chap, in DEVELOPMENT OF CESTODA 89 



nation occurs, at the bottom of which the rostellum, suckers, 

 and hooks are gradually formed, but inside out as compared with 

 the head of the Taenia serrata. At this stage the larva {Cys- 

 ticercus pisiformis) has usually issued from the liver and attached 

 itself to the omentum. The invagination projects into the cavity 

 of the bladder, within which a watery fluid accumulates. Thus 

 the " bladder worm " is formed, the head of which is evaginated 

 if the larva be introduced into the digestive system of a dog. The 

 1 (ladder and neck of invagination are digested, while the head, 

 protected by these, remains, and forms the neck, from which the 

 proglottides are afterwards segmented off. In Taenia {Hymeno- 

 lepis) miirina the whole development may take place in the 

 parental host, the larva living in the villi, the adults in the 

 cavity of the same rat's intestine (Grassi). The different forms 

 of Cestode larvae depend largely upon the presence and degree of 

 development of the caudal vesicle or bladder, which in Scolex 

 polymorphus (Fig. 38) (the young stage of Callidbothrium filicolle 

 Zsch.) is practically absent. If the bladder be small, the larva 

 is known as a Cysticercoid. For example, the common Dipylidium 

 caninum, which lives in the dog, has such a larva, the develop- 

 ment of which is explained and illustrated by Figs. 43 and 

 44. The bladder becomes exceeding capacious in Coenurus and 

 Echinococcus. 



Table for the Discrimination of the more usual Cestodes 

 of Man and Domestic Animals. 1 



I. Scolex in most cases with hooks ; uterus with a median and lateral 

 branches ; yolk-glands simple, median ; genital pore single ; dorsal 

 excretory vessel narrower than the ventral, without a circular com- 

 missural trunk ; eggs without pyriform apparatus (processes of the 

 ovarian membrane) . . . Gen. Taenia L. (s. str.) 



A. Genital ducts pass on the ventral side of the nerve and of the two 



longitudinal excretory vessels . . T. crassicollis Rud. 



B. Genital ducts pass between the dorsal and ventral longitudinal vessels. 



a. Nerve present on dorsal side of genital ducts. 



a. Head armed . . . . . T. solium Rud. 



f3. Head unarmed T. saginata Goeze. 



b. Nerve on ventral side of genital ducts. 



1 See Stiles, Centralbl. f. Bakt. u. Parasitenkundc, 1893, xiii. p. 457 (conf. 

 note, p. 90). 



