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ILLINOIS BIOLOGICAL
MONOGRAPHS

Vol.1 April, 1915 No. 4

Editorial Committee

Stephen Alfred Forbes William Trilease

Henry Baldwin Ward

Published under the

Auspices of the Graduate School by

the University of Illinois

Copyright, 1915
By the University of Illinois

SOME NORTH AMERICAN
LARVAL TREMATODES

WITH EIGHT PLATES

BY

WILLIAM WALTER CORT

Contributions from the

Zoological Laboratory of the I'niversity of Illinois under the direction of

Henry B. Ward, No. 44

THESIS

Submitted in Partial Fulfilment of the Requirements

for the Degree of Doctor of Philosophy

in Zoology in the Graduate School

of the University of Illinois

1914

TABLE OF CONTENTS

. . PAGE

Introduction

General Discussion 7

Forms Previously Described from North America 8

Methods of Study 9

Description of Species
Monostome Cercariae

Cercaria urbancnsis Cort 1914 11

Knoystment of Cercaria urbancnsis 12

Redia of Cercaria urbancnsis 13

Mature Cercaria urbancnsis 14

Aniphistome Cercariae 17

Cercaria inhabilis Cort 1914 18

Redia of Cercaria inhabilis 21

Cercaria diastropha Cort 1914 22

Redia of Cercaria diastropha 23

Critique of Cary's Work on Diplodiscus temporatus 24

Cercaria Caryi Cort 1914. 30

Distome Cercariae 31

Gymnocephalus Cercariae 31

Megalurous Cercariae

Cercaria megalura Cort 1914 31

Redia of Cercaria megalura 32

Echinostome Cercariae 36

Cercaria trivohis Cort 1914 37

Redia of Cercaria trivolvis 39

Cercaria rubra Cort 1914 40

Appendix to Echinostome Cercariae

Cercaria rcflcxae Cort 1914 41

Redia of Cercaria rcflcxae 43

Microcercous Cercariae

Cercaria trigonura Cort 1914 44

Redia of Cercaria trigonura 46

Furcocercous Cercariae

Cercaria douthitti Cort 1914 49

Xiphidiocercariae 52

Polyadenous Cercariae

Cercaria isocotylea Cort 1914 53

Cercaria polyadcna Cort 1914 55

Cercariae Ornatae 57

Cercaria hemilophura Cort 1914 58

Microcotylous Cercariae 59

Cercaria leptocantha Cort 1914 60

Cercaria brevicaeca Cort 1914 61

The Classification of the Cercariae

General Discussion 62

Luhe's Classification of the Cercariae 64

Lebour's Classification of the Cercariae 66

Bibliography _ 68

Explanation of Plates 71

453] LARVAL TREMATODES—CORT

INTRODUCTION

Practically nothing is known of the life-histories of the trematodes
of North America. Even in Europe where many new adults are being
described each year only a few developmental cycles are completely
known. One reason for this is to be found in the difficulties involved.

Two methods of attacking trematode life-history problems have been
employed. One is to attempt to prove specific identity between cercariae
and adults by structural comparisons, and the other is to attempt to
find the relationship experimentally. Positive results from the first
method have been few. The structure of certain types of cercariae gives
no suggestion of the family to which the adult belongs, and in the more
differentiated forms generic identification is very rarely possible. Even
when the comparison is narrowed to very similar cercariae and adults
from limited localities it is safest to consider the results as merely sug-
gestive. Many errors have crept into the literature from too much reli-
ance on this method. In some cases the mere suggestion of probable
identity by one author has been taken by another as if it were an estab-
lished fact. It is only when the adult has been experimentally raised
from the cercariae or the larvae from the eggs of the adult that the
establishment of specific identity can be made sure. The experimental
method of study also has its dangers. The factors involved are so com-
plex and so little understood that only the most carefully controlled
experiments can be considered as conclusive. In some very recent work
as well as in older papers larvae and adults, shown later to be entirely
unrelated, have been joined experimentally.

In connection with efforts to solve developmental problems it is
often argued that given the structure of a cercaria, it is possible to
draw conclusions as to the environment where it encysts, the life it
leads, etc. Further, that if the course of development of one member
of a group is known, it can be concluded that the others follow
the same lines. Dollfus (1914) finds evidence against both generaliza-
tions. He states that after a comparative examination of cercariae and
the environment in which they live, he can assert that cercariae very
similar in structure dwell in different hosts and have very different kinds
of development, and that cercariae very different morphologically live

8 ILLINOIS BIOLOGICAL MONOGRAPHS [454

in identical environments and have similar courses of development. Cer-
tainly in the study of the development of digenetic trematodes it will
be necessary to increase very greatly the number of particular instances
known before the induction of general principles can safely go very
far.

In Europe, where more is known of trematode development than
elsewhere, the foundations of this study were laid in the middle of the
last century by the work of such men as Leuckart, Wagener, Pagen-
stecker, La Valette St. George, de Filippi, and Moulinie. Altho they
made but little progress in the actual working out of developmental
cycles, their descriptions of large numbers of cercariae from molluscs
have formed the basis for later work. In England Nicoll and Marie
Lebour have very recently made some headway in the study of larval
trematodes altho as yet little experimental work has been done. In North
America only a beginning has been made in the study of adult trema-
todes and as yet there are only a very few scattered observations of larval
stages. The present work was undertaken by the writer at the sug-
gestion of Professor Henry B. Ward as an attempt to open up this almost
untouched field in North America.

Most of the descriptions of larval trematodes from North American
molluscs are very inadequate and in many cases it is impossible to tell
to which general group of cercariae the forms belong. In only a very
few instances are either drawings or measurements given. The follow-
ing list is an attempt to bring together all references to date on larval
trematodes from North American molluscs.

1. Cercaria hyalocauda Haldeman (date?) also reported by Evarts
(1880). Host Physa heterostropha Say. Locality (?).

2. Cercaria bUineata Haldeman (1840). Host Limnea catascopium,
Camden, Delaware.

3. Cercariaeum vagens (Leidy) (1847: 220-221)
Syn. Distoma helicis (Leidy 1847:220-221)

" pericardium Creplin (1849)
vagens Leidy (1850: 304-310)
Cercariaeum helicis alternatae Diesing (1855:398)
398)
vagens Diesing (1858:42)
Host Helix alternata and Helix albolabris, Philadelphia, Pa.

4. Cercaria agilis Leidy (1858: 110). Found free in the Delaware
Tiver.

5. Monostoma (Olenocercaria) lucania Leidy (1877:200-201)
Host Planorbis parvus. Philadelphia, Pa.

455] LARVAL TREMATODES—CORT 9

6. Distoma (Gymnoccphala) ascoidca Leidy (1877:201). Host,
Planorbis parvus, Philadelphia, Pa.

7. Distoma ccntrappendiculatum Leidy (1890:416).
Syn., Distoma appendiculatum Leidy (1877:202).
Host, Helix arbora, Philadelphia, Pa.

8. Distama cornifrons Leidy (1878:382-383). Host, Donax fosser,
Cape May, New Jersey.

9. Distoma lasium Leidy (1890). Host, Ilyanassa obsoleta, Beach
Haven, New Jersey.

10. Ccrcaria platyura Leidy (1890:415-416). Free in a pool at
Fort Bridger, Wyoming.

11. Cercaria of Diplodiscus temporatus Stafford, Cary (1909).
Host, Goniobasis virginica, Princeton, New Jersey.

The present paper adds fourteen new species of cercariae from North
American fresh-water snails. A preliminary report, taking up briefly
the structure and activity of these cercariae has already been published
(Cort, 1914).

METHODS OF STUDY

To obtain this material examinations were made of large numbers
of snails from various localities. I want to express my thanks to Dr.
Ruth Marshall, Dr. M. F. Guyer, Dr. G. R. La Rue, Dr. B. M. Allen,
Dr. F. W. Carpenter, Dr. H. S. Pratt, Mr. A. F. Coutant, Mr. Hermann
Douthitt, Dr. C. S. Mead, Dr. A. Richards, Dr. C. C. Nutting, and Dr.
J. E. Ackert for their kindness in collecting and shipping me living
snails. Without their aid it would have been impossible to have ob-
tained the material for this study.

To Professor Henry B. Ward, under whose direction this work has
been carried on, I wish to express my appreciation for his interest and
helpful suggestions.

The following method was employed in the examination of snails
for larval trematodes. The shell was cut or crushed so as to remove
the body, if possible, unbroken. An examination was then made with
the low power of the microscope. If larval trematodes were present,
some of them were almost invariably loosened from the infected part
and scattered around the snail in the water. The digestive gland was
the organ most usually infected, its color being often changed by the
pigmentation of the sporocysts or rediae. When infection was found
a part of the diseased tissue was preserved whole for sectioning, an-

10 ILLINOIS BIOLOGICAL MONOGRAPHS [456

other portion was teased apart to free the larvae for preserving for
toto mounts, and the remainder was used for the study of the living
animals. Much of the anatomy of the cercariae could be made out
from living specimens. In fact this proved to be an extremely impor-
tant part of the study, since some points, for example the smaller
branches of the excretory system and the movement, could only be
observed in this manner.

For preservation of material a number of fixatives were used. In
fixing the freed cercariae for toto mounts the best results were obtained
by the use of hot solutions of Bouin's picro-aceto-formol or corrosive-
v acetic. For sectioning the infected organs were fixed in toto, since as

suggested by Cary (1909:597) this is easier and gives better results
than attempting to section freed individuals. A corrosive-acetic solu-
tion was ordinarily used for fixing this material. Rediae and cercariae
for toto mounts were stained in Mayer's haemalum, Delafield's haema-
toxylin, Conklin's picro-haematoxylin, and Mayer's paracarmine. The
specimens were as a rule considerably overstained in dilute solutions
of the stain and differentiated in HC12 under the microscope. In
mounting it was found convenient to place large numbers of the larvae
on one slide. The infected organs to be sectioned were usually stained
^ in bulk in Ehrlich's acid haematoxylin, cut into sections 5 to 7 micra
in thickness, and differentiated on the slide, part of them being coun-
terstained in eosin.

On account of their great mobility, small size, and remarkable
power of changing their shapes, some cercariae are very difficult objects
to study. No very accurate measurements can be made of living speci-
mens, and in preserved material they are often contracted and dis-
torted. With the living cercariae an attempt was made to get the
range of variability, and in preserved material, whenever possible, the
average measurements of a number of well extended specimens were
taken. The measurements of preserved material are less than those
from living specimens of the same kind, even the suckers shrinking
perceptibly after preservation. For these reasons comparisons of the
cercariae based on size and shape, as Liihe (1909:173) has suggested,
are not always by themselves very reliable criteria for specific deter-
mination.

457] LARVAL TREMATODES—CORT 11

MONOSTOME CERCARIAE

About five per cent., of the largest specimens of Physa gyrina
from a drainage ditch north of Urbana, Illinois, examined in December,
1913, were infected with the rediae and cercariae of a monostome. I
propose to name this species Cercaria urbanensis. The infection was in
the liver of the snail and there were present both rediae in different
stages of development and free cercariae. No sporocysts were found
and none of the rediae contained rediae. It is interesting to note that
in the descriptions of monostome larvae no mention is made of sporo-
cysts, and only one observation of rediae developing from rediae.
Braun (1892:805-806) notes that in certain of the monostomes rediae
are already developing in the free swimming miracidia, which he con-
siders to already represent sporocysts. Looss (1896:197) states that
in material of Cercaria imbricata Looss collected from Bythinia ten-
taculata near Leipzig, rediae were present in which rediae were devel-
oping. From this he concludes that the life-history of this species is
accomplished in the same manner as that of the amphistomes, which
have several generations of rediae.

When freed from the liver of the snail the redia had considerable
power of extension and contraction. The immature ones especially
stretched out the anterior end and reached in all directions. No loco-
motor appendages were present either in young or mature redia and
no locomotion was noted. In fact the redia of Cercaria imbricata Looss
is the only monostome redia that is reported with locomotor append-
ages. According to Looss (1896:196) the appendages in this species
were well marked in the young redia but more or less effaced with
advancing age. In some of the largest rediae of Cercaria urbanensis
very peculiar annular constrictions were noted in different parts of the
body, which divided it into two or three separate regions connected by
very narrow passages (Fig. 12). These constrictions seem to be due
to temporary unequal contraction states in different regions of the
circular body muscles. This condition for the redia of Cercaria urban-
ensis was observed only in preserved material. Leidy (1877:200;
1904:143-144) notes such constrictions in the living redia of a mono-
stome cercaria which he calls Monostoma lucanica.

12 ILLINOIS BIOLOGICAL MONOGRAPHS [458

The methods of locomotion of Cercaria urbanensis, either when
swimming in open water or when upon a substratum, are very striking.
The body when swimming was contracted almost into a round ball and
the tail, which was curled ventrally so that it passed across the ventral
surface of the body, lashed backward and forward with great rapidity.
This method of locomotion was very effective and the cercariae could
be seen even with the naked eye in rapid movement thru the water.
In spite of the lack of a ventral sucker the cercaria was able to move
well on a surface by utilizing two projections which form the posterior
lateral angles of the body. In the process of locomotion the cercaria
took hold with its oral sucker and the body contracted until it was
practically round. Then the sucker let go its hold and the body
stretched out, at the same time extending the posterior projections
until they became little points digging into the substratum. Again the
oral sucker took hold and again the body contracted. By a continued
repetition of these movements this cercaria progressed at about the same
rate of speed as those having two suckers. After each contraction the
posterior projections held the amount gained. During the contraction
and at the beginning of the extension of the body the tail kept lashing
very rapidly, but during the rest of the movement it was held still and
somewhat contracted.

Some of the freed cercariae after moving around for a while
settled down and formed cysts. A cercaria which was moving along
on a surface extended and contracted its body more and more slowly,
until while retaining a hold with the oral sucker the body
became almost round. While in this position the cystogenous material
was extruded, and the . appearance was soon given of a round cyst
with a tail attached. Soon the worm inside freed itself from its con-
nection with the tail and squirmed around in the cyst. Finally the tail
wriggled loose from the cyst and continued swimming around for con-
siderable time, resembling in form and movement a free living nema-
tode. The cysts formed in a watch glass were flattened on the lower
surface and had much the shape of a chocolate drop. The process of
encystment is illustrated by figures 1, 2, 3, and 4. Encystment in the
open has been noted for Cercaria ephemera Nitzch. The encystment
of this species, which was first reported by Nitzch (1807), was the first
described for a cercaria. La Valette St. George (1855:33-34) described
and figured the process for the same species, and in one of his figures
shows the tail attached to the cyst. Von Linstow (1896:377) for Cer-
caria monistomi von Linstow describes the encystment in the same host
as the larval generation.

459] LARVAL TREMATODES—CORT 13

The rediae of Cercaria itrbancnsis (Figs. 10, 11, and 14) were in
various stages of development, varying in length from 0.52 mm. to
1.08 mm. and in width from 0.12 mm. to 0.22 mm. In shape they
are elongate sac-like forms smallest at the anterior and widest a short
distance in front of the posterior extremity. The mouth is at the an-
terior tip and the pharynx varies with the size of the animal. In the
smallest redia studied which had a length of 0.52 mm. the pharynx
was 0.038 mm. in length by 0.048 mm. in width while in one 1.08 mm. in
length it was 0.07 mm. by 0.08 mm. The passage thru the pharynx
immediately opens into the intestine which is at first narrow but soon
widens greatly. In all stages of development the intestine is propor-
tionally very large, having a diameter of from one-third to two-thirds
the width of the body and reaching to within 0.08 mm. to 0.16 mm.
of the posterior end. It is rather clear and transparent and contains
only a small amount of food material floating in a considerable quantity
of fluid. The outer cuticula of the redia is very thin and can hardly
be distinguished as a separate layer. Inside of this is a very strong
layer of circular muscles which encircle the body as separate strands,
each about 0.0018 mm. in width and about the same distance apart.
The strands of the longitudinal layer are very thin and only visible
under favorable conditions. Inside of the muscle layers are several
rows of parenchymatous cells with rather large nuclei. These cells do
not form a circumscribed lining of the body cavity but are irregular
and in young rediae where the lining is several cells thick, strands
extend from them thruout the body cavity forming an irregular net-
work (Fig. 13). Into this loosely filled space between the body wall
and the intestine the cercariae push. In the oldest rediae the wall
has been reduced to one layer and the movements of the cercariae have
broken down the parenchymatous strands and converted the region
between the wall and the intestine into a well defined cavity, which
is more or less completely filled with developing cercariae. In the
posterior end of the redia is the germ gland, in front of this are germ
balls and further forward in the older rediae are differentiated cercariae.
In the youngest redia (Fig. 10) studied the developing embryos were
all back of the middle of the body and the furtherest developed was a
mass of embyronic cells about 0.09 mm. in length and 0.06 in width
and having no tail. In none of the rediae was the body cavity crowded
with cercariae, there being but two or three well developed forms, and
in one mature specimen there were no differentiated cercariae present
leaving the body cavity empty for about two-thirds of its length. These
conditions and the fact that the oldest cercariae in the rediae are not
fully mature and that matured and almost matured cercariae are found

14 ILLIXOIS BIOLOGICAL MONOGRAPHS [460

free in the livers of the infected snails, show that the cercariae as
they develop are continually making their way out of the rediae to
finish their growth in the liver of their host. The most highly differ-
entiated cercaria found in the redia (Fig. 11, 14, cr) was 0.3. mm. in
length and 0.1 mm. in width and had a tail 0.22 mm. in length. The
two lateral eye-spots were developed and the pigment was present
to a considerable extent around them, but was not further scattered
or concentrated to form the so-called intermediate eye-spot. Outlines
of the longitudinal vessels of the excretory system and the anlage of
the reproductive system could be clearly distinguished. The oral sucker
was well defined but none of the rest of the digestive system could
be distinguished.

The mature Cercaria urbanensis (Fig. 5) varies greatly in shape
being at greatest contraction nearly round, about 0.27 mm. in length
and 0.20 mm. in width and when extended 0.54 mm. long and 0.11 mm.
wide. When not in motion the tail is contracted being about 0.2 mm.
long and 0.05 mm. wide at its base. At times of greatest movement
the tail becomes attenuated to about one-half its usual diameter and
often reaches a length of 1.2 mm. It is weakly attached to the dorsal
mid-line of the posterior end of the body and tapers to a sharp point.
There is an inverse ratio between the contraction of the body and the
tail for when the body is most contracted the tail is most extended and
vice versa.

In view of the great power of movement of Cercaria urbanensis
the histological structure of its tail is of considerable interest. Just
inside of the thin cuticula is a layer of circular muscles arranged as
strands separated by about twice their length from each other. Inside
of this is a very strong layer of longitudinal muscle fibers each 0.0026
mm. in diameter. Next comes a single layer of parenchymatous cells
somewhat irregularly elongated with nuclei 0.005 mm. in diameter. Ex-
tending the length of the tail and forming a core are two rows of long
cells which are close together and have their long axes parallel with the
length of the tail. These cells vary in size having a length from 0.028
mm. to 0.035 mm. In a cross section of a tail 0.075 mm. wide one of
these cells measured 0.026 mm. in thickness and 0.014 mm. in width. They
are full of heavily staining granules and their nuclei are 0.007 mm.
in diameter. There is nothing suggestive of a possible function for
these cells. Figure 9, a cross section thru a tail, shows the structures
described above.

