscopic examination, all but five being infested by

Bucephalus or other cercaria forms to the utter destruction of

the proper reproductive tissue. From the shape of the shells

eight of these thirty-one individuals were pronounced males

and two females. Others, also belonging to species in which

the shells of the two sexes are normally characteristic, had

shells of such shape as to render the sex problematical and

to suggest that infestation by Bucephalus or other cercaria,

when early acquired and long continued, may so alter the

form of the shell of the female as to cause it to resemble that

of the male or, if acquired later, may produce an intermediate

form. Moreover, females infested with Bucephalus or other

cercaria rarely (in but three observed cases) carried glochidia,

though examined when the marsupia of other females of their

species were normally inflated with young. This is especially

noteworthy in Lampsilis gracilis,, in which determination of

the sex of the clam by the form of the shell is usually certain.

Of the eighty-nine individuals of this species examined,

thirteen were infested with Bucephalus or other cercaria. Of

these, seven appeared to be males, one was a female with

glochidia in the gill, three were doubtfully regarded as females

although no germinal tissue was discernible, wiiile the sex of

two was problematical.

In thirty-eight of the forty-four species examined the num-

* For the convenience of those who have not followed the recent changes in
unionid nomenclature a list of the names mentioned in this article is given in the
first column below, each name being followed in the second column by the one



A Stiuh/ of the Pdrdnitca of the Uiiioindw. 403

ber of infested males and females for each species did not

dift'er materially. The exceptions are as follows :

Unio heterodon, 4 males, females.

Anodonta suborbiculata, 18 males, 2 females.

Plagiola securis, 5 males, females.

Lampsilis par%'us, males, 31 females.

Lampsilis la-vissimus, 4 males, females.

.
Lampsilis tenuissimus, 5 males, females.

The disparity of the sexes in these few species has no
significance, however, in this connection, for in no case where

both sexes were liberally represented in the host species

could a different capacity for infestation be established for

the two sexes. The only seeming exception, in the case of

previously in common use. The specific names remain the same, except that cornu-
tiis becomes refltxa, Awdi plana is now considered a form of grandis.

Quadrula multiplicaia Unio multiplicatus
tuberculata tuberculatus
metanevra metanever
lachrymosa lachrymosus
asperrima asperrimus
pustulata pustulatus
granifera graniferus
pustulosa pustulosus
plicata plicatus
trigona trigonus
rubiginosa rubiginosus
ebena ebenus

Unio gibbosns gibbosus
complanatus complauatus
heterodon , heterodon

Alasmodonta confragosa Margaritana confragosa
complanata complanata
rugosa rugosa
marginata marginata
undulata nnduhita
tappaniana Unio tappanianus

Strophitus edentulus Anodonta edentula
Anodonta imbecilis iDjhecilis

suborbiculata suborbicnhitu
grandis plana
corpulenta corpulenta

Oblicjuaria retlexa Unio cornutus
Plagiola securis securis

elegans elegans
donaciformis donaciformis

Lampsilis parvus parvus
ellipsis ellipsis
higginsii higginsii
ligamentinus ligamenlinus
luteolus luteohis
nasutus nasutus
anodontoides anodontoides
rectus rectus
ochraceus ochraceus

. ventricosus ventricosus
alatus alatus
la'vissimus la-vissimus
gracilis gracilis
tenuissimus tenuissimus



404 lUinois State Ija})f)r<itor!i of Natural History.

infestation by Bucephalus, has already been explained as the

result of tlie unsexing of the host.

Though this study was instituted primarily with the

trematodes alone in mind, record was made of all parasites

whose presence did not appear to be accidental. I have

presented in Table II. a concise record of these parasites and

of the species and number of their hosts, while the relations

of the one to the other are set forth in the discussion which

follows.

