GENERAL ZOOLOGY 



Vagina 



Seminal 

 receptacle 



Vitelline 

 duct 



Vitelline 

 gland 



Fig. 11.18. Mature proglottid of a tapeworm, Taenia pisifornns, showing details of male and 

 female reproductive systems. Note that there is no uterine pore; fertilized eggs accumulate 

 in the enlarging uterus and are released only by rupture of the shed proglottid. (Redrawn from 

 C. G. Goodchild in F. A. Brown, Jr., el ai. Selected Inverkbrate Types, copyright 1950 bv 

 John Wiley and Sons, Inc., printed bv permission.) 



The internal structure of such a tapeworm is relatively simple, except for 

 the reproductive organs. There is no mouth or digestive system; digested food 

 in the host's intestine is apparently absorbed through the body wall of the 

 parasite. A protonephridial excretory system is present, consisting of flame 

 bulbs and ducts leading to longitudinal vessels which extend throughout the 

 length of the body. The nervous system is little developed except in the scolex; 

 a pair of longitudinal nerve trunks lying near the excretory trunks also extend 

 throughout the length of the strobila. Sense organs and sensory cells are 

 virtually unknown in such tapeworms, although the animal may, by move- 

 ments of the scolex and contractions of the body, respond to external stimula- 

 tion. This degeneration of structures important in free-living animals is 

 common in parasites. The reproductive system is complex, and a complete 

 system of male and female organs develops in every proglottid. In T. solium 

 fully differentiated reproductive systems may be seen in the region from the 

 two-hundredth to the three-hundredth proglottid. Each of these reproductive 

 systems is comparable with the single set of male and female reproductive 

 organs found in a turbellarian or a trematode (Fig. 11.18). The egg is like- 

 wise comparable, consisting of a shell enclosing a zygote and yolk cells. Self- 

 fertilization is probably the most common means of insemination in these 



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