At the posterior lateral angles of the body are projections which
may be extended to aid the animal in locomotion. The tips of these
projections are made firm by structures which appear to be infoldings

461] LARVAL TREMATODES—CORT 15

of the outer cuticula. One of the structures (Fig. 8) is ovoid with
two cuticular walls, having a loosely fitted space between them, and
a very narrow central cavity communicating with the exterior by a
small pore at the tip of the projection. They apparently have no suck-
ing action, since no muscles are present and the central cavity con-
tracts when the projection is extended. They evidently have a function
in locomotion analogous to setae. Similar locomotor projections have
been described for Cercaria ephemera Nitzch and Cercaria imbricata
Looss, and for Leidy's (1877:200) (1904:143-144) Monostoma lucania
from North America they are merely mentioned as conical projections.
Ssinitzin (1905, Plate 4, figs. 75 and 76) figures these structures
in Cercaria ephemera Nitzch as clearly circumscribed projections with
a considerable cavity lined with spines. Figures 6a and b are copies
of his figures. Certainly the structures figured by Ssinitzin differ con-
siderably from those of Cercaria urbanensis. Marie Lebour (1907:442)
in the monostome cercaria from Paludestrina stagnalis which she con-
siders to be Cercaria ephemera describes the posterior locomotor pro-
jections as sucker like structures, which are circular in outline and
divided in two by a bar. Her figure shows them as structures quite
comparable to those of Cercaria urbanensis but very different from*
Ssinitzin 's figures. She assigns no especial reason for considering them
to be sucking structures. The posterior locomotor projections as described
by Looss (1896:194) for Cercaria imbricata offer still greater differences.
The cavity is comparatively large with but one cuticular wall and
divided at its center into two parts by a projection. Figure 7 is a
copy of Looss', Plate 14, Figure 151.

Mature specimens of Cercaria urbanensis are heavily pigmented
especially at the anterior end, and have two lateral pigmented eyes
with lenses and a central anterior spot which is formed by a condensa-
tion of pigment. The eyes are situated dorsad at each side of the large
esophageal commissure and above the obtuse angles formed by the
large nerves which pass forward and backward. Each true eye is
formed by a mass of pigment in the form of a cup the bottom of which
is thicker than the sides. A lens fits into the opening of the cup, leav-
ing a space between its lower surface and the bottom of the cup. An
eye spot has a diameter 0.017 mm. and the depth of the pigment cup
is 0.024 mm. At the anterior end surrounding each eye are scattered
pigment granules extending in all directions and about as clearly defined
ventrally as dorsally. Anteriorly a condensation forms the so-called
anterior eye spot, and scattered granules reach well beyond the limit
of the oral sucker. Posteriorly the pigment granules become more

16 ILLINOIS BIOLOGICAL MONOGRAPHS [462

scattered and continue as two longitudinal lines to the posterior
end of the body on each side of the excretory bladder (Fig. 5,
p. 1). These lines of pigment extend thru the whole dorsal ventral
thickness of the body and are as apparent on one side as on the other.
Along each edge and appearing ventrally are two other irregular lines
of pigment which do not extend as far back. Little pigment flecks
are scattered from these lines out thru the body. In the young cercaria
the pigment develops first around the eyespots in dense masses and
spreads gradually with growth both forward and backward along the
lines mentioned. It is only in the oldest cercaria that it is spread suffi-
ciently to form the anterior spot and the lines extending to the posterior
end.

The digestive system (Fig. 5) of Cercaria urbanensis is like that
described for other monostome cercariae. The mouth at the anterior
tip is sub-ventral and the oral cavity is surrounded by a relatively small
oral sucker, averaging 0.043 mm. in length and 0.049 mm. in width.
The esophagus is very narrow and 0.05 mm. to 0.06 mm. long depending
on the contraction. The cecal bifurcations are close together at their
beginnings but soon spread further apart. They are always within and
slightly dorsad of the large longitudinal excretory vessels and extend
almost to the end of the body. The ceca are not yet functional, being
composed of a solid mass of cells.

The excretory system (Fig. 5, ex) is typical of the group. At the
posterior end is the excretory bladder opening just below the base
of the tail. From this extends forward two large vessels which unite
in the midline just back of the oral sucker. Thruout their whole course
they are filled with small round concretions which disappear in the
process of preservation. It is very difficult to be positive of the rela-
tions of the excretory system of the tail but as nearly as they can be
made out they are as follows: No openings could be found in the
tail and a single vessel passed forward from near the tip becoming
larger nearer the body and opening at the excretory pore.

Almost the whole body of Cercaria urbanensis is filled when mature
with large unicellular cystogenous glands containing small granules.
Only the very anterior tip, the posterior locomotor projections and the
tail are free from them.

Only a few monostome cercariae have been recognized. All of these
except Cercaria lophocera Filippi (1857:5) correspond very closely in
structure to Cercaria urbanensis. Cercaria imbricata Looss is distin-
guished by the fact that the rediae have smaller intestines and lateral
appendages and by the structure of the posterior locomotor projections
of the cercaria (Looss 1896:192-197). Several different forms have

463] LARVAL TREMATODES—CORT 17

probably been described as Cercaria ephemera as it seems improbable
that Ssinitzin and Lebour have described the same form. The only
difference that can be definitely determined by comparing the descrip-
tions of Cercaria ephemera with Cercaria urbane mis is in the structure
of the posterior locomotor appendages of the cercaria. In von Linstow ?s
(1896:376-377) description of Cercaria monostomi both rediae and cer-
cariae are larger than in any of the other species, the arrangement
of pigment is different from that in Cercaria urbane mis, and no poste-
rior locomotor appendages are described. These may have been over-
looked as they are very small and not easily seen unless the animal is
studied alive. Cercaria lophocera described by Filippi (1857:5) from
Italy is entirely different from all other monostomes known.

Two descriptions of monostome cercariae have been made from
North America. Leidy (1877:200-201) describes as M onostoma lucanica,
a form from Planorbis parvus. Unfortunately not enough detail is given
to make the comparison possible. The other form is described by Halde-
man as Cercaria hyalocauda. It has been impossible to find Haldeman's
original description. Evarts (1880) describes this species from Physa-
heterostropha. Altho so little detail is given that a minute comparison
cannot be made between this species and Cercaria urbanensis, certain
points can be definitely made out. Cercaria hyalocauda is about half
again as large as Cercaria urbanensis, and its cyst is much larger than
that of the latter species, being 0.32 mm. to 0.20 mm. It seems evi-
dent that the two forms are not identical.

Altho monostome cercariae have been known since 1817, the life-
history of no one of them has been proven experimentally. Looss (1896 :-
192-193) argues from distribution and structural correspondence that
Cercaria imbricata from Egypt is the larval form of Notocotyle triseri-
ale from the duck. Liihe (1909:178) suggests that Cercaria ephemera
is the larval form of either Notocotyle triserialc or Catatropis verrucosa
(Frol.), but cannot belong to Typhlocoelum flavum (Mehl.) on account
of differences in the digestive systems of the two forms. So little is
known of the monostomes of the United States that it is useless to gen-
eralize in regard to the life-history of Cercaria urbanensis.

AMPHISTOME CERCARIAE

Amphistome cercariae of two species were collected from speci-
mens of Planorbis trivolvis from three localities. Two snails out of
eighteen from Lawrence, Kansas had the livers infected with rediae
and very large pigmented cercariae. Out of large numbers of Planorbis
trivolvis examined from around Urbana, Illinois, one from a small pond

18 ILLINOIS BIOLOGICAL MONOGRAPHS [464

was infected with this same form. The second of these species, a
smaller unpigmented cercaria, was found in one of twenty specimens
of Planorbis trivolvis from a small pond in the suburbs of Chicago.
In all the infected snails mature and immature cercariae were found
free in the liver, the mature forms being nearest the periphery, and
the active rediae contained no fully developed cercariae. There were
no sporocysts present and no rediae in which rediae were develop-
ing, and in none of the infected snails were rediae or cercariae numerous.
Since the large pigmented species is very unwieldy in movement, I
propose to name it Cercaria inhabilis, and the smaller species on account
of the way in which it changes its body shape will be given the name,
Cercaria diastropha.

Cercaria inhabilis swam sluggishly in open water. It contracted
its body and lashed its tail backward and forward, moving in an
unwieldy, irregular fashion. In fact the body was too large in propor-
tion to the size of the tail for rapid locomotion. On a substratum
the cercaria extended and contracted the body but was unable to move
by the aid of its suckers.

When in a state of average contraction, about that of figure 16,
the body of Cercaria inhabilis is pear shaped, tapering in the anterior
half, and wider but of uniform diameter posteriorly. It is the largest
of the cercariae studied, having an average length in mounted speci-
mens of 0.8 mm. and a width of 0.4 mm. The thickness is a little
greater than half the width. The oral sucker is elongate, 0.16 mm. in
length and 0.12 mm. in width with the retrodorsal pharyngeal pockets
which are characteristic of some amphistomes. The acetabulum is very
large averaging 0.23 mm. in diameter; it is at the posterior end of
the body and is turned ventrad.

Two large eye-spots are present just back of the pharynx. They
are located from one-fourth to one-third of the distance from the anterior
to the posterior end, and in a specimen 0.27 mm. wide at this region,
they were 0.065 mm. from the outer margins and 0.13 mm. apart. These
eyes are composed of the lens and the cone of pigment like those
already described for the monostome, Cercaria urbanensis. Figure 17,
a section thru the eyes, shows them in their relation to the nervous
system and other adjacent structures.

In the development of Cercaria inhabilis the pigment starts in the
eyes and is deposited first in a peculiar way over most of the anterior
half of the body. In the youngest cercariae found outside of the
rediae very little pigment is seen and that found near the eyes
(Fig. 18, p.). In forms a little older (Fig. 19, p.), the pigment has

465] LARVAL TREMATODES—CORT 19

begun to spread out a little forward and to the sides, but for the most
part backward along two irregular lines. At first all the pigment
strands were connected in an irregular way, but soon at the ends of
the lines little flecks were scattered out. Backward along the lines
the pigment becomes thicker (Fig. 20, p) and collects at points of union
in the network masses. At an older stage, (Fig. 21, p) the pigment
strands from the two lateral lines become connected and form an irreg-
ular network about the middle of the oral sucker, laterad as far as the
sides, and backward to about the middle of the body. As the cercariae
become older the masses and lines of pigment break up and are more
scattered. In the mature specimen (Fig. 16) the pigmentation shows
as scattered brownish flecks extending thru the whole thickness of the
body (Fig. 17, p) and reaching from the oral sucker back to the middle.

The cystogenous glands in Cercaria inhabilis are as thickly devel-
oped dorsally as ventrally, and extend from the oral sucker to the
acetabulum. Viewed from the surface they appear as small rounded
bodies, 0.012 to 0.016 mm. in diameter, filled with rod-shaped cysto-
genous granules. Figure 17 shows them to be elongate, unicellular, club-
shaped glands with small nuclei.

The tail of Cercaria inhabilis varies from one-third to greater than
the body length. It is attached to the tip of the body above the acetab-
ulum and is easly lost in free swimming animals. The cuticula of the
tail is very thin and no trace could be found of circular muscles. The
longitudinal muscle layer, however, is conspicuous and is formed of a
series of strands each 0.0035 mm. in thickness, which extend from the
base to the tip. Inside of the muscles is a layer of irregular paren-
chymatous cells, with nuclei 0.005 to 0.007 mm. in diameter, and irreg-
ular indistinct cell boundaries. The space between this layer and the
excretory tubule, which courses down the center of the tail, is filled
with large cells with faintly granular cytoplasm, and large nuclei, 0,008
to 0.009 mm. in diameter. These cells are similar to those forming the
core of the tail of Cercaria urbanensis, but inclose no such darkly stain-
ing granules. Several of these cells may occur in one cross section since
they do not seem to be arranged in regular rows. The central excretory
tubule of the tail has a considerable diameter, and contains in its walls
very large scattered nuclei, 0.01 to 0.012 mm. in diameter. A cross
section of the tail (Fig. 22) shows these structures.

A comparison of the nucleoli of the different nuclei of the tail of
Cercaria inhabilis is interesting. Of the three kinds of nuclei present
viz. 1. nuclei of the excretory tubule, 2. nuclei of the large central
cells, 3. parenchymatous nuclei, the first two have large very clearly
defined nucleoli and little if any chromatin scattered outside of it. Of

20 ILLINOIS BIOLOGICAL MONOGRAPHS [466

these two the second is the larger. In the parenchymatous nuclei the
nucleoli are quite small, not definite in outline, and much chromatin
is scattered thru the nucleus.

The excretory vessel in the tail of Cercaria inhabilis is a large
single tube running down almost to the tip, where it divides to open
to each side (Fig. 16). This vessel becomes narrow as it passes from
the tail to the body but immediately widens into the excretory bladder,
which is a triangular vesicle dorsad and anteriad to the acetabulum.
No vessels or flame cells were made out in the acetabulum. Into
each corner of the bladder open vessels which were traced only as far
as the eye-spots. In these vessels were present the round highly refrac-
tive concretions, already noted in the excretory system of Cercaria
urbanensis and by Looss in the amphistome cercariae described by
him (Looss, 1896:181).

The mouth of Cercaria inhabilis is at the anterior tip of the oral
sucker which juts out slightly from the body. The anterior margin
of the mouth cavity is smooth or only slightly roughened. There are
no papillae present around the margin of the oral cavity such as Looss
(1896:179) describes for the amphistome cercaria from Egypt, which
he considers, to be the larva of Gastrodiscus egyptiacus. The mouth
opens into an oral cavity, from which a narrow passage runs directly
backword; this changes into the esophagus at the posterior limit of
the oral sucker. Into each side of the oral cavity open the blind tubes
of the retrodorsal pharyngeal pockets (Fig. 16, pp.). The oral cavity
has a length of 0.059 mm. and the lateral blind tubes are 0.086 mm.
in length. As it passes out of the oral sucker the digestive tube becomes
the thin, straight-walled estfphagus, 0.021 mm. in caliber, which has a
length of 0.1 to 0.15 mm. depending on the state of contraction of
the cercaria. Just before the esophagus bifurcates it is reinforced by
a mass of circular muscle fibers of a sphincter, making a characteristic
structure which superficially resmbles a pharynx. This pseudo-pharynx
is about as long as wide, being 0.038 mm. in diameter, and has a wall
0.009 mm. thick, composed of from 12 to 14 separate layers of mus-
cles. Longitudinal muscles were not made out either in this region or
in the esophagus. The inner lining of the esophagus is non-cellular
and tiny projections extend out into its lumen (Fig. 17, es). After
bifurcation the intestinal ceca run laterally for a short distance and
then turn posteriad to reach within 0.05 mm. to 0.06 mm. of the anterior
margin of the acetabulum. The ceca are fairly wide, 0.028 mm. to
0.036 mm., and the inner walls contain flattened nuclei which jut out
a little into their lumina. Figure 16 illustrates the relations of the
digestive system.

467] LARVAL TREMATODES—CORT 21

In Ccrcaria inhabilis the anlage of the reproductive organs begins
to take definite shape. It is composed of dense masses of small heavily
staining nuclei. In the largest cercariae four areas connected by lines
are marked out. At the middle line of the body just back of the bifurca-
tion of the intestinal ceca, is an elongate mass which reaches up very
close to the ventral surface. This is probably the anlage of the ends
of the ducts of the reproductive system, leading up to the genital pore.
Slightly back of this and close together are two masses, the primordia
of the testes, and further back is the largest densest mass which repre-
sents the ovary and its surrounding structures (Fig. 16, ra).

Along with the cercariae in the livers of the infected snails were
numbers of active radiae all in about the same stage of development
(Fig. 15). "When they were freed from the snail they were very mobile
extending and contracting and making some progress even on the smooth
surface of the watch glass. There were two pairs of locomotor append-
ages and the posterior extremity was attenuated and pointed. The
anterior pair of locomotor appendages was just back of the posterior
limit of the intestine and at normal extension the second pair was
about the same distance back. The tail region was shorter and more
slender. These proportions varied greatly with the contraction of the
animal. In alcoholic material the locomotor appendages of the redia
are often obliterated by the contraction of the muscles.

One of the largest of these rediae (Fig. 15) measures 1.36 mm.
in length and 0.31 mm. in width, and the posterior limit of the volumin-
ous intestine is 0.56 mm. from the anterior tip. The pharynx is very
small in proportion to the size of the body, measuring 0.086 mm. in
length and 0.065 mm. in width. The intestine contains dark brown
materal evidently from the liver of the host. Bunched around the
anterior tip of the redia just back of the oral sucker are elongate uni-
cellular glands of the type often found in rediae, which send forward
ducts to open near the tip.

The cuticula and muscle layers of the rediae of Cercaria inhabilis
are quite thin. The circular muscles are the strongest but do not show
externally. Inside of the muscles except in the region of the anterior
part of the intestine and the germ gland, the wall is made up of a layer
of cells which is thin in the older forms. In none of the rediae are
embryos much differentiated, the largest showing mere stumps of tails,
the beginning of suckers and traces of eye-spots. Since immature cer-
cariae are found outside of the redia it is evident that they make
their way out at a very early stage, and complete their development
free in the snail's liver.

22 ILLINOIS BIOLOGICAL MONOGRAPHS [468

Since Cercaria diastropha resembles Cercaria inhabilis closely the
description will be limited to pointing out the differences. As in Cer-
caria inhabilis different stages of development of the cercaria are found
free in the snail. The mature cercariae extended and contracted their
bodies very actively but none were noted swimming freely. This may
have been due to the condition of the material studied. There was no
cheek on the one examination.

Cercaria diastropha (Fig. 23) is cylindrical in cross section, pointed
anteriorly, and when not contracted or flattened the region in front
of the acetabulum is but little wider than that sucker. In living speci-
mens the body varied from 0.27 to 0.54 mm. in length according to the
state of contraction, and the width changed from 0.20 to 0.08 mm. The
tail is always shorter than the body, being 0.22 mm. to 0.38 mm. in
length and with an average width near its base of 0.054 mm., The tail
is attached dorsad to the acetabulum, which is terminal and forms a
flattened base for the conical body (Fig. 24). The oral sucker is elong-
ate and in a specimen of average contraction has a length as great or
greater than the acetabulum. In an animal about the state of con-
traction of Figure 23 the oral sucker had a length of 0.11 mm. and a
width of 0.065 mm. and the acetabulum had a diameter of 0.105 mm.

The eye-spots are like those already described but are larger in
proportion to the size of the body than in Cercaria inhabilis. Except
for a very limited area around the eyes Cercaria diastropa is entirely
unpigmented.

The body is filled with cystogenous glands from the oral sucker to
the acetabulum.