Aspidogaster conchicola von Baer, the most common parasite

of the Unionidce, is confined for the most part to the peri-

cardial and nephridial cavities of the host. In four hundred

and thirty-five cases it was found in the pericardium only,

in seventy in the kidneys only, and in one hundred and

thirty-four cases both cavities contained the parasite. In

only one host species showing any considerable degree of

infestation, Lampsilis parvus,—where twenty out of thirty-

one individuals examined were parasitized,—were the flukes

restricted wholly, to one cavity (the pericardium), and here

the small size of the host may perhaps account for such

restriction. As a rule, though there are many exceptions,

flukes appear in both chambers only when the parasites are

very numerous ; and as the number in the pericardium is

usually much larger than that in the kidneys, and as the

pericardial infection is the more frequent, it would seem that

only in excessive parasitism is the nephridial cavity invaded.

A single Aspidogaster was found encysted in the lateral waU
of the visceral mass of the host. In four cases only, in all

of which the pericardium was ruptured in opening the shell,

were individuals of this species detected in other than the

usual localities, and then their positions were always such as

to suggest escape from the broken pericardium. This para-

site was most frequently found in the adult stage, but eggs

and embryos in abundance and young of varying sizes were

found when the parasitism was considerable. The presence

of the mature trematode in the pericardium and of eggs

within the nephridia is not infrequent.

Cotylaspis insignis Leidy is found adherent to the surface



A Study of the Para.ntes of tlie Vtiloiildw. 405

of the host in the angle between the inner gill and the

visceral mass. Its range is usually restricted to this axilla,

and the number infesting one host is small. In one case,

however,^that of Anodonta corpidentu, recorded by Dr.

Kofoid,—where the number reached the unparalleled extreme

of ninety-two, the flukes extended well oat upon the inner sur-

face of the gill ; and in another, under my own observation,

some of them were crowded down upon the abdominal surface.

In A. suhorhicvlata, in which Cotylaspis attains its maximum
average, thirty-eight to each host, not only are the axill* and
the adjacent surfaces of both inner gill and visceral mass
invaded, but some are usually found within the tubes of the

inner gill, and occasionally even in those of the outer gill.

Such migration from the usual seat of infestation to immedi-

ately adjacent regions is perhaps to be expected in cases of

overcrowding such as are instanced. In a single Lamps'dU
ellipsis, one Cotylaspis unmistakably occurred in the peri-

cardium along with twenty-three specimens of Aspidogaster.

Since Cotylaspis normally frequents the region of the nephrid-

ial openings, an invasion of the pericardium by way of the

nephridia might not be impossible. All the Cotylaspis found

were adults varying little in size. Eggs were not infrequently

observed in the surface slime collected in the vicinity of the

parasites.

Four forms of Distomidce have been found, probably of as

many different species, all immature, and none sufficiently

developed for specific determination. One of these forms

—

referred to in the table as "Free Distomata"—is found in

loose salmon-colored masses either upon or slightly w'^hin

the tissue of the mantle, along or near the dorsal fold. In

Quadrula, Unio, Phiyiola, and Lampsilis this parasite is most

frequently located immediately between the cardinal teeth,

less commonly between the lateral teeth, or, again, upon

the sides, extending over the external surface of the mantle

on a line parallel to its attachment to the viscera. In

the genera Strophitns and Anodonta the distribution of this

parasite is lateral, as just described, often extending over the

mantle surface like a large widely-open inverted Y, with its



406 Illinois State Laboratory of Natural History.

apex just beloAv the umbo and its arms reaching even beyond
and below the anterior and posterior adductor muscles.*

