UNSEGMENTED WORMS 



199 



Class III. Cestodes (Tape-worm, etc.). — The Cestodes are internal parasites 

 having a complicated life history usually involving two hosts, sometimes three. 

 In the tissues of the first host occurs the "bladder- worm," Cysticercus, or embry- 

 onic stage (Fig. 93, A); in the intestine of a second host the strobila or adult tape- 

 worm (Pig. 93, C) is found. The adult form has no mouth or digestive tract, the 

 animal taking its food by absorption of the digested material in which it is bathed. 

 The anterior end is supplied with hooks, suckers, or both, by means of which it 

 attaches itself to the intestinal wall. Just behind this "head" is a region in which 



Fig. 94. Diagiain of a sexually mature proglottis of TtBnia. A, anterior end; ff, embryos; 

 ex., excretory canals; g.p., genital pore; ov., ovaries (paired); r.s., receptaculum seminis; s.g., shell 

 gland; (, testes; uf., titehis filled with embryos; v, vagina; v.d., vas deferens; y.g.. yolk gland. 



Questions on the figure. — Why is self-fertilization possible in tapeworm? 

 What is the function of the various portions of the reproductive apparatus? 

 Trace the following steps and indicate where each incident happens: formation of 

 eggs and sperm; passage of sperm to vas deferens and into vagina; storing of sperm , 

 in receptaculum seminis; fertilization in the oviduct; addition of yolk; ovum 

 covered with shell secretion; passage into uterus where development proceeds. 



transverse division (Fig. 93, z; and §126) is continually taking place. This results 

 in the continuous formation of new segments or proglottides, the older ones being 

 pushed further from the head by those newly formed. Each proglottis becomes 

 in time a sexually mature hermaphrodite individual. All stages of sexual maturity 

 are found in one strobila or colony, the posterior individuals being most mature. 



