
JOHNSTON AND BECKWITH—LARVAL TREMATODES 573 
cells, spination, aud the flame-cell formula. C. obscurudena Brackett has only 
a sinele pair of flame-cells high in the tail-stem, but the rest of the excretory 
system and gland-cells exclude it from close relationship. 
C. indica. I Sewell (from Indoplanorbis) possesses spination and a caudal 
exeretory system like that of C’. lesseni, but the body excretory system differs some- 
what, there is io posterior commissure, and there are only two pairs of gland- 
cells. before the ventral sucker, Concerning C. indica XXL, Sewell mentioned the 
curious nature of the intestinal caeca, a feature characteristic of (. lessont; it is, 
however, the only feature they haye in common, for C. indica X XT bas four flame- 
eells, well-spaced out, in the caudal trunk—a characteristie of Cotylurus and 
Diplostome cereariae, and very different from the condition in C. lessont and its 
closest, allies. 
DISCUSSION. 
Lt is apparent that the affinities of C. lessoni sre mainly with the six cerearia. 
first mentioned, ie. C, helvetica XV NI, C. pygocytophora, the cercaria of 
Apalemon gracilis, C, burti, C. pseudoburli, and perhaps C, ripen, In 19388, 
Willey and Rabinowitz (1938) proved C. burti to be the larva of Apatemon 
globiceps, the metacercaria being a Tetracotyle occurring in leeches. The meta- 
cercariae of C. helvetica XX XT, C. pseudoburti, C. pyyocylophara and C. riponi 
have not been ceseribed, though it is known that C, psewdoburli does not encyst in 
nymphs of mayflies and dragonflies, zammarids, tadpoles, fish or mice. Dubois, in 
discussing the genus Apatemon (1938, 96) mentioned C. helvetica XX XI as an 
Apatemon larva distinct from that of A, gracilis, whose cercaria was at that time 
known, and whose tetracotyle had been deseribed from leeches by Szidat (1929; 
1981), Lutz’s Dicranocercaria bdellocystis (1984) appears to be a further example 
of an Apatemon cerearia encysting in leeches. Although no other complete life 
eycles have been established for the genus Apetemun, two other Apatemon meta- 
cercaria have been described from fish—the tetracotyle of Apatemon fuligulae by 
Yamagiuti, encysting in Siluridae, and the tetracotyle of A. pelluctdus, encysting 
in Mogqurnda. 
The cercariae listed above, together with (. lessoni, form a particularly well- 
defined group of seven ¢ercariae (if C. ¢¢pont is included) so elosely allied that 
their differences are unlikely to be more than inter-specifie, As a group, these 
“ Apalemon cercaria’’ are characterized by certain features which distinguish 
them clearly from cercariae of related yvenera, such as those of Cotylurus and 
Diplostomum. The Apatemon cercariae are extremely sinall and very active. 
The body is characteristically pear-shaped, but may assume a variety of forms 
because of the remarkable contractility of the body; both Miller (1926, 41) and 
Dubois (1929, 95) comment on the great extensibility of the forms they deseribe. 