On account of the freedom from pigmentation it was possible to
work out the excretory system further in Cercaria diastropha than in
Cercaria inhabilis. Figure 23, ex, shows the relations of this system.
The tail vessel and the bladder are alike in both forms. The much
convoluted crura of Cercaria diastropha which are large and contain
scattered concretions, extend as unbranched vessels up to the region
of the eyes. There they receive small branches from all parts of the
body. It was possible to trace the largest of these branches altho on
account of the cystogenous glands, the flame cells and the smallest
ducts were not found. Into the tips of the crura open on each side
two vessels, one from the side of the oral sucker and one from the
posterior end. The posterior branch is soon divided into an outer and
an inner vessel, which subdivide to reach all parts of the posterior body
region.

469] LARVAL TREMATODES—CORT 23

The digestive system in Cercaria diastropha is similar to that of
Cercaria inhabilis, but the oral sucker and its pouches -are larger in
proportion to the size of the acetabulum, and the intestinal ceca reach
nearer the posterior end of the body.

In Cercaria diastropha the anlage of the reproductive organs is
further developed than in Cercaria inhabilis. It is differentiated into
four clearly separated areas, which bear the same general relation to
the adult organs as in the other species. They are not however con-
nected with strands of nuclei and are not in exactly the same relative
position as in Cercaria inhabilis.

The above descriptions show that Cercaria inhabilis and Cercaria
diastropha differ considerably in the size and shape of the body, the ratio
in size of the suckers and in the position of the acetabulum, the amount
of pigmentation, and in the anlage of the reproductive organs.

Rediae of Cercaria diastropha (Fig. 25) were in different stages
of development. No rediae were found in which other rediae were
developing and in none were the cercariae further dveloped than in the
rediae of Cercaria inhabilis. The rediae were very mobile, having remark-
able power of changing their shape, and with the aid of the posterior
locomotor appendages could move fairly well. In the younger rediae
the body would sometimes be extended to five or six times the length
when contracted. This mobility is correlated with the extereme devel-
opment of the circular muscles, which show clearly as annular bands
(Fig. 25, ab). In the young living redia part of the excretory system
could be made out. One longitudinal trunk from the anterior extremity
was traced until it met two trunks from the posterior end.

In the youngest redia studied there were practically no germ
balls in the body cavity ; the length of a toto mount slightly contracted
was 0.45 mm. and the greatest width 0.13 mm. The intestine extended
to a point 0.22 mm. from the anterior end and was relatively voluminous.
The oral sucker had a length of 0.43 mm. and a width of 0.032 mm.
One of the largest specimens measured 0.78 mm. in length and 0.16
mm. in width. The intestine reached one-third the body length. The
oral sucker was about the same size as in the younger specimens, the
length being 0.044 mm. and the width 0.038 mm. From the above
description it is evident that the redia of Cercaria diastropha is very
much like that of Cercaria inhabilis. The greatest difference is in a
greater mobility correlated with a greater development of the circular
muscles in the former.

Altho a large number of adult trematodes belonging to the family
Paramphistomidae has been described, I have found in the literature

24 ILLINOIS BIOLOGICAL MONOGRAPHS [470

mention of only three cercariae belonging to this group. The cercaria
of Diplodiscus subclavatus has been known for a long time, having been
first described by de Filippi and best described by Looss (1892 :162-166).
Of the two other forms one was first described by Sonsino (1892) as
Cercaria pigmentata, and later shown experimentally by Looss ( 1896 :-
185-191) to be the larval form of Amphistomum conicum (Paramphis-
tomum cervi). The other was also described by Looss (1896:177-185)
as the larva of Gastrodiscus aegyptiacus. This conclusion, however, only
rests on the structural comparison of the cercaria and the adult.

The five amphistome cercariae now known belong to two different
sub-families of the Paramphistomidae. The cercaria of Paraphistomum
cervi differs from the others in lacking the pockets of the oral sucker, and
in having a connection between the longitudinal crura of the excretory
system. It belongs to the sub-family Paramphistominae. The other four
of these cercariae are much alike and belong to the sub-family Diplodis-
cinae. They all have the retrodorsal pockets of the oral sucker, and
the muscular enlargement of the esophagus at the bifurcation of the
intestinal ceca. Cercaria diastropha differs considerably from the other
three in its small size, lack of pigmentation and in the proportionally
large size of the oral sucker. The cercaria of Diplodiscus subclavatus,
that of Gastrodiscus aegyptiacus and Cercaria inhabilis are very similar.
The first of these differs from the other two in the large size of the
pharynx of its redia and in the small size of the intestine. Cercaria
inhabilis is larger than Looss' cercaria of Gastrodiscus aegyptiacus. My
measurements show that the oral sucker of this species is twice as
large as his, and the acetabulum is very much larger. There is noth-
ing in Cercaria inhabilis to correspond to the papillae found by Looss
around the mouth of his form, and the intestine of the redia is much
larger in his species.

The only adult trematode which I have found in the literature
from the United States which resembles these cercariae in structure is
Diplodiscus temporatus Stafford. This form has the retrodorsal pharyn-
geal pockets found in the cercariae and also the muscular thickening
of the esophagus. The arrangement of the reproductive organs is such
that those of the adult might be derived from the anlage of either of
the above cercariae. As far as conclusions from comparative structure
are concerned either Cercaria inhabilis or Cercaria diastropha might
be the larvae of Diplodiscus temporatus. Infection experiments alone
can clear up this point. It is evident, that Cary (1909) is mistaken
in the larval form which he assigns to Diplodiscus temporatus.

Cary (1909) described as belonging to the life-history of Diplodis-
cus temporatus Stafford sporocysts and rediae, both containing cercariae

471] LARVAL TREMATODES—CORT 25

from Goniobasis virginica obtained near Princeton, New Jersey. In 1911
Cary sent me some of the material which he had used in the prepara-
tion of this paper, including specimens of Diplodiscus temporatus from
his experimental tadpoles. A study of this material and a careful anal-
ysis of Cary's account has convinced me that he has described in this
paper, two different species of larval trematodes neither of which
belong to Diplodiscus temporatus.

The snails of the species Goniobasis virginica which he collected
from the Delaware and Raritan canal near Princeton in the fall of 1908,
contained rediae in which cercariae were developing, but those collected
from the same locality in the spring of the following year and those
from the Delaware river near Trenton, contained sporocysts in which
cercariae were developing. Cary assigns both these stages unhesitat-
ingly to the same species, for no other reason so far as can be judged
than that they were collected from the same species of snail from the
same general locality. That in the same species of trematode, cercariae
should be found developing from both sporocysts and rediae is without
parallel. Further in his own descriptions Cary shows that he is deal-
ing with two separate types of larvae. In connection with his account
of the development of the cercariae in the sporocysts, (p. 643), he
writes of the cercaria.

"In the dorsal part of the sucker (oral sucker) there is developed
the dart (Stachel). This lies in a thin structureless sheath between
the muscle cells. It is shaped like a short arrow with a comparatively
broad head."

Neither in the description nor in the figure (Cary, 1909, PL 30,
Fig. 6) of the cercaria which develops from a redia is a dart shown.
Further a comparison of the cercariae developing from rediae with the
others developing from sporocysts from the material which Cary sent
me, shows that they are entirely different in practically every character.
Figures 26 and 27 are drawings made to scale of these two types of
cercariae. According to Liihe's (1909) classification of the cercariae
these forms would fall into two entirely unrelated groups. The smaller
one with the boring spine which develops in the sporocysts (Fig. 26)
very evidently belongs in the Xiphidiocercariae (Liihe, 1909:189-200)
while the larger form agrees with the characteristics of the Gymno-
cephalous cercariae (Liihe, 1909:182-186). That two such diverse cer-
cariae should develop into the same adult is utterly impossible. Since
in his infection experiments Cary uses only the cercariae which develops
from rediae, he certainly can have no evidence that the cercariae
which develops from sporocysts and are entirely different from the
first type have any connection whatever with Diplodiscus temporatus.

26 ILLINOIS BIOLOGICAL MONOGRAPHS [472

Therefore Cary's whole discussion in the embrological part of the paper
(pp. 617-647) which is based on the study of the sporocysts and the
cercariae developing in them cannot without further evidence be given
a place in the life-history of Diplodiscus temporatus. Since it proves
the thesis that Cary sets out to make that the embryo in the sporocysts
develops from parthenogentic eggs, this account is a very important
contribution to trematode embrology.

Another point in Cary's paper which is very striking is the
great difference between the cercaria which he describes as belonging
to Diplodiscus temporatus and other amphistome cercariae, especially
that of Diplodiscus suoclavatus, of which Looss has worked out the
life-history (Looss, 1892). A comparison of these two forms shows
striking differences in the shape and size of the body, in the size and
position of the acetabulum, in the excretory system especially that of
the tail, and in their activities and encystment. (Compare Cary, 1909,
PL 30, Fig. 6 and Looss, 1892, PL 20, Fig. 20). That cercariae belong-
ing to the same genus should be so different in structure is contrary
to all the accepted views of trematode development. The few observa-
tions found in the literature on the life-histories of closely related
species of trematodes show a closer structural correspondence betweeen
the cercariae than the adults.

A study of Cary's material shows that he is in error in the descrip-
tion and drawing of the largest of the two cercariae in certain funda-
mental points. In his drawing (plate 30, Fig. 6) and description the
digestive system of the cercaria corresponds to that of the adult Diplo-
discus temporatus, in having pharyngeal pouches and in the muscular
enlargement of the esophagus at the point of division into the intestinal
ceca. In his material the cercaria has no pharyngeal pouches and the
clearly circumscribed pharynx which is followed by an enlarged portion
of the esophagus is entirely different from his description and drawing
for this form. Compare figure 27 and Plate 30, Figure 6 of Cary's
paper. In fact, the digestive system of his so-called cercaria of Diplo-
discus temporatus does not in reality correspond to that of the adult
as his drawing and description suggest. Cary is also in error in his
description of the tail of this cercaria, since the material which he
sent me shows that it is much longer than he figures it and has a
truncated end (Fig. 27). Neither does the reproductive anlage of this
cercaria agree with his description and drawing (p. 606, PL 30, Fig. 6),
for instead of three definitely circumscribed areas two in front of the
acetabulum and one behind, it really consists of a small mass just in
front of the excretory vesicle connected with a mass in front of the
acetabulum by a line of nuclei.

473] LARVAL TREMATODES—CORT 27

When the adult Diplodiscus temporatus is compared with the larger
of the cercariae which Cary assigns to it certain differences of structure
are noted so fundamental that it seems impossible that the two forms
can be the same species. Compare figures 27 and 28. The adult is a
typical amphistome with the conical body terminating in a very large
acetabulum, while the cercaria is widest toward the anterior end, flat-
tened and its acetabulum which is only a little larger than the oral
sucker is just back of the center of the body. Certainly very remark-
able changes in shape and position of the organs would be necessary
before the cercaria which developed in rediae in Goniobasis virginica,
which Cary described, could metamorphose into an adult Diplodiscus
temporatus. Cary makes no attempt to bridge this gap altho it would
seem from his infection experiments that intermediate stages should have
been obtained. It is especially hard to believe that the ventral sucker
could have migrated from the middle of the body to the posterior
end and have become so much larger in proportion to the oral sucker,
and that the digestive system could have changed so fundamentaly
as would have been necessary. In those species of trematodes in which
the development is known the digestive and excretory systems are very
much alike in the cercariae and in the adults. The principal changes
come in the development of the reproductive organs and the correspond-
ing enlargement of body regions, usually the post-acetabular.

The infection experiments that Cary conducted to prove the con-
nection between this cercaria and Diplodiscus temporatus seem con-
vincing until they are carefully analyzed. To carry conviction they
should have been better controlled, described in more detail and the
stages of development worked out. That I may do Cary no injustice
I will quote in full his account of the experiments that he used to
prove the connection between the cercaria and the adult (Cary, 1909:
612-1613).

"On Oct. 30 a number of tadpoles of Rana catcsbiana were secured
and several put in each jar containing infected snails. The tadpoles
came from a pond in the grounds of the Biological Hall of the Univer-
sity of Pennsylvania, where Goniobasis is not found, so it seemed improb-
able that they would be infected with the parasite that was found about
Princeton. As an added precaution all of the tissues of three of the
tadpoles were carefully examined without finding parasites of any kind.
When the tadpoles had been for a week in the jar containing the
encysted cercariae, a dead individual was found. This one was exam-
ined for the presence of parasites with the following results: Nine
worms, which from the condition of the sexual organs could be recog-
nized as young, were found in the intestine of this tadpole. The other

28 ILLINOIS BIOLOGICAL MONOGRAPHS [474

organs of the body were entirely free from parasites. The worms in
the intestine were about 2.5 mm. in length and 1 mm. in diameter at
the posterior end. All of them were found in the last third of the
intestine of the tadpole, scattered throut that part of its length. Dur-
ing the time that they were under examination, as was also true in.
every other instance, the worms remained attached to the intestine of
the host by a large posterior sucker. The anterior part of the body
was in almost constant motion.

"Others of the tadpoles used in the experiment died from time to
time and were examined for the presence of worms. The result of the
examination was identical in everj^ case. A greater or less number of
worms, ranging from ten to thirty-one, was found in the posterior part
of the intestine. In no case were any of the worms, or any other recog-
nizable parasites, found in any of the other organs of the tadpoles.

1 ' The intestines of two of the tadpoles at the time of death contained
worms still within the cyst. Among the others, individuals of different
ages could be recognized so it was definitely established that the tad-
poles could serve as the host for the sexually mature worms.

' ' Since it seemed probable from the number of worms found in each
of the dead tadpoles that they had in many instances been the direct
cause of the death of the host, some larger tadpoles were secured for
further experiments. These were put into jars containing encysted
cercariae; but after they had remained there for a few days they were
transferred to a jar in which there had been no snails. A tadpole from
this jar was killed each week to note the development of the parasites.
The conditions of the environment proved unfavorable for the tadpoles
and the last one of them died on Jan. 19, 1909, after having been
infected with Diplodiscus nine weeks. The worms which were taken
from the intestine of this tadpole had fully developed sexual organs,
but, so far as could be determined from the condition of the jar, no
eggs had been laid, or at least no embryos had been developed."

An analysis of these experiments shows a number of weaknesses.
The fact that the tadpoles came from a pond in the grounds of the
Biological Hall at the University of Pennsylvania offers no check on
the results. The location of the pond at Philadelphia and not at Prince-
ton and the fact that Goniobasis does not occur there, can hardly be
called evidence that Diplodiscus temperatus is not present in great num-
bers. The only check that Cary gives on his experiments is the exam-
ination for parasites of three tadpoles out of the whole lot. These he
reports free from all parasitic infection. This does not prove that the
others were uninfected. The three examined may have been without
parasites while the rest were infected, or it is even possible that the

475] LARVAL TREMATODES—CORT 29

parasites may have been overlooked. Any man even if somewhat accus-
tomed to examinations for parasites is likely to overlook them, especially
if they are small and the infection light, until a chance finding directs
his attention to a particular organ. Therefore it would seem that the
above experiments were not sufficiently controlled to prove that no infec-
tion of Diplodiscus temporatus was present in the tadpoles previous to
the experimental feeding.

For infection he puts the tadpoles in the jars with infected snails
and when a week later one of the tadpoles proved to be infected with
nine immature specimens of Diplodiscus temporatus he concluded that
they had developed from the larvae in the snails. A comparison of
the structure of the two forms shows how improbable this is. The
largest cercaria of this type in the alcholic material, which Cary sent
me, had a body, 0.40 mm. in length and 0.20 mm. in width and an
acetabulum 0.065 mm. in diameter. The only measurements that Cary
gives in his description, which were evidently taken from a living speci-
men, are 2 mm. in length for both body and tail and 0.15 mm. in width.
The tail is usually at ieast as long as the body, so that would make
the body of the cercaria not over 0.1 mm. in length. Cary gives no
measurements for the acetabulum but his drawing (PI. 30, Fig. 6) shows,
it less than half the width of the body.

Since the cercariae must have had about the size, shape, and propor-
tions given above at the time this tadpole was supposed to have eaten
the cysts, their metamorphosis surely must have been extraordinary to
have developed in a week into the immature specimens of Diplodiscus
temperatus, which Cary found in the tadpoles. He describes these forms
as 2.5 mm. in length, 1 mm. in diameter at the posterior end, and as
being attached by the typical large posterior sucker. The posteror
sucker of the young Diplodiscus temporatus (Fig. 28) has practically
the width of the posterior end, which in this case would be almost 1 mm.
Therefore if Cary's contention be correct his cercariae in one week
almost tripled their length, changed the whole shape of the body, and
increased their width five times. The acetabulum must in some way
have jumped from the center of the body to the posterior extremity
and increased its diameter ten times. Since such a transformation is
impossible one is forced to conclude that the tadpoles used in the experi-
ments were already infected with Diplodiscus temporatus, and that
there is no connection between this species and the cercariae used in the
experiment. The fact that according to Cary every tadpole examined
was infected with Diplodiscus temporatus cannot be taken as attesting
the success of the experiments but merely the general uniformity of
the original infection. A detailed comparison of figures 27 and 28

30 ILLINOIS BIOLOGICAL MONOGRAPHS [476

shows how fundamental are the differences between the cercariae used in
the experiments and Diplodiscus temporatus.

The second experiment is even less convincing than the first because
no check whatever is given and the source of the tadpoles is not indi-
cated, altho it is perhaps to be taken for granted that they are obtained
from the same source as the first batch. These tadpoles were killed each
week to note the development of the parasites and stages of Diplodiscus
temporatus were found. If the development had followed the course
that the writer maintains it would have been possible with this material
to find transitional stages in change of shape, supposed migration of the
acetabulum, etc. This Cary has not done and even the possibility seems
to have escaped his notice, since it is not mentioned in the paper.

The following points have been proved in the above discussion.

1. That Cary described two entirely different species of cercariae
as belonging to Diplodiscus temporatus;

a. Those with stylets, which develop in sporocysts,

b. Larger forms without stylets which develop in rediae.

2. That since the second type only were used in the infection ex-
periments, no connection between the first type and Diplodiscus tem-
poratus can have been shown.

3. That the infection experiments were not sufficiently controlled
to be conclusive.

4. That the cercariae used could not have possibly developed into
Diplodiscus temporatus, since the two forms differ so fundamentally in
structure.

Since the stylet form of Cary's two species, which I have named
Cercaria caryi, is very small, and no living material is available, it does
not seem wise at the present time to attempt a detailed description.
Figure 26 gives the most salient features. From the presence of the
Stylet, the small number of the stylet glands, and the small size of the
acetabulum it may be placed with the Xiphidiocercariae in Liihe's group
of the Cercariae microcotylae (Liihe, 1909: 196-198).

I was fortunate enough to obtain further material of the larger
form, so that a detailed description of it is possible. On account of the
great length to which the tail is sometimes stretched I shall describe it
as Cercaria megalura.

477] LARVAL TREMATODES—CORT 31

DISTOME CERCARIAE

The great bulk of known cercariae belong to this division. In ray
material are eleven distome cercariae representing eleven of the principal
sub-groups.

