PLATYHELMINTHES 



171 



tubular structure, — the vitellarium or yolk-gland, the secretion from 

 which surrounds the eggs in the cavity of the shell-gland, the latter a 

 small body consisting of compactly arranged gland-cells and located 

 just above the vitelhne gland. From the shell-gland the eggs pass 

 through a narrow duct into the uterus, a simple tubular organ ascending 

 directly in the middle of the segment and closed at its distal end. The 

 "uterus becomes much dis- 



L 



tended from the accumulation 

 of eggs and develops nu- 

 merous lateral branches to 

 which the other sexual organs 

 gradually give place until Ht- 

 tle remains of them but ves- 

 tiges of the vas deferens and 

 vagina. The egg-engorged 

 organ, with its lateral cecal 

 pouches, may rupture, or the 

 integument of the segment 

 itself may give way, permit- 

 ting the eggs to escape directly 

 into the intestinal contents. 

 As a rule these terminal or 

 *'ripe" segments are passed 

 to the outside of the body of 

 the host with the feces where, 

 by their disintegration, the 

 eggs are set free. 



The eggs of cestodes are 

 globular or more or less oval 

 in shape and are provided with shells of variable thickness (Figs. 96 

 and 110). Beneath the shell is a translucent yolk which surrounds an 

 inner covering containing the onchosphere (hexacanth) or . six-hooked 

 embryo (Fig. 112, a). In some forms the eggs as found in the feces 

 often have the outer shell absent. 



Life History. — Species of Tseniidse in which the development is 

 known undergo a complex series of metamorphic changes, involving 

 larval and sexually mature parasitism in hosts of differing species. 

 After the egg, either free or with the segment entire, has been ingested 

 by a proper larval host, the shell and embryonic envelope are digested 

 away by the gastric Juices, and the onchosphere is freed (Fig. 112, a). 

 At this stage the embryo is provided with three pairs of booklets by 

 which it penetrates the intestinal wall and, probably by blood and 

 lymph currents, may be carried to certain parts of the body specifically 

 essential to its further development. Thus passively lodged, it loses 



Fig. 90. — Segment' of Taenia saginata, with 

 sexual organs matured. Ovaries in lower portion 

 to right and left; yolk gland in extreme lower 

 portion; shell gland between yolk gland and ova- 

 ries; uterus, tubular organ extending upward; 

 vagina, extending from glands to genital pore at 

 left margin; testis, bodies distributed throughout 

 segment-, vas deferens, convoluted organ extend- 

 ing laterally to genital pore. Excretory vessel 

 united by transverse commisures. Lateral longi- 

 tudinal nerves shown by heavy lines. 



