ON TEMNOCEPHALA. 299 



matter, and become enclosed in a chitinous shell secreted by 

 the shell-glands. The egg is now compared with the animal, 

 a very large structure (as much as a sixth of the length of the 

 animal), and greatly distends the walls of the uterus. When 

 extended (PI. XXII, fig. 18) it has a short stalk, by means of 

 which it becomes attached to the shell of the crayfish, and is en- 

 closed in viscid matter, which when it hardens serves to cement 

 the eggs together. The eggs are found in considerable numbers 

 from October to February, attached chiefly to the under surface 

 of the abdomen, some also at the sides of the mouth and the 

 lower edges of the branchiostegites. The development of the 

 embryo has not yet been studied ; there is, as in other ecto- 

 parasitic Trematodes, no metamorphosis. Temnocephalae, 

 perfect in every respect, being found still enclosed in the egg. 



Affinities of Temnocephala. 



Though most nearly related to the Tristomidae, Temno- 

 cephala presents so many special peculiarities that it becomes 

 necessary to regard it as the type of a distinct family. The 

 principal characteristic features in its structure may be sum- 

 marsied as follows : 



The cephalic end of the body is produced into four, five, or 

 six slender, filiform tentacles, which are capable of being used 

 for prehension and touch, and in locomotion take the place of 

 anterior suckers, their adhesive powers being increased by the 

 secretion of certain special unicellular glands. There is a 

 single, large, radiated posterior sucker without hooks. The 

 body presents traces of a rudimentary form of segmentation 

 in the shape of incomplete transverse dissepiments formed by 

 specialised portions of the parenchyma muscle. The intestine 

 is constricted at regular intervals by these septa ; its epithelium 

 is not ciliated. There are three pairs of longitudinal nerve- 

 trunks, a dorsal, a dorso-lateral, and a ventral, connected by 

 numerous commissures. The excretory system opens by two 

 apertures, placed far forwards on the dorsal surface. There is 

 a single genital aperture leading into a genital cloaca, into 