This trematode has not in my experience been found singly,

the number associated having varied from four to many
hundreds. They are habitually loosely adherent by their

suckers to the mantle surface and to each other, but may be

slightly insinuated within the loose tissue of the mantle,

especially when found between the hinge teeth. The position

of this parasite is usually marked by rusty stains in and
upon the nacre, by malformation of the shell or of the hinge

teeth, and not infrequently by a number of dark, poorly

formed pearls. Though these conditions of the superimposed
shell do not always accompany infestation by this trematode,

and though similar abnormalities are found without its pres-

ence being discerned, yet these malformations are very con-

stant where the mass of the parasites is considerable, and
the size and location of the ferruginous stain or injury corre-

spond to those of the infesting colony. When but few are

present and there is no injury to the nacre, the irritation is

no doubt too slight or too recent for much interference with

the normal secretion of the mantle. A malformation char-

acteristic of the presence of this parasite but unaccompanied
by it, would seem to imply desertion for another host. Such
implication is strengthened by the fact that in the case of

some of the host species, individuals are frequently found in

which none of these salmon-colored masses of trematodes are

present, but which nevertheless present malformations of

considerable size in which the rusty, altered, and diseased

nacre is covered Avith a normal layer of later deposit. The
parasite is, moreover, uniformly immature, no matter at

what season it is observed.

The other three species of Distomum were found encysted

in the following situations respectively: (1) in the peri-

cardium of a single individual in each of the species Quadrula
ruhiginosa, Plagiola elegans, and Lampsilis anodontoides

;

(2) in the ventral muscular margin of the mantle in four

*H. L. Osborn ("98, Zobl. Bull., Vol. I., No. fi) describes in like manner this para-
site and its mode of infestation in Anodonta plana (= grandis) and Strophiius
eaentulus from Chautauqua Lake, N. Y.



A Study of the Parasites of the Unionidce. 407

individuals of LainpslUs liganientinKs ; and (3) within the

ovary of a single specimen of LanipsUiH ventricosiis. These
are evidently all immature forms, the clam serving them only

as an intermediate host.

Bucej^hahis polynwrphus von Baer and two other cercaria

forms were found within the viscera of the host. These

usually occurred in such abundance as to obliterate totally

the normal tissue of the sexual glands, rendering the whole

abdomen as granular as fish roe, or fibrous with the sporo-

cysts of the Bucephalus. Extensive infestation with the

latter parasite also involves the nephridia, which may be

much swollen, their ducts being nearly obliterated by the

tangled fibers of sporocysts. This unsexing of the host, and
the accompanying changes in the form of the shell have

already been referred to.

Various species of Atax are common ectoparasites of the

Unionida. Their favorite situations are upon the body

surfaces, between the gills or between the gills and abdomen,

between the labial palps, or among the papillae fringing the

mantle edges at the inhalent siphon. Their eggs are laid

either in the body wall, the gills, or the mantle.

Dr. Eobert H, Wolcott, to whom a part of the Atax

material collected in the course of this study was sent, kindly

furnished the determinations of the species of this genus

included in the following table, which indicates the host

species from which these different parasites were derived,

and also the total number of individuals of each species of

Atax found in a given host species.

Conchophthiriis hirtns Ehrbg. and C. anodontce Ehrbg.,

ciliated Infusoria, inhabit the slime investing the body sur-

faces. In the accompanying tables no attempt is made to

separate the two species.

Chtetofjaster limtuei von Baer, an oligochaete, is also found in

the slime of the various surfaces and in the kidneys.

The frequent presence of leeches and planarians upon the

shell and on the mantle edges, at times indeed within the

branchial chamber, was regarded as accidental, and they are

consequently not included in the accompanying tabulations.



408 Illinois State Lahoratori/ of Natural History.

HOST SPECIES.



A Stiidtf of the Parasites of tlie l^nionidce. 409

Table 11. Spkcific



410 Illinois State Laboratory of Natural History.

species, Alasmodonta rugosa and Lavijysilis ligamentinns,

show no infestation with the generally prevalent Aspidogaster,

and this is especially noteworthy in the case of the latter,

whose infestation with other parasites is both frequent and

severe.

Table III. brings into comparison the number of kinds of

parasites found infesting individuals of each host species and

Table III. Degree of Infestation of Individual Hosts.

HOST SPECIES.

Names.

Quadrula multiplicata . .