GYMNOCEPHALOUS CERCARIAE

Since beyond the fact that they develop in rediae the cercariae
placed in this subdivision agree only in the absence of certain characters,
it is without doubt an unnatural group. However in the present state of
our knowledge it is convenient to retain it. Of my material only Cer-
caria megalura belongs here. This species and its allies differ so much
from all the other cercariae of the gymnocephalous group, that I pro-
pose to make them the basis of a new sub-group, to which the name
Megalurous or heavy-tailed cercariae may be given.

MEGALUROUS CERCARIAE

From 73 specimens of Pleurocera elevatum from the Sangamon
river near Mahomet, Illnois, examined during November and December,
1913, one was found with the liver packed with rediae in which devel-
oped a very peculiar kind of cercaria. Comparison showed this form
to be the same as the larger cercaria which Cary assigns to Diplodiscus
temporatus. Since Cary's account is not very complete, obscured by a
mistaken viewpoint, and incorrect in many particulars, a further de-
scription of this species seems advisable. Altho many of the cercariae
seemed fully matured, none were found free in the organs of the snail
and no rediae were found which contained rediae.

Living rediae and cercariae of Cercaria megalura moved actively.
The redia was very active and the region back of the posterior locomotor
appendages on account of its mobility and attenuation resembled a tail.
The anterior portion of the body also could be extended and contracted
freely and with the aid of the locomotor appendages locomotion was
possible. The cercaria was unable to use its tail for swimming in open
water but on a substratum it moved fairly rapidly with the aid of the
suckers. With the acetabulum attached the anterior end would reach
out and the oral sucker take hold. The acetabulum would then loosen

32 ILLINOIS BIOLOGICAL MONOGRAPHS [478

its grip, arid the body contract until the suckers were close together.
The acetabulum would again take hold just back of the oral sucker,
which would in its turn become loosened and extended. Locomotion
consisted in a continued repetition of these movements. During this
process the tail was contracted and took no part in the movement. At
certain times a cercaria became attached by the posterior tip of the tail,
which is furnished with an adhesive organ. The animal then extended
to five or six times its usual length, and became greatly attenuated.
While in this position the cercaria moved continually with a wriggling
\/ motion. Looss (1896:202) noted a similar activity in Cercaria distoma-

tosa Sonsino from Cleopatra bidimoidcs Bourg., from Cairo, Egypt, a
form very much like Cercaria megalura. This cercaria became attached
by the extremity of the tail to the surface of the water or to some bodies
such as plants or branches of trees very near the surface of the water,
and moved in a serpentine manner like a tubificid worm. This compari-
son would apply equally well to Cercaria megalura. What relation this
peculiar habit has to the future development of the cercaria is not
known.

No cercariae were found encysting altho large numbers had ex-
truded cystogenous material in the form of a sort of open tube around
the body (Fig. 30). In fact this seemed to be the normal procedure
when the animal came in contact with fresh water. Cary (1909:609-
610) performed experiments with some of his specimens of Cercaria
megalura which showed that this extrusion of cystogenous material was
due to the change from the conditions in the liver of the snail to fresh
water. Looss (1896:201-203) found no free individuals of Cercaria
distomatosa in which the cystogenous material was still in the glands,
and he also mentioned that when the cercariae were taken up in a pipette
they became encysted as quick as a flash. He noted further that they
encysted also in the open, loosening their hold and dropping to the
bottom after having lived for a period free.

The redia (Figs. 31 and 36) of Cercaria megalura is an elongate sac
slightly tapering toward the anterior end, with the posterior locomotor
appendages about six-sevenths of the distance from the anterior to the
posterior extremity. It is widest just in front of the appendages and
tapers almost to a sharp point posteriorly. The birth pore is on the
dorsal side just back of the pharynx.

The mouth of the redia is at the anterior tip and the oral cavity
opens into a short, narrow passage, which widens out almost immediately
into the voluminous digestive tract. This extends back of the posterior
locomotor appendages nearly to the end of the body. In cross sections
(Fig. 35) the intestine occupies from one-third to two-thirds of the body

479] LARVAL TREMATODES—CORT 33

cavity depending on the amount of food material present and the
pressure from the developing cercariae. In the inner lining of the
intestine were found flattened, scattered nuclei, but no cell boundaries
could be distinguished. The body cavity occupies most of the entire
region from just back of the oral sucker up to the posterior tip, but
does not extend into the posterior locomotor appendages and the tail-like
posterior extremity. These regions are filled with parenchymatous
tissue in which definite cell boundaries could be determined (Fig. 35).
The wall of the body cavity of even the youngest redia is very thin, and
in the inner lining of flattened pavement cells with flattened nuclei, the
cell boundaries could be distinguished only with difficulty. A small
germ gland consisting of but few differentiated cells is present at the
posterior extremity of the body cavity.

All ages of rediae were present in the snail from those in which
the oldest contained cercaria was scarcely differentiated at all, to those
in the body cavities of which there were from four to eight almost fully
matured cercariae. The youngest redia studied (Fig. 36) which con-
tained no cercariae having the cystogenous glands at all developed, was
0.53 mm. in length and 0.12 mm. in greatest width. The posterior loco-
motor appendages were 0.097 mm. from the posterior end, and the
intestine extended to within 0.076 mm. of the posterior extremity. The
pharynx was slightly elongated, being 0.049 mm. in length and 0.043
mm. in width. The body cavity contained a number of developing
embryos only one of which was far enough along to be recognized as a
cercaria. In this embryo (Fig. 36, cv) the tail was a mere stub hardly
marked off from the body. The length of the body was 0.33 mm. and its
width 0.054 mm., while the tail had a length of 0.038 mm. and a width
of 0.032 mm. The sucker, digestive system, and reproductive anlage
could be made out.

From one of the largest rediae studied the following measurements
were taken (Fig. 31). The length was 1.16 mm., and the width 0.19
mm. The posterior locomotor appendages were 0.27 mm. from the pos-
terior extremity, and the intestine, which filled about half the body
cavity, extended to within 0.086 mm. of the posterior tip. There were
four fully matured cercariae, one of which while still in the redia had
extruded its cystogenous material, and a number of developing embryos
of all ages. The pharynx of this redia was 0.054 mm. in length, and
0.049 mm. in width, showing almost no development in size from the
very youngest redia.

The length and width of the body of Cercaria megalura (Figs. 29
and 30) vary greatly with the state of contraction. The tail varies
from one-half the length of the body when the animal is moving on a

34 ILLIXOIS BIOLOGICAL MONOGRAPHS [480

substratum to ten times that length when the cercaria is attached. The
body is also about four or five times as long when extended as when
contracted. At average extension the body is slightly pointed anteriorly
and the acetabulum is but little more than half the distance from the
anterior to the posterior end. The preacetabular region is a little wider
than the postacetabular, and the postacetabular tapers slightly toward
its truncated posterior extremity.

In mounted specimens the average length of the body is 0.4 mm.,
and the widest diameter of the preacetabular region averages 0.13 mm.,
while that of the pqstacetabular is 0.09 mm. The width of the tail at its
base varies from 0.03 mm. to 0.054 mm., depending on the state of
contraction. The oral sucker is lightly elongate, having an average
length of 0.05 mm. and a width of 0.045 mm. The acetabulum is slightly
larger and circular, averaging 0.054 mm. in diameter.

The structure of the tail of Cercaria megalura (Figs. 30 and 32)
is interesting on account of its remarkable power of extension and the
modification of its posterior end for attachment. It is truncated and
there is an invagination at its tip. Into this inpushing open a clump of
from 15 to 20 unicellular club-shaped glands. These glands have an
average length of 0.017 mm. and width of 0.009 mm., and contain nuclei
measuring 0.005 mm. in diameter. It is probable that these glands
secrete some substance which makes possible the adhesion of the end
of the tail. The tail is strongly attached at its base and except near the
tip appears to be filled with vesicles, which are stretched out when the
tail is extended and compressed when it is contracted. Figure 32
represents a cross section of the tail. The cuticula is very thin, the
muscle layers reduced, and *the great bulk of the tissue is made up of
parenchymatous cells, the nuclei of which are surrounded by small
masses of protoplasm. These cells are connected by strands of proto-
plasm and the large intercellular spaces which are filled with clear fluid
give the appearance of vesicles. It is the looseness of the tissue of the
tail which makes possible its remarkable changes in shape.

The cystogenous glands of Cercaria megalura are very highly de-
veloped and when the cystogenous material is still present in them they
from the bulk of the whole body. These glands (Fig. 33 cs) are large,
elongate, club-shaped cells, most of which open on the ventral surface,
a few only opening dorsally. They are full of tiny, rod-shaped, cysto-
genous granules. The cells have an average length of 0.036 mm. and a
width of 0.012 mm., and contain at about their centers large nuclei.
They are present all thru the body of the cercaria from the posterior
limit of the oral sucker almost to the posterior extremity. A comparison
of cross sections of two cercariae at the region of the esophagus shows

481] LARVAL TREMATODES—CORT 35

the striking changes made in the glands and all the tissues by the
extrusion of the cystogenous material (Figs. 33 and 34). The cysto-
genous glands in figure 34 have become much reduced in size, their cyto-
plasm is only slightly granular, and even their nuclei appear to have
shrunk. In fact they are distinguished only with difficulty from the
body parenchyma. The extruded material forms a layer about 0.0075
mm. in thickness, closely adhering to the animal. After the extrusion
of the cystogenous material the cercaria becomes somewhat shorter,
broader, and thinner than before.

The mouth of Cercaria megalura is subterminal and the oral cavity
leads into a narrow prepharynx about the length of the oral sucker but
varying greatly with the state of contraction. The pharynx is small,
measuring on the average 0.022 mm. in length and 0.020 mm. in width,
and opens by a narrow passage into a much wider portion of the eso-
phagus. This soon divides into the narrow intestinal ceca which reach
almost to the posterior end of the body. The lining of the enlarged
portion of the esophagus and the intestinal ceca is formed by flattened
pavement cells with flattened nuclei, which are illustrated in figures 33
and 34. Figures 29 and 30 both show the relations of the digestive
system.

At the posterior end of the body is found the pyriform excretory
vesicle which also has its walls formed of flattened cells with flattened
nuclei. From the anterior end of the vesicle pass forward two longi-
tudinal vessels which can be followed to the region of the pharynx
(Fig. 30). The excretory pore was not distinguished. In Cercaria dis-
tomatosa Looss (1896:200) described the excretory system as having a
short common trunk in the tail, leading from the bladder and opening
to the outside by two short ducts. Cary in his material of Cercaria
megalura located the excretory pore as dorsal just at the base of the
tail, and found no extension of the excretory system into the tail. The
most careful examination of the anterior part of the tail of Cercaria
megalura in living specimens, toto mounts, and sections showed no trace
of excretory tubules.

The anlage of the reproductive organs (Fig. 30 ra) appears as a
small mass of nuclei just in front of the excretory bladder; from this
a line of nuclei extends forward to join another small nuclear aggrega-
tion just in front of the acetabulum. No definite outlines of organs
could be determined. The mass just anterior to the acetabulum repre-
sents to the ends of the reproductive ducts.

In the literature are found descriptions of only two cercariae re-
sembling Cercaria megalura: Cercaria distomatosa Sonsino best de-
scribed by Looss (1896:197-204), and a cercaria from the Hawaiian

36 ILLINOIS BIOLOGICAL MONOGRAPHS [482

Islands from Melania baldwini Annecy and Melania newcombii Lea,
described briefly by Lutz ( 1893 :327 ) . Since in these forms there are no
fixed larval organs such as spines, stylets, etc., to compare, specific dif-
ferentiation is difficult. Cercaria distomatosa, however, seems to show
some constant differences from Cercaria megalura. In Cercaria disto-
matosa the rediae reached a length of 1.8 mm., had a pharynx 0.07 mm.
long, and a digestive system extending only to the posterior locomotor
appendages, which are about three-fourths of the distance from the
anterior to the posterior end. Also rediae were found in which rediae
were developing. In Cercaria megalura as noted above the digestive
system was more voluminous and reached almost to the posterior end of
the body, and the posterior locomotor appendages were further back,
being about six-sevenths of the distance from the anterior to the posterior
end. Also Looss' measurements for Cercaria distomatosa are slightly
larger than those for Cercaria megalura, and the judge from his figure
and description, the tail is considerably smaller. He found the final tub-
ules of the excretory system in the tail, which were not found in my
species. The Hawaiian cercaria described by Lutz (1893:337) seemed to
be similar to Cercaria megalura and Cercaria distomatosa in having a
long tail by which it can become attached, in the general relationship of
the body structures, and in the form of the cyst. A more detailed com-
parison is impossible on account of the meagerness of Lutz's description.
It is evident that these three forms are very closely related. Cary's
attempt to relate Cercaria megalura to Diplodiscus temporatus is the
only suggestion as to the life-history of a member of this group, but as
shown above, this cannot be accepted.

ECH1NOSTOME CERCARIAE

Cercariae belonging to the family Echinostomidae are very easily
recognizable on account of their structural correspondence to the adults.
It is impossible to subdivide them into smaller natural groups. The
following may be given as a brief statement of characters for the
Echinostome cercariae.

1. Distome cercariae developed in rediae.

2. Rediae have collar, birth-pore, and posterior locomotor append-
ages.

3. Cercaria with digestive system consisting of prepharynx,
pharynx, long esophagus, and intestinal- ceca reaching the posterior end
of the body.

4. Anterior end of cercaria with collar and circle of spines.

483] LARVAL TREMATODES—CORT 37

5. Excretory system opening on each side of the anterior part of
the tail ; excretory bladder small, crura large, reaching to oral sucker.

6. Tail powerful, longer than body.

In the material used in this study were two echinostome cercariae.

The first species was found in several specimens of Planorbis trivol-
vis, examined during November, 1913, from a small pond near Urbana,
Illinois. The infection consisted of rediae containing cercariae in the
livers, and encysted cercariae in the body cavities of the snails. Planor-
bis trivolvis is then able to serve both as intermediate and secondary
intermediate host for this species. I propose for this cercaria the name
Cercaria trivolvis. The second of these larvae was found in a few out
of thirty-six specimens of Campeloma subsolidum from Hartford, Con-
necticut. The snails in this case I regard simply as the secondary
intermediate host since no rediae were found. From the fact that the
encysted cercaria had a pinkish tinge produced by pigment granules
in the postacetabular region the name Cercaria rubra is proposed for
this species.

Cercaria trivolvis (Fig. 39) completes its development before leav-
ing the redia. Therefore very few cercariae were found free in the
liver of the host. That a certain time is spent in free life is suggested
by the fact that altho free swimming cercariae were kept under observa-
tion for a whole day none were seen to encyst. There seemed to be no
connection between infection in the liver and cysts in the body cavity,
since altho a few snails had both types of infection the majority had
only one.

This cercaria (Fig. 39) moved actively both in open water and on
a substratum. The tail was powerful and extended when the animal
was swimming to two or three times the body length. For the swimming
movement the cercaria bent ventrad almost double, with the posterior
half of the body almost directly dorsad of the anterior. The tail which
extended beyond the anterior end lashed vigorously and propelled the
animal rapidly. When a cercaria came in contact with a surface it
took hold with its suckers and moved actively with a creeping movement
similar to that already described for Cercaria megalura. The structure
and position of the crown of spines suggests that it would be of consider-
able aid to the animal in making its way thru connective tissue.

The cysts of this species were oval, having an average length of
0.16 mm. and width of 0.15 mm. The cyst wall varied in thickness
from 0.007 mm. to 0.012 mm.

Cercaria trivolvis (Fig. 39) is elongate, pointed anteriorly, and has
its greatest width in the region of the acetabulum. It has an heart-

38 ILLINOIS BIOLOGICAL MONOGRAPHS [484

shaped anterior end with a crown of thirty-seven spines. Well extended
toto mounts have an average length of 0.38 mm. and a width of 0.12
mm. The tail is large and powerful in proportion to the size of the
body, and in a state of average extension has a length of 0.50 mm.
When contracted it may have a length considerably less than that of
the body and when the eercaria is swimming it often reaches to two or
three times that length. It ends in a sharp point and at the tip for
about 0.05 mm. to 0.06 mm. it is narrow and is composed only of the
muscle layers, lacking the parenchymatous core.

The oral sucker is almost exactly spherical and has an average
diameter of 0.043 mm. The acetabulum is a little larger, being 0.049
mm. in diameter, and is situated two-thirds of the distance from the
anterior to the posterior end.

The body from the anterior end to the acetabulum is set thickly
with rows of small spines, only visible under the highest powers of the
microscope.

The crown contains thirty-seven spines of equal size, arranged in
two alternate rows, broken in the middle of the ventral surface. They
are arranged regularly except for the two or three nearest the mid-line
on the ventral side, which point in.

The body from the oral sucker to the attachment of the tail is filled
with cystogenous glands. They are unicellular and club-shaped and all
open on the dorsal surface.

The oral sucker is followed by a typical prepharynx about 0.022
mm. in length. The pharynx is round, on the average 0.017 mm. in
diameter. The esophagus and intestinal ceca are not yet functional,
but appear merely as columns of granules enclosed in membranes and
containing irregular spaces representing the beginning of the lumina.
The esophagus is long, reaching almost to the acetabulum, and the ceca
reach nearly to the posterior end of the body.

Cercaria trivolvis (Fig. 39) has the typical echinostome excretory
system. The portion in the tail was made out with great difficulty and
is apparently not functional. A vessel passes back from the excretory
bladder for one-fifth or one-sixth of the length of the tail and sends out
two lateral branches which open to the outside. Dorsally at the base
of the tail is the adult excretory pore which apparently at this stage
gives passage to the outside for the waste material, since altho the
vesicle kept filling and emptying the vessels of the tail did not change
their caliber. In the oldest specimens the crura are large and distended
from the bladder to the region of the pharynx with regularly-shaped,
highly refractive granules. For the most part these are round or oval,
but some of them appear to be compounded of from two to four of the

485] LARVAL TREMATODES—CORT 39

round ones. At the region of the pharynx the excretory tubules are
much smaller and curve around forming a characteristic loop, from
which a small vessel on each side could be traced back to the level of the
acetabulum.

The nuclei which form the anlage of the reproductive organs of
Cercaria trivolvis are not yet fully divided into separate masses. In
the midline of the body just in front of the excretory bladder and
behind the acetabulum is an elongate mass of these nuclei from which
a line can be traced forward to a smaller mass in front of the acetabu-
lum. The posterior mass probably develops into the ovary and testes,
and the anterior mass represents the ends of the reproductive ducts.

Rediae of Cercaria trivolvis (Figs. 37 and 38) were present in the
infected snails in various stages of development. The youngest were
unpigmented but in the older ones orange colored pigment had devel-
oped in the outer wall, which rendered the largest rediae almost opaque.

The smallest redia found had a length of 0.30 mm. and a width of
0.065 mm. The posterior locomotor appendages were 0.22 mm. from
the anterior end and the region back of them was attenuated and
pointed. The oral sucker had a length of 0.041 mm. and width of 0.043
mm. and the intestine, which was very narrow and elongate, reached to
the posterior locomotor appendages. No germ balls were present in the
body cavity which was but little larger than the intestine, and the region
of the germ gland was not visibly differentiated.

