
JOHNSTON AND BECKWITH—LARVAL TREMATODES 581 
though ventral to the caeca and exhibiting a tendeney to be arranged as In 
C. ameriannae are posteriot to the ventral sucker; secondly, that there is one 
fewer flame-cell on each posterior collecting tubule, the formula being either 
2[(2+1) + (241+ (2))] =16, or 2[(2 +1) + (14+ 2+ (2))] = 16; and 
thirdly, that caudal bodies are present in the tail-stem. In none do the caeca 
suddenly distend. 
C. yegena Cort and Brackett, from Stagnicola, is very similar structurally 
to the last three, though shehtly smaller; like them it has four eland-cells behind 
the ventral sucker, caudal bodies, and ihe same exeretory formula. C. marite- 
burgensis Porter from Linnaéa, is another form closely related to C. yogena, 
possessing the same excretory formula, and differing mainly in certain details 
of spination and proportions of the tail. 
C. micradena Cort and Brackett (from Stagnicola), the larva of Diplostomum 
micradenum, with its metacervaria in tadpoles, and C. tetradena Johnston and 
Beekwith (1945), from Plotiopsis, are both smaller than C. ameriannae, but 
resemble it closely in a number of points; |hey differ m the shape of the cacea, the 
arrangement of the gland-cells, and in the possession of one more flame-cell on 
each posterior tubule. In C. micradena also the gland-cells are very sinall. 
C. macradena Cort and Brackett, from Stagnicola, which has the same excretory 
formula as C, micradena, is of a size similar to that of C. wmeriannae, but the 
spines are nowhere arranged in definite transverse rows, the gland-cells are much 
larger tha those of C. ameriannae and are situated differently, and the four 
flame-vells in the tail are situated close to the junction of tail and body. 
C. longifurca Cort and Brooks, from Limndteu, and (. mereianae Cort and 
Brooks, from Planorbis, each with two small pairs of gland-cells, differ markedly 
In spination, size (both are much smaller than @. ameriannae), shape of the caeca, 
and proportions of the tail, and in the possession of cither one (C. longifurca) or 
three (€. marecianae) more flame-cells ou each side, 
Cercaria Alariae-mustelae Bosma, from Planorbula, has two pairs of gland- 
cells, latera-posterior to the acetabulum ; bnt the ecrcaria is so very much smaller. 
and differs in so many other respeets from C. mnerannde that no close relationship 
seems possible. Cercaria F’ Harper, closely resembling the above, is also debarred 
from near relationship with C. ameriannae. The eerearia of Slrigca tarda 
Steenstrup has four pre-aeetabular gland-cells, but the excretory formula is 
2] (1+ 14-1) + (2+ (2))] = 14, and it is said to develop into a tetracotyle 
within the primary host, Limnaca. 
Henee, it ts clear that while the systematic position of C. ameriannae cannot be 
fully established, its affinities are with the Diplostomes. Some Diplostomes. in 
whieh the fore and hind-bhody ave seareely differentiated externally have been 