,

tuberculata
metanevra
lachrymosa , . .

.

asperrima
pustulata
pustulosa
plicata
trigona
rubiginosa
ebena

Unio gibbosus
complauatus

Alasmodonta confragosa.
complanata
rugosa
margin ata .

Strophitus edentulus
Anodonta imbecilis

suborbiculata .

corpulenta . . .

.

Obliquaria reflexa
Plagiola elegans

donaciformis . .

.

Lampsilis parvus
ellipsis

ligamentinus ..

luteolus
anodontoides .

rectus
ventricosus. . .

.

alatus
gracilis

Totals

50 a

fO o.

28
41

36
29
21

20
87

89
27
40
23
35

137
35
64
18
51

36
49
20
45
32
48
37
31

14

91

25
62
49

108
60
89

15771

Percentage of Individuals
Infested.

39
93
78
76

100
90
68
79
37
38
30
60
88
34
56
72
92
72
94

100
96
59
21

59
68
100
56
68
87
89
87
95
97

. 74

29
20
33
55
33
25
32
43
33
35
30
40
64
26
44
28
31
58
18

"'9"

50
17
48
58

43"

52
18
24
25
13

16

32 25 10 5 0.6

20



A Study of the Parasites of the Uniooiidce. 411

the number harbored by each species considered as a unit.

In this table and in the succeeding one the data regarding

host species in which less than fourteen individuals were

examined are not included. While the comparisons between

the species included are thus rendered the more reliable, it

will be seen that the general conclusions deduced from Tables

III. and IV. only confirm the findings of the more general

statistics of Table II. While nine kinds of parasites are here

listed for the JJnionidce, no species of the family was found

to harbor more than seven, and the average was but four or

five. Moi'eover, in but four species

—

Quadrula lachrymosa,

Q. ehena, Q. pustidata, and Anodonta suborhicidata—were

individuals found with the laiaximum variety of parasites

listed for its species, and in these the maximum variety is

four or less. It is perhaps futile to imagine what variety of

parasites an individual host might successfully sustain, but

it is noticeable that in this table the mean individual infesta-

tion lies closer to the species minimum than to its maximum.
A close inspection of the data of all examinations further

confirms the inference that the individual host is unable to

realize the maximum capacity of its species for infestation,

since in no case is the presence of an unusual number of one

parasite accompanied either by like severe infestation by

another or by a considerable variety of parasites. It is true

that one individual of Lampsilis gracilis with sixteen speci-

mens of Aspidogaster in the pericardium and six in the

nephridia, harbored also two of Cotylasjjis and one each of

Atax and Bucephalus, and that one Lampsilis ventricosus

infested with thirty-one specimens of Aspidogaster contained

large numbers of Bucephalus ; but these are exceptional

cases, and even in these individuals, when we consider the

size of the host and the established maximum capacity of

their species, the extreme limit can hardly be said to be

reached. The ectoparasites probably require but little from

their hosts, but they rarely occur in numbers upon clams

exhausted by Bucephalus.

Table IV. gives the percentage of the hosts which wei'e

infested with Aspidogaster, Cotylaspis, and Atax, the most



412 Illinois State Laboratory of Natural History.

abundant parasites, and also the maximum, minimum, and

average number of these parasites found in the hosts. The

frequency of occurrence of any one of these parasites in

relation to the total infestation of a species is to be learned

by a comparison of Tables III. and IV. Thus in a total of

fifteen hundred and seventy-seven examinations,—in which

seventy-four per cent, were in some measure infested (Table

Table IV. Comparison of Infestation by Different Parasites.

HOST SPECIES.



A Stiidi/ of the Ptirdsiti's of the Unionidce. 418

IIL),^foi*ty-one per cent, were parasitized by Aspidogaster,

eighteen per cent, by Cotylaspis, and thirty-seven per cent,

by Atax—as shown in Table IV. It may be seen from the

tables under discussion, as well as from Table II., that there

is a marked difference between the several host species in

capacity for infestation in both kind and degree.

