III. CESTODA 



249 



When a circle of hooks is present it is on the anterior end and is moved by a 

 epecial apparatus, the rostellum, a plug of complexly arranged muscles (fig. 226) 

 which can arch and flatten the central area. Each hook has its point outwards 

 and its base with two roots, one of which rests on the rostellum; the protrusion 

 of the rostellum forces the points outwards into the mucous membrane of the 

 intestine. In some Tcenice without hooks (T. saginata) the rostellum is replaced 

 by a sucker-like depression. Since the rostellum arises in development from a 

 similar cup, it may be a modified apical sucker, but it is doubtful how far com- 

 parisons may be made with the oral sucker of the trematodes. 



The animals are almost always hermaphroditic and the gonads are 

 equal in numbers to the proglottids, so that these were formerly regarded 

 as sexual individuals of a colony, each with its own reproductive apparatus. 

 Two types must be recognized. In the one the presence of vitcllaria and the 

 separate openings of uterus and vagina recall the conditions in trema- 

 todes, while in the second the uterus ends blindly and the vitellaria are 

 modified into a small albumen gland. Since vagina and vas deferens 

 almost always open together, self-impregnation is possible, but cross- 

 fertilization of separate proglottids has been seen. The general features 

 of the two types may be made out from figures 227 and 228. 



FIG. 224. FIG. 225. FIG. 226. 



FIG. 224. Apical view of head of Tcenia solium (from Hatschek). 



FIG. 225. Head of Tetrarhynchus viridis (after Wagner) . Dissected to show the 

 internal parts of the proboscides (o) and the ganglion (g). 



FIG. 226. Schema of action of rostellum. On the right the hooks are exserted for 

 adhesion, on the left retracted, r, rostellum; s, sheath; /, longitudinal muscles. 



In the Bothriocephalidae numerous testes (fig. 227, /;) are scattered through 

 the parenchyma. , The small vasa deferentia unite repeatedly to a chief canal 

 which opens in the middle line, near the anterior margin of the proglottid, the 

 terminal portion, the cirrus being retractile into a cirrus pouch (cb). The ovary 

 is two-lobed (ov) and is near the posterior margin of the proglottid. It forms 

 eggs poor in yolk. The scattered vitcllaria are voluminous. The unpaired 

 oviduct and the vitelline duct unite to form the shell gland, in which each egg 

 unites with a number of 'yolk cells' to form a compound egg (see p. 240). The 

 uterus (u) leads from the gland to the exterior. There is also a vagina (va) 

 leading from the oviduct to the exterior. In distinction to the Trematodes, the 

 vagina empties with the uterus apart from the male organs. The general 

 differences in the Taeniidae may readily be made out from fig. 228, the chief 

 points being the replacement of the vitellaria by an albumen gland, the blind 

 uterus, and the termination of male and female ducts (vagina) at the side of the 

 proglottid. 