In a redia (Fig. 38) 0.41 mm. in length and 0.081 mm. in width
the posterior locomotor appendages were 0.34 mm. from the anterior
end. A ridge extended around the body like a welt 0.086 mm. from
the anterior end. The body cavity at this stage had become well devel-
oped and contained germ balls, none of which however were in front of
the posterior extremity of the intestine. The pharynx had a length of
0.043 mm. and a width of 0.046 mm. and the intestine, which was wider
than the earlier stage and contained dark material, reached more than
one-half the body length.

The great majority of rediae found were well advanced in devel-
opment and contained mature cercariae in their body cavities (Fig. 37).
The body cavity had increased in size extending from the birth-pore
almost to the posterior end, and into the posterior locomotor append-
ages. In each redia were germ balls and from two to four mature
cercariae, the bodies of which were from one-third to one-half the total
length of the redia. In a mature redia 0.81 mm. in length and 0.15 mm.
in width, the posterior locomotor appendages were 0.62 mm. from the
anterior end. The pharynx was 0.041 mm. long and 0.043 mm. wide
and the intestine which was somewhat distended with food material and

40 ILLINOIS BIOLOGICAL MONOGRAPHS [486

pushed ventrad by the cercariae, extended for more than one-third of
the length of the body.

The pharynx is about the same size in the youngest and the oldest
rediae, and the actual size of the intestine differs but little, altho its
ratio to the size of the body is much less in the older form. The birth-
pore is located on the dorsal side of the body a little back of the pharynx.
In the oldest rediae the anterior collar is not present and the posterior
locomotor appendages are much reduced.

Cysts of Cercaria rubra (Fig. 41) were present in the tissue above
the gills of six of the thirty-six specimens of Campeloma snhsolidum
from Hartford, Connecticut. The cysts were large, round, thick-walled,
and of very uniform size, measuring 0.195 mm. to 0.205 mm. in diame-
ter. The cyst wall was transparent and had a thickness of 0.016 mm.
The worm almost completely filled the cyst with practically the whole
dorsal surface against the wall, and the posterior end overlapping the
anterior.

Several cysts were opened and the worms freed. A study was
made of these while living but none were preserved.

The living cercariae (Fig. 40) were on the average 0.50 mm. in
length and 0.15 mm. in width at the region of the heart-shaped anterior
end. They tapered slightly posteriorly, had a width at the acetabulum
of 0.13 mm. and the end was bluntly rounded.

The oral sucker of Cercaria rubra was a most exactly round having
a transverse diameter of 0.043 mm. and the acetabulum which is two-
thirds of the distance from the anterior to the posterior end was larger,
measuring 0.065 mm.

The collar which is typical of the Echinostomes is very well defined
in this species and has arranged around its edge in two alternating rows,
forty-three spines, which vary only from 0.018 mm. to 0.022 mm. in
length. In the middle of the ventral surface as is usual there is a
depression and a break in the rows of spines. The four median spines
on each side of this space are not in regular line with the others and
point inward. Besides these eight there are seventeen spines in the
upper row and eighteen in the lower row (Figs. 40 and 42). The
surface of the body as far back as the acetabulum was covered thickly
with rows of spines pointing backward. They were 0.005 to 0.007 mm.
in length. The rows were 0.008 mm. apart and the spines were set
thickly in the rows.

The digestive system (Fig. 40) offered nothing peculiar. The pre-
pharynx had a length about equal to the diameter of the oral sucker,
and the pharynx had a diameter of 0.025 mm. The short esophagus and

487] LARVAL TREMATODES—CORT 41

intesinal ceca contained the granules and their regular spaces described
for Cercaria trivolvis.

Only the main branches of the excretory system could be traced.
The crura were packed very full of concretions and the bladder was
narrow.

Since this species was only studied alive the anlage of the repro-
ductive system could not be distinguished.

In comparison with the large numbers of adult Echinostomes known
the descriptions of but few larval forms are to be found in the literature.
Cercaria rubra differs markedly from all of these in the number and
arrangement of the oral spines. Cercaria trivolvis on the other hand
agrees very closely with Cercaria echinata von Siebold. This form is
not sufficiently described to make a detailed comparison possible. Liihe
(1909:188) gives to it thirty-seven spines of equal size in the crown,
altho the earlier writers counted only thirty-six. The intestine of the
redia is shorter in Cercaria echinata than in Cercaria trivolvis, and the
size of the oral sucker in comparison to the acetabulum is less in the
former than in the latter species.

A number of suggestions have been made in regard to the adults
corresponding to the European species of echinostome cercariae. None
of these proposed relationships have been proved by experiments. Liihe
(1909:65) recently made the following statement in regard to this
group:

"von den bisher mit Namen unterschiedenen Cercarien (samtlich
aus Susswasserschnecken) ist noch keine mit volliger Sicherheit auf
eine bestimmte Art zu beziehen."

In England two echinostome cercariae have been assigned to adults
on the basis of morphological comparisons. The cercariae were not
given special names; the adults are Echinostomum leptosomum Creplin
by Lebour (1907:447-451) and Echinostomum secundum Nicoll by
Nicoll (1906:517-518).

None of the adult echinostomes from North America agree in the
number and arrangement of the anterior spines with the two cercariae
described above.

The following form, for which I propose the name Cercaria reflexae,
and which was found in, some of the specimens of Lymnaea reflexo from
Chicago, 111., will be treated as an appendix to the Echinostome cercariae.
The livers of the snails infected with Cercaria reflexae were packed with
rediae in various stages of development of which the greatest numbers
were large and full of numerous mature cercariae. Sections of the liver
showed that but very few cercariae were free in its tissues but large

42 ILLINOIS BIOLOGICAL MONOGRAPHS [488

numbers were freed when the organ was removed, from the snail. Also in
the body cavities of numbers of the same snails were found encysted cer-
cariae of the same species. That the cercariae were continually making
their way out and encysting in new snails was shown by the fact, that
while during the first few days after the snails came into the laboratory
only a few contained the encysted cercariae, later all were infected.

The movement of Cercaria reflexae both in open water and in a
substratum was exactly like that of Cercaria trivolvis.

The body varied greatly in size and shape. When contracted for
locomotion it was nearly as wide as long. A fairly well extended
mounted specimen (Fig. 43) is pointed anteriorly, widest at about the
level of the acetabulum and narrower at the posterior end. The anterior
end does not suggest the echinostome collar, and there is no crown of
spines. One well extended specimen had a length of 0.46 mm. and a
width at the acetabulum of 0.135 mm., with a thickness of a little more
than half the width.

The tail at average extension has a length a little greater than the
body, and an average width at its base of 0.05 mm. to 0.06 mm. The
tail is provided with a dorsal and a ventral fin-like ridge which is nar-
row at its base and at its widest part equals about one-half the diameter
of the tail.

The oral sucker has a diameter of 0.046 mm. and the acetabulum
which is just back of the middle of the body is 0.06 mm. in width.

The surface of the body back to the region about half way from the
acetabulum to the posterior end is covered with small spines arranged
in rows and set closely together.

The whole body . from the oral sucker to the posterior extremity
contains large unicellular cystogenous glands which open dorsally, and
fill the bulk of the body toward the dorsal side. These are like the
cystogenous glands already described for Cercaria megalura and the
amphistomes.

The oral cavity is followed by a short prepharynx and a small
pharynx, 0.022 mm. in diameter. The esophagus and the intestinal ceca
were very small and could only be followed in sections. The esophagus
reaches almost to the acetabulum and the intestinal ceca to the posterior
end.

The excretory system of Cercaria reflexae (Fig. 43, ex) is very
much like that of Cercaria trivolvis. In the former species the vessels
from the bladder to the tail are larger and serve to carry away the
excretory products, for when the excretory bladder became contracted
they became distended but soon again became reduced. The excretory
crura too are smaller than in the other species. Opening into the

489] LARVAL TREMATODES—CORT 43

crura could be traced smaller vessels leading from the posterior end of
the body, and forming a loop at the region of the pharynx similar to
that of the echinostomes.

The nervous system shows very prominently both in living and
preserved specimens and its larger branches can easily be traced. It
shows nothing peculiar. From the large masses on each side of the
prepharynx two branches pass out to the oral sucker. Laterally there
are two branches on each side in the pharyngeal region, and strands
can be traced almost to the posterior end of the body.

That the specimens of Cercaria reflexae which came under my ob-
servation were well along in development is attested by the condition of
the reproductive anlage (Fig. 43). It is divided into four definite
areas. In the region of the mid-line just in front of the excretory
bladder are three masses of nuclei, one in front of the other and close
together. The posterior two which are smaller than the other probably
represent the anlage of the testes, while the anterior larger mass of
nuclei the anlage of the ovary and the structures surrounding it. Just
in front of the acetabulum is a mass of nuclei representing the future
ends of the reproductive ducts. The course of the ducts connecting
these areas could not be traced.

The rediae of Cercaria reflexae (Fig. 45) were present in different
stages of development in the infected snails, altho there was a great
preponderance of the fully matured forms. The smaller rediae were
very active in extending and contracting their bodies, and were able
to make progress with the aid of their posterior locomotor appendages.
The largest forms were sluggish and reduced to mere sacs containing
cercariae.

The smallest rediae have much the same structure as those of
Cercaria trivolvis. One of them measured 0.28 mm. in length and 0.054
mm. in width and had the posterior locomotor appendages 0.22 mm.
from the anterior end. The anterior ridge or collar was very promi-
nent, 0.022 mm. from the anterior end, and the pharynx had a diameter
of 0.043 mm. A number of small germ balls were present in the body
cavity and the intestine extended to the region of the posterior locomotor
appendages.

A redia somewhat larger than the one just described is shown in
figure 44. In this form the cercariae are beginning to be developed into
recognizable form.

The nervous system was quite well developed in the young rediae
of this species. In one very immature specimen studied while alive
two large nervous masses (Fig. 46, n) could be distinguished on each
side of the posterior part of the oral sucker, which sent two branches

44 ILLINOIS BIOLOGICAL MONOGRAPHS [490

forward and two backward soon to become lost in the body wall. Very
few observations on the nervous system of the redia are found in the
literature. Looss (1896:199) traced the nervous system of the redia of
Cercaria distomatosa about as far as it is followed in the above descrip-
tion. He speaks of this as the best developed of the nervous systems
described in the redia up to that time. The degree of development of
the nervous system as he suggests is correlated with the degree of
mobility of the redia.

In the largest of the rediae (Fig. 45) almost all the germ balls
have developed into mature cercariae, of which from twenty to thirty are
present. The body cavity is much enlarged, the cercariae having pushed
clear into the posterior tip, into the posterior locomotor appendages
and up around the oral sucker. The rediae are mere shells, the anterior
collar having been obliterated and the posterior locomotor appendages
much reduced. The length of one of the largest rediae found was 2.26
mm., the width 0.30 mm. and the posterior locomotor appendages were
1.65 mm. from the anterior end. The oral sucker is very little different
in size from that in the younger specimens, its diameter being 0.056
mm. The intestine was long and slender and reached to a point 1.15
mm. from the anterior end.

That Cercaria reflexae is closely related to the echinostomes is shown
by a comparison with Cercaria trivolvis. The rediae of the two species
have much the same general structure. In fact it would be practically
impossible to distinguish between very young rediae of the two forms.
Further, the locomotion and general body structures of the two cercariae
are similar. The arrangement of the anlage of the reproductive organs
of the two forms is much* alike. That Cercaria reflexae is simply an
immature echinostome cercaria in which the crown of spines has not
developed is improbable from the fact that its other structures are
those of a well advanced cercaria, the anlagen of the reproductive
organs especially being considerably differentiated. No record has been
found of any species either cercaria or adult which agrees with Cercaria
reflexae.

MICROCERCOUS CERCARIAE

Among thirty-six specimens of Campeloma subsolidum from Hart-
ford, Connecticut, four were found to be infected with rediae and very
short tailed cercariae. The infection was in the tissues of the body
above and at the bases of the gills. This form, which has a very short
triangular tail, I propose to name Cercaria trigonura.

The cercariae were free in the tissues of the snail, were numerous,
and were all in the same stage of development. When freed, they ex-

491] LARVAL TREMATODES—CORT 45

tended and contracted their bodies rapidly, the preacetabular region
being the most active, but they were unable to swim. The tail, bent
ventrad and pushing against the substratum, aided somewhat in loco-
motion, and at times the oral sucker was used for attachment. By this
peculiar method the animal was able to make a little progress with a
great deal of effort. Leuckart (1886:86) notes the same type of move-
ment for Cercaria limacis Moulinie, a stumpy tailed cercaria from lAmax,
cineria.

"Ausser Stande zu schwimmen, benutzen diese Wurmer den fast
hertzformigen Schwanzanhang beim Kriechen als Nachschieber. "
Some of these cercariae were kept alive and active in tap-water for
three or four days.

Cercaria trigonura (Figs. 48, 50) has an elongate, cylindrical body,
averaging in mounted specimens 0.24 mm. in length and 0.06 mm. in
width. The oral sucker measures on the average 0.049 mm. in length,
and 0.039 mm. in width and the acetabulum which is slightly back of the
center of the body is smaller, being 0.04 mm. in diameter. The cuticula
is thin and at the anterior end beset with tiny spines, which are numer-
ous over the oral sucker, thin out posteriad and disappear entirely back
of the pharynx.

The tail of Cercaria trigonura is short, easily detached, grooved
ventrally, and has the extremity bluntly pointed (Figs. 48, 50). It
has an average length of 0.052 mm. and a width of 0.024 mm. Under
the cuticula is a thin layer of circular muscles which are supplemented
ventrally by a number of stronger longitudinal fibers that extend from
the base to the tip. The size of these strands and the fact that they
have no opposing muscles on the dorsal side would account for the
fact that the tail is usually bent ventrad. The bulk of the tail is formed
of loose parenchymatous tissue consisting of scattered nuclei, connecting
protoplasmic strands and good sized vesicles.

On the ventral surface of Cercaria trigonura just at the base of
the tail is a slit-like opening, which extends forward a short distance
and dorsad reaches up into the body. Opening into this cavity are
large numbers of unicellular glands which stain very heavily with
haematoxylin. Figure 48 pg shows the relation of this posterior glandu-
lar structure. It is interesting that the shape and position of the tail
give it the appearance of a short trough ready to carry off the secre-
tions of this gland. The position and structure of the posterior gland
suggest that it may function for adhesion. No activity which suggests
such a function has been observed and for none of the other stumpy
tailed cercariae has the description of a posterior glandular structure
been found.

46 ILLINOIS BIOLOGICAL MONOGRAPHS [492

Set in the dorsal part of the oral sucker and protruding for about
one-fifth of its length is a cephalic spine or stylet (Figs. 47, 49). The
spine has a length of 0.018 mm. and a width of 0.004 mm., is sharply
pointed and slightly thickened about two-thirds of the distance from
the base to the tip. Ventrally no thickening is present.

The cephalic or stylet glands (Fig. 48, sg) fill the space
dorsad and anteriad to the acetabulum and extend forward to just back
of the nervous system. The individual glands are small, averaging
0.024 mm. in length and 0.012 mm. in width, finely granular and con-
tain small nuclei. Their ducts pass forward dorsally over the oral
sucker and open around the cephalic spine in the anterior pit. The
numbers were so large that no accurate count could be made. They
form a single mass unbroken in the median line.

The mouth is subterminal and back of the oral sucker is a small
pharynx, 0.012 mm. in length by 0.01 mm. in width. The prepharynx
is short and the esophagus and intestinal ceca are entirely undeveloped.

The excretory pore of Cercaria trigonura opens dorsally at the
posterior extremity of the body just at the base of the tail. Leading
up to it is a narrow tube near the dorsal surface which expands into
the large bicornuate excretory vesicle, the horns of which reach on each
side of the acetabulum. Vessels could be traced along the sides of the
body from the tips of the horns of the vesicle up to the pharynx.
Figures 48 and 50 give the relations of the pore vesicle and vessels.
The vesicle is lined with a thick layer of granular, cuboidal, epithelial
cells with large nuclei (Fig. 54, ex). An excretory vesicle with thick-
ened walls of the character .just described has been found in the other
so-called stumpy-tailed forms.

The anlage of the reproductive organs shows as a mass of nuclei
dorsal to the posterior part of the acetabulum (Fig. 48, ra).

Along with the cercariae in the tissues of Campeloma subsolidum
were large numbers of rediae of different sizes. They were found in
every snail infected with Cercaria trigonura and were not found in any
instance where this species was not present. The only rediae that
showed any activity were very small immature forms which were present
in considerable numbers and in the same stage of development. They
were active, extending and contracting and twisting and turning in
all directions. Part of them were sharply pointed and the others bluntly
rounded posteriorly. The first type (Fig. 51) was the most common
and the structure of these will be described in some detail. The avera-
age of the mounted specimens of the small rediae is 0.019 mm. in length
and 0.04 mm. in width. The pharynx is small, 0.03 mm. in width

493] LARVAL TREMATODES—CORT 47

and set back slightly from the anterior end so that the anterior part
of the oral cavity is in front of it. The intestine is narrow and elongate
reaching nearly to the end of the body and almost filling the small
body cavity. Inside of the cuticula and muscle layers the wall is made
up of embryonic nuclei with poorly defined cell boundaries. There
are present no developing embryos and the germ gland is not clearly
differentiated.

Very few rediae were found showing intermediate stages between
the small type just described and the largest forms. The one shown
in Figure 53 with only five germ balls of any size in its body cavity is
0.43 mm. in length and 0.09 mm. in diameter. The pharynx has the
same width as length, 0.043 mm., and the pouch shaped intestine, which
is 0.11 mm. long and 0.038 mm. at its greatest width, extends to a point
only about one-third of the distance from the anterior to the posterior
end. The body cavity is large and contains oval germ balls, the largest
of which is 0.076 mm. long and 0.06 mm. wide. The wall of the cavity
is about two cells thick and the germ gland is clearly defined con-
taining both single germ cells and balls of several cells. The pharynx
in this form is also set back from the anterior end.

The largest rediae (Fig. 52) are all rounded at their posterior
extremities and their body cavities are very large with thin walls. The
germ gland is much reduced. At one side near the anterior end is located
the birth pore with protruding lips. The rediae vary in size up to the
largest measured which is 0.73 mm. in length. The largest ones have
from 25 to 30 germ balls of about the same size in their body cavities.
The intestine is smaller in proportion to the size of the body but of
about the same absolute size as in the smaller forms, and the oral sucker
is very clearly set back from the anterior end. The outer layers at the
anterior part are wrinkled so that the appearance is given of a series
of horizontal folds from the anterior tip back to the middle of the body.

Besides the large rediae with the body cavity full of germ balls
there are present numbers of large rediae with few or no germ balls and
much constricted and twisted.

That these rediae are the nurse generations of Cercaria trigonura
is very probable from their being found in every instance with that
form. All the most immature rediae are about the same size and appear
to belong to the same brood, yet there is no evidence as to their origin.
Neither is there any evidence where the cercariae free in the tissues
come from, for in none of the rediae can cercariae be distinguished.
The old rediae shells may have been the nurses of these cercariae and
rediae but even when a few embryos are found in these, they are merely
large little differentiated germ balls. It is possible that in these snails

48 ILLINOIS BIOLOGICAL MONOGRAPHS [494

were represented several infections and that the approach of winter,
for the examinations were made in November, may have arrested the
development of the second brood.