The range in numljer of parasites infesting one host, and

their average number, may depend to some extent upon the

size of the host,

—

Anodonta suhorhiculata and .1. corpnlenta

showing high numbers and Strophitus edentula and Anodonta

inibecilis low,—and this applies with force in case of occupa-

tion of the pericardial and nephridial cavities by AspidogaHter,

where the volumes of the organs closely limit the possible

number of invading parasites. But size is not the sole

determining factor, else Lampsilis luteolus and L. <niodontoi-

dcs, L. li(nnnentiniis and L. alatus, QiKidrula mnltiplicata and

Lampsilis ventricosvs, and Quadnda plicata and La))ipsHis (/nt-

rilis should harbor similar, rather than so widely different,

numbers of parasites, and little Lampsilis parvus should not

show such large infestation and such a wide range in the

number of parasites harbored.

The tables seem to indicate in the different species a

general correspondence between the frequency of infestation,

the variety of parasites, and the average number of individual

parasites harbored by a given host. Thus Quadnda tuher-

rnlata, Anodonta suborhiciilata, A. corpulcnta, Lampsilis

ellipsis, L. ventricosus, L. alatus, and L. gracilis, all figuring

largely in the tables, are frequently parasitized, carry a large

variety of parasites, and, in proportion to their size, a high

average number individually ; while the statistics concerning

Quadrula mnltiplicata, Q. trigona, Q. ehena, Unio gihbosus,

Ohliquaria redexa, and Plagiola elegans show a like uniformity

in infrequent infestation, little variety in kinds of parasites,

and a low average number harbored by the individual host.

In the light of the latest views upon the natural classifica-

tion of the Unionidce it may be said that closely related

species exhibit somewhat similar capacities for infestation.

In general the species of Anodonta and also those of Lamp-



414 IllinoiH State Lahoratoi-tj of Natural History.

silLs are of large parasite capacity ; those of Unio (restricted)

and of Plagiola are of low capacity ; while within the genera

Qiiadrnhi and Alasinodonta we find wide extremes of infesta-

tion. Within the limits of the above genera this correspond-

ence is more or less evident between members of groups of

closely related species, especially when taken in considerable

numbers in similar situations. For example, we may note

the correspondence between Lampsllis ligamentinus and

L. luteolus; Quadrida chena, Q. trigona, and Q. ruhiginosa;

Lampsilis alatus and L. gracilis ; and Quadrula lachrymosa

and Q. asperrima.

Seasonal changes have been found to modify the distribu-

tion of the parasites in the case of Atax and Conchophthirus

only. As the water grew colder in late October and Novem-
ber, the examinations of Unionidce from the Cedar Eiver

gave relatively fewer adult Atax and more abundant eggs.

The presence of these eggs was regarded as potential infesta-

tion, and therefore these data may properly be included in

the tabulation. The reliability of the tabulations may be

somewhat vitiated by the fact that Conchophthirus is much
more plentiful in the warmer months, during which the

greater part of my collections were made.



A Study of tkr I'drasitcH of the Unionidfe. 41')

