198 



ZOOLOGY 



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

 having a complicated life history usually involving two hosts. In the tissues of 

 the first host occurs the "bladder- worm," Cysticercus, or embryonic stage (Fig. 93, 

 A) ; in the intestine of a second host the strobila or adult tape-worm (Fig. 93, Q 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 or suckers by means of which it attaches itself to the 

 intestinal wall. Just behind this "head" is a region in which transverse division 



fl 3. 



FIG. 94. Diagiam of a sexually mature proglottis of Toenia. A, anterior end; e, embryos; 

 ex., excretory canals; g.p., genital pore; ov., ovaries (paired); r.s., receptaculum seminis; 5.?., shell 

 gland; t, testes; ut., uterus filled with embryos; v, vagina; v.d. t 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. 



(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. At the posterior 

 end of an old colony the proglottides (Figs. 93, 94) are rilled with the developing 