The only possible group in Liihe's (1909) classification of the cer-
cariae where Cercaria trigonura might fit is with the Microcercous cer-
cariae. An analysis of the forms coming under this category shows that
they cannot form a natural group. Dollfuss (1914) in the preliminary
account of his work on this group separates from it the Cotylocercous
cercariae which he considers to be a natural subdivision.

He gives the following characters for the Cotylocercous cercariae.

1. Cercariae developing in simple sporocysts parasitic in marine
gastropods.

2. Oral sucker with stylet ; stylet glands fill a large part of the
anterior region of the body.

3. Bladder large not bifurcate occupying almost all the posterior
region of the body; wall formed by a single layer of large granular
cells, which have the appearance of glands.

4. Tail very short, wide at least at its base, consisting of a cup with
thick walls of large cells, which functions as a sucker.

Dollfuss is in doubt in regard to the specific distinctness of some
of the members of this group. He therefore designates them together
provisionally under the name Cercaria pachycerca Diesing "sensu lato
et var." Under this designation he includes Cercaria brachyura Lespes,
Cercaria cotylura Pagenstecker, and several new forms. Besides these
are included in the Cotylocercous cercariae as undoubtedly distinct
species Cercaria linearis Lespes and Cercaria buccini sp. inq. Lebour.
Dollfuss states in regard to this group that the cercariae are so alike
in structure that the adults must be closely related.

Aside from the cercaria of Catoptroides macrocotyle Liihe (Phyllo-
distomum folium Ssinitzin,) and metacercariae, which have been related
to the Microcercous cercariae without any good reason, there are left
after the separation of the Cotylocercous cercariae, five more or less well
known forms.

1. Cercaria limacis (MoulhnV (1856:83, 163-164) from sporo-
cysts in the terrestrial molluses Arion rufus L. and Limax cinereus 0. P.
Miiller.

2. Cercaria micrura de Filippi (1857:5-7) (larva of Sphaerosto-
mum bramae (0. F. Miiller) from sporocysts in the freshwater snail
Bithynia tentaculata.

3. Cercaria myzura Pagenstecher (1881:25-26) from rediae in the
fresh-water mollucs Neritina (Theodocia) fluvialvis L.

495] LARVAL TREMATODES—CORT 49

4. Cercaria trigonura mihi from rediae in the fresh-water snail
Campeloma subsolidum.

5. Cercaria columbellae Pagenstecher (1862:305-306) from rediae
in the marine mollucs Columbella rustica L.

Of the two of the above cercariae developed in sporocysts only
Cercaria micrura agrees in structure with the characters given for the
Cotylocercous cercariae. The only reason for not including it in this
group is apparently that it is a fresh-water form. Just why it should
be excluded from this group for this reason when it agrees with them
in structure is not clear.

Cercaria myzura Pagenstecher, Cercaria columbellae Pagenstecher,
and Cercaria trigonura mihi are the three stumpy-tailed forms which
develop from rediae. Two are from fresh-water and one is marine.
Both of Pagenstecher 's forms are so insufficiently known that but little
structural comparison is possible. Cercaria myzura and Cercaria colum-
bellae both have the truncated tail like Cotylocercous group.

Cercaria trigonura is unique among the stumpy-tailed forms in
having a large posterior gland opening at the base of the tail and a
bicornuate excretory vesicle. It differs from all except Moulinie's (1856 :-
83, 163-164) Cercaria limacis in having a blunted tail, which is not mod-
ified as a sucker.

FURCOCERCOUS CERCARIAE

Sporocysts containing small forked-tailed cercariae with eye-spots,
were found in five out of thirty-eight specimens of Lymnaea reflexa from
a small pond in the suburbs of Chicago, Illinois. I propose to name this
form Cercaria douthitti.

The livers of the snails infected with Cercaria douthitti were filled
with a tangled mass of elongate, cylindrical sporocysts. The walls of
the sporocysts were so thin and in such close contact with the liver
lobes (Fig. 63, sw), that it was impossible to free individual sporo-
cysts and to follow them to any length. They were irregular tubes of
varying caliber, had no free, mobile, club-shaped ends sticking out, and
were filled with large numbers of embryos in various stages of develop-
ment. (Fig. 64). The walls of the sporocysts were made up of a very
thin fibrous layer on the inside of which were scattered nuclei (Fig 63).

None of the cercariae were found outside of the sporocysts in the
snail, but when the liver was dissected large numbers worked their way
out. Their progress from place to place was quite erratic altho they
moved their bodies and tails vigorously. When in locomotion the body
and tail were both somewhat contracted and both moved back and forth.

50 ILLINOIS BIOLOGICAL MONOGRAPHS [496

The movement of the tail was not a lashing as in many forms, but a
vibration in which the middle region was the most active. The cerearia
kept catching hold with its acetabulum and extending its anterior end,
but was not able to take hold with the oral sucker, which even in the
older specimens was not fully developed. Sometimes when a cerearia
under observation was pressed lightly with a cover-glass it would catch
hold with the ventral sucker and the vibrations of the tail would cause
it to swing round and round on the sucker as pivot.

Cerearia douthitti is a very small form, cylindrical in cross section,
slightly wider at the center and tapering from the acetabulum to the
posterior end to a width equal to about that of the tail. The body has
an average length in well extended mounted specimens of 0.19 mm. and
a width of 0.067 mm. The tail is bifid and even when contracted is
equal to about one and one-half the length of the body. The lobes form
less than one-third of its length and are definitely constricted from the
main portion, making the tail appear as if jointed (Fig. 55). The
main stem of the tail has an average length of 0.22 mm. and a width
of 0.025 mm., while the lobes have about half that width and an average
length of 0.089 mm. When the cerearia is alive the tail has a range
of variation from less than once to about twice the length of the body.
Underneath the thin cuticula and the muscle layers of the tail is a layer
of unicellular club-shaped glands ( ?), which lie close together and have
their ducts extending forward to open to the outside along the sides.
These glands were only seen in the living specimens. The central
core of the tail is composed of parenchymatous tissue, thru which flows
the caudal branch of the excretory system. The base of the tail fits into
a depression at the posterior end of the bodv, which is open ventrally
(Fig. 57).

The oral sucker of Cerearia douthitti is proportionally large, meas-
uring on the average 0.057 mm. in length and 0.045 mm. in width. It
is a mass of embryonic cells which are separated from the surrounding
tissue by a fibrous sheath, except that the ducts of the cephalic glands
pass thru to open at the anterior tip. There is no differentiation into
muscle fibers and no mouth or oral cavity is marked. Cerearia ocellata,
a European forked-tailed form which corresponds in structure very
closely to Cerearia douthitti is described as having a definite mouth
opening, by La Valette St. George (1855:22-23). He also gives measure-
ments much smaller for the oral suckers than those of Cerearia douthitti,
making it less than half as large as the acetabulum (0.013 mm. to 0.033
mm.) Luhe (1909:206) questions his measurements, and since in La
Valette St. George 's drawing that region is not clear, it may well be that
he did not grasp the true proportions of the oral sucker, and that it is as

497] LARVAL TREMATODES—CORT 51

large in his form as in Cercaria douthitti. The acetabulum is fully func-
tional as noted above. It is very small, on the average 0.025 mm. in dia-
meter, situated just back of the middle of the body.

The eye-spots are in front of the middle of the body and located
nearer the dorsal side. The pigment masses which compose them are
in the shape of concavo-convex discs, 0.007 mm. in diameter, placed so
that the concave surfaces are toward the sides of the body. Fitting into
the concave side of each is a small lens. In the region of the eyes and
lying just ventrad and behind them is the central nervous system. A
cross section of the cercaria in the region of the eye-spots brings out
these relations clearly (Fig. 56, e).

The region back of the center of the body of Cercaria douthitti
is almost completely filled with large unicellular cephalic glands, the
usual number of which is eight. The most anterior reach to the middle
of the body and from all, large ducts extend forward in two groups.
These two groups of ducts pass thru the sheath of the oral sucker pos-
teriorly and traverse this organ to open at the anterior tip of the body
(Fig. 59, 60, 61, 62). No cephalic spine is present altho these glands
appear to be analogous to stylet glands. Cercaria ocellata La Valette St.
George (1855:22-23) is the only other furcocercous cercaria in which
such glands are described. The glands are flasked-shaped and have a
length varying from 0.04 mm to 0.05 mm. and a width of from 0.025
to 0.03 mm. The thickness is about equal to the width. A traverse
section (Fig. 58) thru the acetabular region of Cercaria douthitti shows
how much of the body space is taken up by the cephalic glands.

At the posterior end of the body of Cercaria douthitti is a small
excretory vesicle from which two crura could be traced forward only as
far as the acetabulum. Backward from the bladder two small vessels
pass into the tail (Fig. 57, ex). These soon unite into the central caudal
vessel which divides to run down the lobes and opens at their tips (Fig.
55). No concretions of any kind were present in the excretory system.

Just in front of the excretory bladder and wedged between the tips
of the cephalic glands, is a small mass of nuclei the anlagen of the repro-
ductive organs.

The furcocercous or forked-tailed cercariae are very imperfectly
known. The anatomy of only a few of the known forms is at all well
worked out and the life-history of no one of them has been determined.
At least a dozen species have been reported as distinct, some of which,
however, have been described very briefly in the older accounts. Suffi-
cient evidence is not available to justify any conclusion as to the natural
or artificial character of this group.

52 ILLINOIS BIOLOGICAL MONOGRAPHS [498

Only one of the forked-tailed cercariae, Cercaria ocellata La Valette
St. George corresponds at all closely in structure to Cercaria douthitti.
The structure of this form has been fairly well worked out by La Valette
St. George (1855:22-23) and Moulinie (1856:172-173). Cercaria ocellata
agrees with Cercaria douthitti in the large unicellular glands of the pos-
terior body region, in the presence of eyespots, in the length and jointed
character of the tail, and in fact that the forked portion is only one-third
of its total length. The ratio in size of the suckers may agree as stated
above.

Cercaria ocellata differs in several particulars from Cercaria douth-
itti. The former is almost twice as large, is found in a different host,
in a different continent, and has narrow fin-like extensions on the divided
lobes of the tail.

No suggestion can be made as to the life-history of Cercaria douth-
itti. Its structure is such as not even to suggest to what family of dis-
tomes the adult belongs. In fact hardly a suggestion has been made in
regard to the life-histories of the forked-tailed cercariae and no experi-
ments that I can find have been carried on to trace their development.
Certainly further studies are needed on their structure and develop-
ment.

XIPHIDIOCERCARIAE

Luhe (1909:189) defines Diesing's (1855) group of the Xiphidio-
cercariae or stylet cercariae as follows :

Slender-tailed distome cercariae with a boring spine on the rounded
anterior end. Eyes lacking p develop in sporocysts ; encystment in a sec-
ondary intermediate host.

Since this group is formed on likenesses in but few larval char-
acters it can be considered only within wide limits as expressing rela-
tionship. On account of their small size and also since many of them
are known only from the older accounts many of the forms of this group
are very insufficiently described. Five new forms are added to this
group by the study of my material. In the following account the new
American forms will be compared with the most closely related of the
already known species, and where it is possible an attempt will be made
to fit them into natural groups.

Two of these forms with the related European species agree so
closely in structure that a new group, the Polyadenous cercariae will
be formed for them.

499] LARVAL TREMATODES—CORT 53

POLYADENOUS CERCARIAE

In eighteen per cent., of 170 specimens of Planorbis trivolvis col-
lected from the drainage ditch north-east of Urbana, Illinois, the livers
were filled with elongate, cylindrical sporocysts very much twisted
together. The sporocysts were not branching but it was very diflficut
to trace out the individual sacs. When this could be done they were
found to be of about uniform caliber and various lengths (Fig. 66).
Two that were measured were 1.48 mm. and 1.9 mm. in length and
varied in diameter from 0.13 mm. to 0.17 mm. The walls were thin
and contained flecks of orange pigment, which were very dense in the
oldest specimens. Many of the sporocysts contained large numbers of
actively moving cercariae which would escape and swim about freely
when the liver was teased apart. The wall of the sporocyst was com-
posed of a layer of pavement epithelium with flattened nuclei. No
thickenings were found in the wall and no traces of germ gland, altho
small germ balls were free in the cavity. Cercariae at all stages of devel-
opment were found in the sporocysts (Fig. 65). I propose to call this
species Cercaria isocotylea from the fact that the acetabulum and the
oral sucker are very nearly equal in size.

Cercaria isocotylea (Fig. 68) is oval elongate, slightly pointed
anteriorly, and of uniform width from the region of the pharynx back
to the acetabulum. The length and the width varied with the contrac-
tion state within rather wide limits. From the measurements of mounted
specimens of moderate contraction, the length averages 0.17 mm. and
the width 0.06 mm. The cross section is oval and the thickness a little
greater than half the width. The tail is small in proportion to the size
of the body and set in a groove on the ventral side of the posterior end.
When contracted it may be less than one-half the body length, but when
the cercariae is swimming it may be extended to greater than that length.
Under ordinary circumstances it has an average length of 0.01 mm.
and a width at its base of 0.02 mm.

Cercaria isocotylea moved actively both in open water and on a
substratum. When swimming it turned so that the ventral side was
up, the body was contracted and bent slightly ventrad. The tail became
much extended and lashed rapidly backward and forward. It did not
however have the power of moving the animal definitely in one direction
for any length of time, and locomotion was very erratic. Whenever
while swimming the cercaria came in contact with a surface, the tail
ceased its lashing and the body began to stretch and reach around until
the oral sucker could obtain a hold. Then the animal would creep along
with the aid of its suckers. Sometimes after the cercariae had come

54 ILLINOIS BIOLOGICAL MONOGRAPHS [500

in contact with a surface and extended its body, the tail resumed its
iashings and swimming was started again with the body extended. This
was simply preliminary to the contraction of the body and the resump-
tion of the usual swimming position.

The oral sucker of Cercaria isocotylea (Fig. 68, os) has an average
length in mounted sections of 0.04 mm. and a width of 0.037 mm., while
the acetabulum which is spherical in the living animal, is about two-
thirds of the distance from the anterior to the posterior end and has a
diameter of 0.036 mm.

Set in the dorsal wall of the oral sucker is a large sylet (Fig. 67)
which is sharp pointed and has a thickening two-thirds of the distance
from its base to its tip. It is flattened ventrally, has a length of from
0.028 mm. to 0.030 mm. and a width at its base one-sixth of its length.

The whole surface of the body back to the anterior margin of the
acetabulum is thickly set with tiny cuticular spines which are 0.003
mm. to 0.004 mm. in length. Back of this region the cuticula is entirely
smooth.

On each side of the stylet open the ducts of the stylet glands, which
form a group on each side. The glands are elongate, sac-shaped, uni-
cellular and faintly granular, with a length of from 0.02 to 0.025 mm.
and a width of from 0.011 to 0.013 mm. They form two clumps of from
six to eight in a clump, in the region just in front and to each side of
the oral sucker.

In this stage of development of the cercariae no cystogenous glands
are present.

Except for the mouth, oral cavity, very short prepharynx, and
pharynx the digestive system of Cercaria isocotylea is undeveloped. The
mouth is very small having a transverse diameter of 0.012 mm. and the
round pharynx is 0.016 mm. in width.

The excretory pore opens on the dorsal side just at the base of the
tail. The bladder is bicornuate consisting of a median part and two
lateral horns which reach on each side up to the middle of the acetabulum.
From the anterior and posterior regions on each side can be traced small
vessels which unite into a short common duct to open at the tips of the
horns. (Fig. 68, ex)

The anlage of the reproductive organs is not divided into definite
parts. It consists of a mass of small nuclei which lies dorsad and just
in front of the anterior margin of the acetabulum. This connects with
a larger mass which is dorsal to the posterior part of the acetabulum by
a broad band running around dorsal to the left margin of the sucker.

Among the specimens of Lymnaea reflexa from Chicago was one
which contained a number of elongated unpigmented sporocysts, which

501] LARVAL TREMATODES—CORT 55

were found in the liver with the rediae of Cercaria reflexae. These
sporocysts were much like those of Cercaria isocotylea, and were
so thin walled and so closely interwoven with the lobes of the liver that
none were isolated for accurate measurements. Altho large numbers
of the cercariae in the sporocysts were mature, few were free in the liver
of the host. When, however, the liver was taken out of the snail many
were freed and swam actively about. I propose the name Cercaria poly-
adena for this species from the fact that the body of the cercaria contains
such large numbers of gland cells.

The position of Cercaria polyadena in swimming was similar to that
of Cercaria isocotylea. The tail was, however, somewhat stronger than
in the latter species and the animal was able to move forward definitely
and fairly rapidly. Whenever the cercaria came in contact with a sur-
face, it immediately settled down, took hold with its suckers, and crept
along.

The tail was very easily detached from the body and would con-
tinue swimming for some time with a wriggling motion. Whenever an
actively detached tail came in contact with the substratum, it ceased
wriggling and alternately extended and contracted as if it were still
attached to a living cercaria. One of the detached tails kept up active
movement for over fifteen minutes and then was stopped by the drying
up of the water around it.

Small thin walled cyst containing tailless individuals of Cercaria
polyadena were found scattered in with the material preserved for study
(Fig. 71). It is probable that these cysts were formed after the liver
was removed from the snail, and that Lymnaea reflexa is not the sec-
ondary intermediate host of this species. The formation of the cyst
was not observed but that the glands were ready for secretion is shown
by the fact that the extrusion of the cystogenous material was observed
in one individual that was flattened under a cover slip. The cysts
were round and varied in diameter from 0.15 mm. to 0.16 mm., and
the transparent cyst wall varied in thickness from 0.005 to 0.007 mm.

Cercaria polyadena (Fig. 70) is very variable in shape, living speci-
mens changing from 0.12 mm. when contracted to 0.30 mm. at greatest
extension. When most contracted the tail may be less than one-half
the body length, but when the animal was swimming it reaches to
0.30 mm. The average measurements of five mounted individuals in
about the state of contraction of the figure, give the length of 0.18
mm., the width of 0.07 mm., the length of the tail 0.12 mm. and its
width 0.017 mm. The tail is attached in a groove on the ventral sur-
face of the posterior end, and altho it is small for the size of the body
it is relatively larger than in Cercaria isocotylea.

56 ILLINOIS BIOLOGICAL MONOGRAPHS [502

The oral sucker has an average diameter in mounted specimens
of 0.040 mm. and the spherical acetabulum which is three-fifths of the
distance from the anterior to the posterior end is 0.025 mm. wide.

In the dorsal wall of the oral sucker is set a stylet like that of Cer-
caria isocotylea. It is 0.028 to 0.03 mm. in length and thickened as in
the other species.

The whole surface of the body is set with tiny spines contained
entirely within the cuticula and not set in rows; these thin out some-
what in the postacetabular region.