Table V. indicates tlie range of the different parasites in

the several localities supplying the material. In all proba-

bility the blanks opposite the more usual parasites are due to

the absence of the proper host species or to the examination

of an insufficient number of these species rather than to

peculiarities in the localities themselves, for the variety of

parasites listed for any situation varies with the number of

individuals and the variety of species examined from each

locality. Thus the absence of Atax from the Schuylkill and

its occurrence in the tributary French and Pickering creeks

may be accounted for by the fact that with the exception of

a single occurrence, this parasite was never found by me, in

any locality, in the particular species examined from the

Schuylkill. Again, an examination of twenty individuals of

Anodonta corpulenta from Abbey Creek, la., made since these

tabulations were completed, has increased the list of unionid

parasites from that stream to six, adding Aspidogaster, Coty-

lasjtis, free I)istomata, Bucephalus, and Conchophthirus, the

smaller number of parasites reported in the table being due

in a large degree to the particular species of Union'uhe

previously examined from the stream. An examination of

Table YI. shows, however, that there is quite a great varia-

tion in the infestation of the same species in different local-

ities. This variation is the greatest, as Avould be expected, in

the host species least frequently parasitized, and especially

in the case of those parasites that are infrequent or unusual

in a given host. Again in the case of the larger streams as

compared with the smaller ones, whenever a given host is

especially plentiful and Union'uhe in general are abundant

the infestation is relatively larger and a greater variety of

parasites occur. For example, in the Illinois and Cedar

rivers, both large streams, a large proportion of the Union idee

are excessively parasitized, but in the Spoon River, a smaller

stream, only such species are extremely infested as are

abundant or dominant, as, for example, Quadrula tuherculata,

Unio fj'ihhosns, and Lampsilis (jrac'ilis. The fact that Unto

complanatus from the Schuylkill River is but slightly parasit-

ized in comparison with individuals from its tributaries.



416 Illinois State Laboratory of Natural History,

Table VI. Comparison of



A Stiidi/ of the Paras'itcH of the I'liioii'Khr. 417

French and Pickering creeks, and from the Susquehanna

Pliver, may be due to the very peculiar conditions, referred to

on page 400, which exist in the Schuylkill. Again, in

Ahismodonta marfi'matu the extensive amount of parasitism

in the material from Pickering Creek as compared with that

from French Creek, is explained by the fact that this is an

abundant and dominant species in Pickering Creek, while it

is relatively infrequent in French Creek.

Some attempt was made to discover whether purely local

conditions in the habitat, such as the character of the bottom

and association with other species of Vnioii\d<r, bear any

relation to the character and degree of infestation. The

examination of representatives of nine species taken at one

time from a restricted locality below a bar in the Illinois

Eiver, where Vnion'uJce were unusually abundant and at least

twenty-nine species represented, gave results which did not

differ materially or in any one direction—save in the slightly

larger infestation to be expected because of the hosts'

unrivaled opportunities for infestation—from those obtained

from the same species collected in other localities,

A purely qualitative examination of the food of the various

species of Union'ulce showed no differences that could be

correlated with their capacities for infestation. The nature

of the food would hardly determine to auy appreciable degree

the parasites of other organs than those closely connected

with the alimentary canal, and least of all those whose lodg-

ment would be effected by mere entrance to the branchial

chamber.

Ill conlusion, the results arrived at by the foregoing studies

may be summed up as follows : The host species seem to

exhibit unlike capacities for infestation, both as to the num-

ber of individuals and the kinds of parasites present. It

appears that the differences shown are attributable only in

a minor degree to the age and size of the host, the size of the

stream, and the density of the unionid population. They

are not sufficiently accounted for by the seasonal variation,

—which is shown to exist to some degree at least in the case

of certain parasites,—nor by the very slight difference in



418 Illinois State Laboratory of Natnrid History.

general structure between the various host species. The

evidence therefore seems to indicate that the capacity for

infestation in each host species is to a large extent a specific

characteristic.

I am indebted to Prof. S. A. Forbes, Director of the Illinois

State Laboratory of Natural History, for opportunities of

study and of publication ; to Dr. C. A. Kofoid, Superin-

tendent of the Illinois Biological Station, for many sugges-

tions of value ; and to Mr. C. A. Hart, of the Laboratory and

Station staff, Dr. H. A. Pilsbry, of the Philadelphia Academy
of Sciences, and Mr. C. T. Simpson, of the United States

National Museum, for assistance in the determination and

nomenclature of the Unionidce.

Just before going to press the nomenclature and systematic

arrangement of the host species have been revised by Mr.

Hart to bring them into accord with the more natural classi-

fication which is now being elaborated for this group.

CoENELL College, Mt. Vernon, Ia.

January, 1899.