Two groups of voluminous ducts pass up dorsad to the oral sucker
from the stylet glands, which fill most of the space from the acetabulum
to the pharynx. The diameter of one of these ducts is from 0.004 mm.
to 0.005 mm. The stylet glands are divided into two groups of from
ten to twelve in a group, and the individual glands vary in length from
0.017 to 0.022 mm., and in width from 0.010 mm. to 0.014 mm.

Cystogenous glands are present both on the dorsal and ventral sides
of the body from the pharynx to the posterior end.

In Cercaria polyadena the oral cavity is followed by a short pre-
pharynx 0.015 mm. in diameter. No traces were seen either of the
esophagus or the intestinal ceca.

The excretory bladder is bicornuate and the excretory pore is
located dorsally at the base of the tail. The two horns of the vesicle
extend up to the posterior lateral margins of the acetabulum, and each
receives a small vessel. This receives a long vessel from the region of
the pharynx and a short vessel from the posterior end. The lining
of the vesicle is formed of a layer of slightly flattened cuboidal epi-
thelial cells with prominent nuclei and well defined cell boundaries.

The anlage of the reproductive organs is not differentiated into its
individual parts. It is represented by an elongate s-shaped mass of nuclei
lying dorsad to the acetabulum and extending backward beyond its
posterior margin.

Cercaria polyadena and Cercaria isocotylea are very much alike and
form the nucleus of a group the members of which present such uni-
formity of characters that they must be considered to be related. The
name Polyadenous cercariae may then be proposed as the name of a
natural group the members of which correspond closely to Cercaria poly-
adena.

The following are the characters of the Polyadena cercariae :

1. Development in gastropods in elongate sac-shaped sporocysts.

2. Tail slender and less than the body length except when very
much extended.

503] LARVAL TREMATODES—CORT 57

3. Acetabulum back of the middle of the body and smaller than
the oral sucker.

4. Stylet about 0.030 mm. in length, six times as long as broad,
and with a thickening one-third of the distance from the point to the
base.

5. Stylet glands, six or more on each side between the acetabulum
and the pharynx.

6. Excretory bladder bicornuate.

7. Very short prepharynx and small pharynx present. Esophagus
when developed short to of medium length. Intestinal ceca (when pres-
ent reaching to posterior end of body).

Two European fresh-water cercariae, Cercaria limnaeae ovatae
von Linstow and Cercaria secunda Ssinitzin, without doubt belong to
this group. They both agree in all known particulars with the char-
acterization given above. No mention is made, however, in von Lin-
stow's (1884) account of stylet glands.

There are definite specific differences between the four forms which
constitute the Polyadenous cercariae. Of the two American forms
Cercaria polyadena has a larger body and tail, a smaller oral sucker, and
a larger number of stylet glands. Cercaria limnaeae ovatae is the largest
of the group, has much larger suckers than any of the others, and is
developed in larger sporocysts. The closest correspondence is between
Cercaria polyadena and Cercaria secunda Ssinitzin. These two species
are certainly very closely related. Cercaria secunda is, however, larger
in size and has slightly larger suckers and fewer stylet glands than
Cercaria polyadena.

Some suggestion can be made in regard to the type of adults into
which the cercaria of this group develop. Cercaria limnaeae ovatae
has been assigned to Opisthioglyphe rastellus (Liihe, 1909:108) and
Ssinitzin (1905) suggests that Cercaria secunda may be the larva of a
Plagiorchis species. It would seem probable from the above facts and
the structure of the excretory and digestive systems that the Polyadenous
cercariae belong in Liihe 's subfamily Plagiorchiinae which contains
Opisthioglyple as well as Plagiorchis. The further development of the
two American forms is entirely unkown.

CERCARIAE ORNATAE

In 5 per cent, of the specimens of Physa gyrina from Rockford,
Illinois, the body contained a tangled mass of elongated, orange pig-
mented sporocysts. The tubes did not branch, they were of varying

58 ILLINOIS BIOLOGICAL MONOGRAPHS [504

caliber and club-shaped ends protruded from the mass. They were very
much twisted together and none of the individual sacs were disentangled
without breaking. The jutting ends moved slightly, swaying backward
and forward.

Thruout their whole length the sporocysts (Fig. 77) were stuffed
with cercariae of different ages, and mature forms in large numbers
wormed out of the broken places in the sporocysts. The swimming
movement of the cercaria offered nothing peculiar and since the oral
sucker did not appear to function, creeping was not very effective.
The name Cercaria hemilophura is proposed from the fact that a fin-
Jike projection extends for half the length of the tail.

Cercaria hemilophura (Fig. 76) is oval elongate in shape and widest
at about the middle. The average length in well extended mounted
specimens is 0.38 mm. and the width 0.14 mm., with a thickness of about
one-half the width.

The tail at average extension is about the length of the body,
0.36 mm. being the average in toto mounts and with a width of 0.048
mm., but it can be extended to almost twice that length. Along the
ventral surface of the posterior half of the tail extends a fin-like projec-
tion, which at its widest is about half the width of the tail.

The oral sucker has a length of 0.065 mm. and the acetabulum,
which is just back of the middle of the body, has a diameter of 0.049 mm.

The stylet (Fig. 75) is small, tapers regularly to a point and has
no thickened region. It measures 0.020 mm. in length and 0.005 mm.
in width at its base.

The whole surface of the body is covered with very small cuticular
spines pointing backward, which are very dense in the preacetabular
region but thin out slightly posteriorly. They are contained entirely
within the thickness of the cuticula and have a length of from 0.0055
mm. to 0.0065 mm.

The whole body contains large numbers of small cystogenous glands
filling almost all the available space.

Stylet glands could not be distinguished.

The digestive system of Cercaria hemilophura is very clearly differ-
entiated. The oral cavity is followed by a very short prepharynx and
a good sized pharynx 0.033 mm. in diameter. From the pharynx a large
esophagus reaches back almost to the acetabulum. The esophagus is thin
walled but the intestinal ceca are lined with cuboidal cells which at this
stage fill most of the lumina.

It was possible in many cases to trace the branches of the excretory
system to the flame cells. The bladder is club-shaped, extending about
three-fourths of the distance from the posterior end to the acetabulum,

505] LARVAL TREMATODES—CORT 59

and widens slightly at its anterior end. Into it flow two vessels from
the region of the oral sucker on each side, which are met by vessels from
the posterior end at the region of the acetabulum. Small branches lead
from the flame cells and connect with these branches. Figure 76 shows
the excretory system of Cercaria hemilophura.

The anlagen of the reproductive organs are represented by a two
lobed mass of nuclei dorsal to the acetabulum.

According to Liihe 's classification of the Xiphidiocercariae Cercaria
hemilophura would belong with the group Cercariae ornatae. He gives
the following definition for this group :

' ' Distome Cercarien mit Bohrstachel, deren schlanker Ruderschwanz
einen Flossensaum besitzt. ' '

He included in this group Cercaria ornata La Valette and Cercaria
prima Ssinitzin. This is certainly not a natural subdivision, since the
three forms are very different in other structures. The presence of
such a character as the "Flossensaum" hardly forms the basis for a
natural group. Since it has been developed also in such widely different
groups as the monostomes and the echinostomes. At the present state
of our knowledge it seems impossible to relate Cercaria hemilophura
to any natural group. Neither is there any suggestion as to the further
development of this species.

MICROCOTYLOUS CERCARIAE

The tissue above the gills in three out of thirty-six specimens of
Campeloma subsolidum from Hartford, Conn., was heavily infected with
oval thin-walled sporocysts. The sporocysts had granular, somewhat
opaque walls, varied in shape from almost round to elongate oval, and
contained small cercariae in different stages of development. None of
the cercariae were fully mature and none were found free in the tissues
of the host. There was little movement of the cercariae either within
the sporocysts or when freed. I propose the name Cercaria leptacantha
for this species on account of the small size of the stylet.

The sporocysts (Fig. 81) varied from 0.26 mm. to 0.41 mm. in length
and from 0.15 mm. to 0.26 mm. in width.

The body of Cercaria leptacantha (Fig. 80) is oval elongate and
almost circular in cross-section. The average measurements of five well
extended mounted specimens are 0.12 mm. in length and 0.063 mm.
in width. The tail is not fully developed still remaining as a direct
continuation of the body and having little power of movement. It is
less than the length of the body and slender, averaging 0.081 mm. in
length and 0.016 mm. at its greatest width.

60 ILLINOIS BIOLOGICAL MONOGRAPHS [506

In none of the individuals studied were the suckers fully developed
or functional. The oral sucker shows a beginning of a mouth cavity
but the acetabulum is merely a rounded off mass of embryonic nuclei.
In mounted specimens the oral sucker averages 0.027 mm. in diameter
and the acetabulum, which is back of the middle of the body, has a
width of 0.024 mm.

On the surface of living specimens of Cercaria leptacantha were
scattered highly refractive round globules of different sizes, which
appeared like water or oil droplets. These bodies, which resemble the
concretions in the excretory systems of certain cercariae, were so prom-
inent that they could be seen thru the walls of the sporocysts, but dis-
appeared in the preservation of the material.

The small characteristic stylet (Fig. 79) is 0.011 mm. to 0.13 mm.
in length, and 0.0025 mm. in thickness at its base.

Two kinds of glands were present in Cercaria leptacantha. The
first type consists of small irregular shaped bodies with granular con-
tents at the anterior lateral margins of the acetabulum. The other
type are almost clear globlet shaped stylet glands, four on each side
arranged along the body lateral to the acetabulum, with ducts from
their outer margins leading up to the cephalic spine. No ducts were
found for the first type of gland, but from their granular contents they
may be cystogenous in character.

The digestive system of Cercaria leptacantha is represented only
by a short prepharynx and a small pharynx 0.09 mm. in diameter.

Of the excretory system only the elongate, club-shaped bladder can
be distinguished.

The anlage of the reproductive organs is represented merely by a
large mass of small nuclei dorsal and posterior to the acetabulum.

Cercaria leptacantha belongs to a group of very small cercariae
which Luhe (1909:196) calls Cercariae Microcotylae. It is possible that
they form a natural group. They are, however, so insufficiently known
that no final judgment can be passed on their relationships. At present
it seems best to follow Luhe in considering them a provisional group,
with Cercaria microcotyla Filippi as the type and the following char-
acteristics.

1. Developed in gastropods in round or oval sporocysts which are
seldom more than twice as long as wide.

2. Cercariae under 0.2 mm. in length.

3. Acetabulum back of the middle of the body and smaller than
the oral sucker.

507] LARVAL TREMATODES—CORT 61

4. Stylet glands not more than four on each side and arranged in
rows on each side of the acetabulum.

5. Digestive system undeveloped except for a short prepharynx
and a small pharynx.

Three European and three Egyptian cercariae are sufficiently known
to be included with any certainty in this group. The European forms
are Cercaria pugnax La Valette, Cercaria microcotyla Filippi, and Cer-
caria siibulo Pagenstecher (for description of these forms see Liihe, 1909 :-
196-198), and the Egyptian forms are Cercaria celluosa sp. inq., Cercaria
pusilla sp. inq., and Cercaria exigua sp. inq. all described by Looss
(1896:227-232). Insufficiently known forms which from their small size
and the shape of the sporocysts may belong to this group are Cercaria
chlorotica Diesing, Cercaria alba Ercolani, and Cercaria punctum Erco-
lani. Cercaria parva Ercolani in which the oral sucker is smaller than
the acetabulum agrees in its other characters with the members of this
group.

The Microcotylous cercariae are best distinguished from each other
by the size and shape of their stylets. Cercaria leptacantha agrees most
closely with the Egyptian species Cercaria exigua. It is larger than this
species however, the suckers differ in size and the ratio of size, and the
stylets differ in size and shape.

Only one suggestion is found in regard to the adults of this group.
Looss (1896 :232) considers that the three cercariae of this type described
by him may belong to some small distomes found in Egypt in the intes-
tines of chameleons and lizards. He offers no particular grounds for
this hypothesis.

Last will be considered a form of the Xiphidiocercariae which
seems to be different from all forms previously described. The livers
of three out of ninety-one specimens of Physa anatina from Manhattan,
Kansas, were infected with sausage-shaped sporocysts (Fig. 72) which
contained cercariae in different stages of development. I propose for this
species the name Cercaria brevicaeca from the fact that the intestinal
ceca are very short.

Cercaria brevicaeca moved clumsily and irregularly while swimming
and did not creep by aid of its suckers. The infection was very slight
and all the observations were made from living specimens. None of the
cercariae were observed to live more than two or three hours after removal
from the snail.

62 ILLINOIS BIOLOGICAL MONOGRAPHS [508

Cercaria brevicaeca (Fig. 74) was elongate oval in shape and its tail,
which was very easily lost, had about the same length as the body.
At average extension the body had a length of about 0.30 ram. and a
width of 0.14 mm. The tail did not change its shape very greatly and
ranged in length only from 0.22 mm. to 0.38 mm. with a width at its
base of 0.038 mm. It was attached in a groove in the ventral side of
the posterior end and when contracted had a tendency to curl at its
tip.

The oral sucker of Cercaria brevicaeca had an average diameter
of 0.082 mm. and the acetabulum, which is just back of the middle of
the body, was slightly larger, 0.087 mm.

The stylet (Fig. 73) had a length of 0.018 mm. and was slightly
thickened 0.007 mm. from its point.

The body back to the acetabulum was covered thickly with rows
of very tiny spines.

In the region between the acetabulum and the pharynx were two
clumps of from ten to twelve stylet glands, varying in length from
0.026 mm. to 0.035 mm. and in width from 0.018 mm. to 0.025 mm.
The ducts from these glands united into two groups one on each side
which passed dorsad of the oral sucker to open beside the stylet.

Almost every bit of available space behind the pharynx was filled
with cystogenous glands, which from the surface appeared as round,
granular bodies 0.014 mm. to 0.017 mm. in diameter.

The oral cavity was followed by a short prepharynx and a small
pharynx, 0.030 mm. in diameter. The short, narrow esophagus divided
just in front of the acetabulum into short intestinal ceca which did not
reach beyond the acetabulum. The lumina of the ceca showed only
as irregular, elongate spaces in the granular contents.

Of the excretory system only the peculiarly shaped vesicle could
be made out. This was composed of a pyriform median portion, and
two more narrow lateral parts which almost completely surrounded the
acetabulum. The pore opened dorsad at the base of the tail.

No cercariae were found in the literature closely corresponding with
Cercaria brevicaeca. Especially unique is the shape of the excretory
vesicle.

THE CLASSIFICATION OP THE CERCARIAE

At the present state of our knowledge it is impossible to fit most
of the cercariae into the general trematode classification, for, except
in those forms like the echinostomes or the amphistomes, where the larvae
are much like the adults, or in a group like the family Gorgoderinidae,
where the life histories of several species have been worked out, little

509] LARVAL TREMATODES—CORT 63

is definitely known of the relation of the cercariae types to the adults.
Therefore for convenience it has seemed advisable to build up a tentative
classification of the cercariae, treating them almost as if they were an
independent class of the animal kingdom. Of necessity such a classifica-
tion must be based pretty largely on superficial characters. As our
knowledge increases wherever possible natural groups must be substituted
for the artificial, and as more and more larvae are connected with the
adults, the classification of the cercariae will gradually be merged with
that of the adults.

In order to understand clearly the classification that has been made
for larval trematodes, a careful analysis of the characters used for com-
parison must be made. Cercarial characters can be roughly divided
into two main groups: (1) adult characters, and (2) larval characters.
By adult characters of a cercaria are meant those which foreshadow
adult structure. It is by the use of these characters as a basis that
the greatest progress in natural classification can be made, since the more
the adult characters are developed, the more will the cercariae resemble
the adults, as in amphistome and echinostome larvae. For example the
digestive and the excretory systems of the cercariae of these groups are
much like those of the adult. Sometimes in the larva definite specific
peculiarities of the adult can be distinguished in detail. Thus Looss
( 1896 :192-197 ) in attempting to prove by morphological comparison that
Monostomum verrucosum Froel (Notocotyle triseriale Diesing) and
Cercaria imbricata Looss belong to the same species, advances as his
strongest argument, that in the mature cercaria are found around the
excretory pore plications arranged as the rays of a circle like those
found in the adult. A combination of adult characters will often give
a clue to the family or even in a few cases to the genus to which the
cercaria belongs. Allowance must be made however for the fact that
adult characters may be somewhat modified in the development of the
cercaria. For example the loss of the tail modifies the excretory system,
and changes in shape and proportion of the body change considerably
the relative lengths of the different parts of the digestive system.

Larval characters of cercariae may be defined as those which are
not carried over into adult life. Many structures are developed to meet
the exigencies of larval conditions, and are merely temporary. In many
cercariae much dependence must be placed on such characters in classi-
fication, for often as in the forked-tailed and stylet cercariae, adult
characters are very little differentiated and the whole structure is
very largely dominated by larval characteristics. This brings up the
question as to how far such characters can be considered as expressing
relationship. Like structures in cercariae either show relationship or

64 ILLINOIS BIOLOGICAL MONOGRAPHS [510

convergence due to adaptations to similar environments. Such an adap-
tation as the development of a boring spine which occurs in cercariae
widely different in other characters, can hardly be considered as show-
ing close relationship. When, however, cercariae are very similar in a
number of larval characters such likeness can hardly be ascribed to
convergence, and even if adult characters are not sufficiently developed
for comparison, such forms can with reasonable certainty be placed
together in natural groups.

Another question which must be considered in classifying certain
cercariae, is whether the larvae of closely related adults might not be
different on account of modifications in larval life. From consideration
of conditions in other groups this would seem very possible. The little
evidence that we have, however, seems to indicate that the cercariae of
closely related forms are more alike than the adults. Liihe (1909:175)
suggests this as one reason for the small numbers of cercariae known
in comparison with the adults.

"Vielfach sint iibrigens die Cercarien verschiedener Trematoden-
Arten einander so ausserordentlich ahnlich, dass ihre sichere Bestim-
mung, wenigstens bei uriseren jetztigen Kenntnissen, nicht moglich ist,
and manche alte Art-namen haben dadurch die Bedeutung von Gruppen-
statt von Artbezeichnungen gewonnen."

The statement of these problems shows how merely tentative at the
present state of our knowledge must be considered any classification of
cercariae.

The most extensive classification of the cercariae is that of Liihe
(1909:173-210). His main subdivisions are for the most part based
on the recognition of the relationship of the cercariae to the larger
recognized adult groups. In the subdivision of the distome cercariae,
however, his classification is to a considerable extent purely artificial,
being based on the character of the tail. A summary of Liihe 's classifi-
cation follows.

luhe's classification of the cercariae

A. Lophocercariae

Cercariae with longitudinal projections along the sides of the body.
Ex. Cercaria crista La Valette.

B. Gasterostome cercariae

Two long projections from the end of the body. Mouth opening
in the middle of the ventral surface. Intestine simple sac-shaped.
Ex. Bucephalus polymorphus Baer.

C. Monostome cercariae

Ventral sucker lacking. Ex. Cercaria urbanensis Cort.

511] LARVAL TREMATODES—CORT 65

D. Amphistome cercariae

Ventral sucker at the posterior end of the body. Ex. Cercaria
inhabilis Cort

E. Distome cercariae

Ventral sucker some distance in front of the posterior end of the
body.

1. Cystocercous cercariae

Base of the tail forms a space into which the body can
be drawn. Ex. Cercaria macrocerca Filippi.

2. Rhopalocercous cercariae

Tail having as great or greater width than the body. Ex.
Cercaria isopori Looss.

3. Leptocercous cercariae

Tail straight, slender, and narrower than the body.

a. Gymnocephalous cercariae

Anterior end rounded, without stylet or boring spine.
Ex. Cercaria megalura. Cort.

b. Echinostome cercariae

Anterior end with a collar and crown of spines. Ex.
Cercaria trivolvis Cort

c. Xiphidiocercariae

Anterior end with stylet. Ex. Cercaria isocotylea Cort.

4. Trichocercous cercariae

Tail set with spines. Ex. Cercaria setifera Moulinie.

5. Furcocercous cercariae

Tail forked at its end. Ex. Cercaria douthitti Cort.

6. Microcercous cercariae

Tail stumpy. Ex. Cercaria brachyura Lespes.

7. Cercariaeae

Tail entirely undeveloped. Ex. Leucochloridium para-
doxum Carus.

8. Rattenkonigcercariae

Cercariae with tails joined, forming a sort of colony.

In the present state of our knowledge it seems to me that for the
comparison of forms, no general grouping can be suggested which
will be of more help to workers. It must be recognized, however, that
many of the groups are purely artificial.. The Gymnocephalous cercariae
have little in common but negative characters, and the stylet cercariae
form a very heterogeneous mass. These and other groups are merely
temporary arrangements for convenience and must be split up or rear-
ranged into more natural groups as soon as our knowledge permits.

66 ILLINOIS BIOLOGICAL MONOGRAPHS [512

Marie Lebour (1912) tried to substitute for Liihe's classification one
based on other characters. According to her classification the cercariae
are divided into two large groups depending on whether they develop
in rediae or sporocysts, and within these groups are formed small groups
of apparently related forms. As an attempt to form closely related
groups this work is very suggestive, but as a working classification
it has very limited value. In the first place there is little evidence
for the belief that development in sporocysts or rediae expresses funda-
mental relationship. Such widely divergent forms develop from spor-
ocysts as Bucephalus, tailless cercariae, stylet cercariae, etc. The most
important part of Miss Lebour 's work is her attempt to build up natural
groups of closely related forms centering around some well known
species, as for instance, her Spelotrema group centering around Spelo-
trema excellens. The following is the essential part of Lebour 's classi-
fication.

Lebour 's Classification of the Cercariae

A. Gasterostomata

Cercariae develop in sporocysts. Mouth at middle of ventral sur-
face. Ex. Bucephalous polymorphus Baer.

B. Prostomata

Mouth at anterior end.
1. Distome cercariae

Two suckers.

a. Cercariae developed in sporocysts.

(1) Gymnophallus group

Cercaria tailless. Ex. Cercaria glandosa Lebour.

(2) Fork-tailed cercariae

Tail forked at its end. Ex. Cercaria douthitti
Cort.

(3) Spelotrema group

Free swimming stage with stylet. Encysted
cercaria tongue-shaped, covered with spines, with
long prepharynx and esophagus, short ceca not
reaching to the end of the body. Ex. Cercaria of
Spelotrema excellens. (?)

(4) Stumpy-tailed cercariae

Tail broad and stumpy. Ex. Cercaria brach-
yura Lespes.

(5) Lepodora group

Body covered with spines. Intestinal ceca
reaching nearly to posterior end of body.' Tail

513] LARVAL TREMATODES—CORT 67

present in very young forms, but cast off before
encystment, which takes place within the sausage-
shaped sporocysts. Ex. Cercaria of Lepodora
rachiaea. (?)
b. Cercariae developed in rediae.

(1) Cercariae neptuneae

Tail very thick and large, two eye-spots present.
Excretory vesicle very thick walled. Ex. Cer-
caria neptuneae Lebour.

(2) Acanthopsolus group

Two eye-spots, intestinal ceca reaching nearly
to the end of the body, thin tail developed in
young forms but cast off before it is full grown.
Ex. Cercaria of Acanthopsolus lageniformis. (?)

(3) Echinostomum group

Cercariae with anterior collar and crown of
spines. Ex. Cercaria trivolvis Cort.
2. Monostome group

One sucker present. Ex. Cercaria urbanensis Cort.

It is in attempts to join a few closely related cercariae into groups
which are probably natural, rather than in further broad generalizations
which must be based on artificial characters, that hope for advance in
the classification of cercariae lies. As more and more life-histories are
worked out, such groups can be fitted into their place in the adult classi-
fication, until the relationship of all the cercariae groups to the adults
will be known.

Note. All of the species of Cercaria discussed in detail in this paper were
originally described in my preliminary report (Cort, 1914).

68 ILLINOIS BIOLOGICAL MONOGRAPHS [514

BIBLIOGRAPHY

Braun, Max

1879-1893. Vermes. A. Trematodes. Bronn's Klass. u. Ordnung. d. Thier-
reichs, Bd.4, Abt.l, pp. 306-925.
Cary, L. R.

1909. The life-history of Diplodiscus temporatus Stafford. With especial
reference to the development of the parthenogenetic eggs. Zool. Jahrb.,
abt. f. Anat. u. Ont., 28:595-659.
Cort, W. W.

1914. Larval Trematodes from North America Freshwater Snails. Prelim-
inary Report. Jour. Paras., 1 :65-84.
Creplin, F. C. H.

1849. Nachtrage von Creplin zu Gurlt's Verzeichnisse der Thiere, in welchen
Entozoen gefunden worden sint. Arch. f. Naturg., 1 : 52-80. (Cited after
Stiles and Hassall.)
Diesing, K. M.

1855. Revision der Cercarieen. Sitzungsb. d. k. Akad. d. Wissensch. Wien,
Math. Naturw., 15:377,-400.

1858. Berichtigungen und Zusatze zur Revision der Cercarieen. Sitzungsb.
d. k. Akad. d. Wissensch., 31 :239-290.
Dollfus, R.

1914. "Cercaria pachycerca" Diesing et les Cercaires a queue dite en
moignon. IX Congres International de Zoologie. Session de Monaco.
Compte rendu sommaire des traxaux du Congres. pp. 683-685.
Everts, H. C.

1880. Cercaria hyalocauda Haldeman. Am. Month. Micr. Jour., 1 :230-232.
de Filippi, Ph.

1857. Troisieme memoire pour servir a l'histoire genetique des trematodes.
Mem. de l'Acad. sc. Turin, 18:201-232.
Haldeman, S. S.

1840. Supplement to number one of "A Monograph of the Limniades, or
Fresh-water univalve shells of North America," containing descriptions
of apparently new animals in different classes, and the names and char-
acters of the sub-genera in Paludina and Anculosa. Wagner Free Inst.
Phila., Separate pp. 3.

515] LARVAL TREMATODES—CORT 69

La Valette St. George, A. J. H. von

1855. Symbolae ad trematodum evolutionis historiam. Berolini, Separate, (•)

PP. 38.
Lebour, M. V.

1907. Larval Trematodes of the Northumberland Coast. Trans. Nat. Hist.

Soc. Northumberland, Dur. Newc, 1 :437-454.
1912. A Review of the British Marine Cercariae. Parasitology, 4:416-456.
Leidy, J.

1847. On an Entozoon from the pericardium of Helix alternata. Proc. Acad.

Nat. Sc. Phila., 3:220-221.
1850. Descriptions of two new species of Distoma with the partial history

of one of them. Jour. Acad. Nat. Sc. Phila. 1 :30i-3io.
1858. Contributions to Helminthology. Proc. Acad. Nat. Sc. Phila., 10:110-
112.

1877. On Flukes infesting Molluscs. Proc. Acad. Nat. Sc. Phila., 29 :2oo-202.

1878. On Donax fossor. Proc. Acad. Nat. Sc. Phila., 30:382-383.

1888. On the Trematodes of the Muskrat. Proc. Acad. Nat. Sc. Phila.,

40:126-127.
1890. Notices on Entozoa. Proc. Acad. Nat. Sc. Phila., 42:410-418.
1904. Researches in helminthology and parasitology. With a bibliography
of his contributions to science arranged and edited by Joseph Leidy, Jr.
Smithson. Misc. Col., Washington (1477) 46:1-281.
Leuckart, R.

1886-1901. Die Parasiten des Menschen und die von ihnen herriihrenden
Krankheiten. Ein Hand- und Lehrvuch fur Naturforscher und Aerzte.
1 Bd. Abt. 2. pp. 1-897.
von Linstow.

1884. Helminthologisches. Archiv f. Naturgesch., 50 Jahrg., 1:124-145.
1896. Helminthologische Mittheilungen. Arch f. mik. Anat., 48:375-397.
Looss, A.

1892. Amphistomum subclavatum und seine Entwicklung. Leuckart's
Festschr., pp. 147-167.

1896. Recherches sur la faune parasitaire de l'Egypt. Premiere partie.
Mem. de lTnst. Egypt, 3:1-252.
Luhe, Max.

1909. Parasitische Plattwurmer. 1, Trematodes. Die Susswasserfauna
Deutschlands, 17:1-217.
Lutz, A.

1893. Weiteres zur Lebensgeschichte des Distoma hepaticum. Centr. f.
Bakt. u. Parasitenk., 13:320-328.

Moulinie, J. J.

1856. De la Reproduction chez les Trematodes endoparasites. Mem. d.
l'lnst. Genevois, 3:1-279.

70 ILLINOIS BIOLOGICAL MONOGRAPHS [516

NlCOLL, W.

1906. Some new and little-known Trematodes. An. Mag. Nat. Hist., 17:513-

527.
Nitzch, Ch. L.

1807. Seltsame Lebens- und Todesart eines bisher unbekannt Wasser-

thierchens. In Kilian, Georgia, oder Der Mensch im Leben und in

Staate, pp. 257-262; 281-286. (Cited after Braun.)
1817. Beitrag zur Infusorienkunde, oder Naturbeschriebung der Cercarien

und Bacillarien. N. Schrift. d. Naturf. Gesellsch., Halle, vol. 3. (Cited

after Braun.)
Pagenstecher, H. A.

1862. Untersuchungen iiber niedere Seethiere aus Cette. Ztsch. f. wissensch.

Zool., 12:265-311.
1881. Allgemeine Zoologie oder Grundgesetze des thierischen Baues und

Lebens. Part 4, pp. 959.
Sonsino, P.

1892. Studi sui parassiti molluschi di acqua dolce dintorni di Cairo in

Egitto. Leuckart's Festschr., pp. 134-147.
Ssinitzin, D. Th.

1905. Contributions to the Natural History of Trematodes. The Distomes

of Fish and Frogs in the vicinity of Warsaw. Separate, pp. 210. (Rus-
sian.)

517]

LARVAL TREMATODES—CORT

71

EXPLANATION OF PLATES

Unless otherwise stated all figures are drawn with a camera lucida.

The lettering on some toto drawings of the cercariae was omitted by
error. The descriptions in the text will be sufficient to make the struc-
ture clear.

ABBREVIATIONS USED

ab annular bands of muscle

ac acetabulum

acr anterior collar of redia

bp birth pore of redia

eg cystogenous glands

ccg cephalic glands

cm cystogenous material

cr cercaria in redia

dc ducts of cephalic glands

e eye-spot

es esophagus

ex excretory system

exv excretory vessel

gb germ ball

* intestinal cecum of cercaria

ir intestine of redia

la locomotor appendages of redia

Ic

large central cells of tail

Is

lobes of snail's liver

»ll

muscle layer

n

nervous tissue

OS

oral sucker

P

pigmentation

Pb

pharyngeal bulb

PC

parenchymatous cells

Pi

pigment line

pr

pharynx of redia

ra

reproductive anlage

s

stylet

sc

cercaria in sporocysts

sg

stylet glands

sw

wall of sporocyst

vi

excretory vessels of tail

72 ILLINOIS BIOLOGICAL MONOGRAPHS [518

EXPLANATION OF PLATE

Figures i to 4. Free hand drawings of stages in the process of encystment of

Cercaria urbanensis. X about 70.
Fig. 5. Mature Cercaria urbanensis, ventral view. Cystogenous glands not shown.
X 140.

Posterior locomotor projection of Cercaria ephemera. From Ssinitzin.

Posterior locomotor projection of Cercaria imbricota. From Looss.

Posterior locomotor projection of Cercaria urbanensis. X 433.

Cross section of the tail of Cercaria urbanensis. X 433.

Immature redia of Cercaria urbanensis... X 88.

Mature redia of Cercaria urbanensis. X 88.

Mature redia of Cercaria urbanensis, showing constrictions. X 44.

Cross section of immature redia of Cercaria urbanensis. X 433.

Fig.

6.

Fig.

7-

Fig.

8.

Fig.

9

Fig.

10.

Fig.

11.

Fig.

12.

Fig.

13.

PLATE I

519] LARVAL TREMATODES—CORT 73

PLATE II

74 ILLINOIS BIOLOGICAL MONOGRAPHS [520

EXPLANATION OF PLATE

Fig. 14. Immature redia of Cercaria urbanensis. X 88.

Fig. 15. Redia of Cercaria inhabilis. X 88.

Fig. 16. Mature Cercaria inhabilis, ventral view. Cystogenous glands not shown.

X 88.
Fig. 17. Cross section of Cercaria inhabilis in region of eye-spots. X 195.
Figures 18 to 21. Anterior body region of different stages of Cercaria inhabilis,

showing changes in pigmentation. X 140.
Fig. 22. Cross section of the tail of Cercaria inhabilis. X 433.

PLATE II

521] LARVAL TREMATODES—CORT 75

PLATE III

76 ILLINOIS BIOLOGICAL MONOGRAPHS [522

EXPLANATION OF PLATE

Fig. 23. Mature Cercaria diastropha, dorsal view. Cystogenous glands not shown.
X 88.

Fig. 24. Free hand drawing of Cercaria diastropha from the side. X 75.

Fig. 25. Redia of Cercaria diastropha. X 88.

Fig. 26. Cercaria caryi, ventral view. From Cary's material. X 140.

Fig. 27. Cercaria megalura, ventral view. From Cary's material. X 140.

Fig. 28. Diplodiscus temporatus, ventral view. From Cary's experimental tad-
poles. X 140.

PLATE III

523] LARVAL TREMATODES-CORT 77

PLATE IV

78 ILLINOIS BIOLOGICAL MONOGRAPHS [524

EXPLANATION OF PLATE

Fig. 29. Cercaria megalura before the extrusion of cystogenous material, ventral

view. X 195.
Fig. 30. Cercaria megalura after the extrusion of cystogenous material, ventral

view. X 195.
Fig. 31. Mature redia of Cercaria megalura. X 88.
Fig. 32. Cross section of tail of Cercaria megalura. X 433.
Fig. 33. Cross section of Cercaria megalura before the extrustion of cystogenous

material. X 276.
Fig. 34. Cross section of Cercaria megalura after the extrusion of cystogenous

material. X 276.
Fig. 35. Cross section of immature redia of Cercaria megalura. X 433.
Fig. 36. Immature redia of Cercaria megalura. X 88.

PLATE IV

525] LARVAL TREMATODES—CORT 79

PLATE V

80 ILLINOIS BIOLOGICAL MONOGRAPHS [526

EXPLANATION OF PLATE

Fig. 37. Mature redia of Cercaria trivolvis. X 88.
Fit'. 38. Immature redia of Cercaria trivolvis. X 88.
Fig. 39. Mature Cercaria trivolvis, ventral view. Cvstogenous glands not shown.

XI95.
Fig. 40. Cercaria rubra freed from cyst, ventral view. X 195.
Fig. 41. Cercaria rubra inside of cyst. X 195.
Fig. 42. Anterior end of Cercaria rubra, dorsal view. X 195.
Fig. 43. Cercaria reflexae, ventral view. Cystogenous glands not shown. X 88.

PLATE V

527]

LARVAL TREMATODES-CORT 81

PLATE VI

82 ILLINOIS BIOLOGICAL MONOGRAPHS [528

EXPLANATION OF PLATE

Fig. 44. Immature relia of Cercaria reflexae. X 88.

Fig. 45. Mature redia of Cercaria reflexae. X 44.

Fig. 46. Nervous system of immature redia of Cercaria reflexae. X 276.

Fig. 47 and 49. Stylet of Cercaria trigonura, side view and ventral view. X 433.

Fig. 48 and 50. Cercaria trigonura, side view and ventral view. X 195.

Fig. 51. Very immature redia of Cercaria trigonura. X 195.

Figs. 52 and 53. Rediae of Cercaria trigonura, containing germ balls. X 140.

Fig. 54. Cross section of Cercaria trigonura in region of acetabulum. X 433.

PLATE VI

529] LARVAL TREMATODES—CORT 83

PLATE VII

84 ILLINOIS BIOLOGICAL MONOGRAPHS [530

EXPLANATION OF PLATE

Pig- 55- Cercaria douthitti, ventral view. X 195.

Fig. 56. Cross section of Cercaria douthitti thru eye-spots. X 433.

Fig. 57. Diagram of the excretory system of Cercaria douthitti in the region where

the tail joins the body. X about 400.
Fig. 58. Cross section of Cercaria douthitti thru the acetabulum. X 433.
Figures 59 to 62. Cross sections of Cercaria douthitti showing the passage of the

ducts of the cephalic glands thru the oral sucker. X 433.
Fig. 63. Section thru a sporocyst of Cercaria douthitti while still included in the

snail's liver. X 433.
Fig. 64. A portion of a sporocyst of Cercaria douthitti. X. 44.
Fig. 65. A portion of a sporocyst of Cercaria isocotylea. X 44.
Fig. 66. Sporocyst of Cercaria isocotylea. X 44.

Fig. 67. Stylet of Cercaria isocotylea from ventral and side view. X 433.
Fig. 68. Cercaria isocotylea, ventral view. X 311.

PLATE VII

531] LARVAL TREMATODES—CORT HS

PLATE VIII

86 ILLINOIS BIOLOGICAL MONOGRAPHS [532

EXPLANATION OF PLATE

Fig. 69. Stylet of Cercaria polyadena, ventral view. X 433.

Fig. 70. Cercaria polyadena, ventral view. Cystogenous glands not shown. X311.

Fig. 71. Cercaria polyadena in cyst. X 140.

Fig. 72. Sporocyst of Cercaria brevicaeca. X 88.

Fig. 73. Stylet of Cercaria brevicaeca, ventral view. X 433.

Fig. 74. Free hand drawing of Cercaria brevicaeca, ventral view. Cystogenous

glands not shown. X about 150.

Fig. 75. Stylet of Cercaria hemilophura, side view. X 433.

Fig. 76. Cercaria hemilophura, ventral view. Cystogenous glands not shown.

X 140.

Fig. 77. A portion of a sporocyst of Cercaria hemilophura. X 44.

Fig. 78. Cercaria hemilophura, side view. X. 140.

Fig. 79. Stylet of Cercaria leptacantha, ventral view. X 433.

Fig. 80. Immature Cercaria leptacantha, ventral view. X 433.

Fig. 81. Sporocyst of Cercaria leptacantha. X 88.

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