CHARACTERISTICS OF THE BOTHRIOCEPIIALIM. 675 



if present at all (Trisenophorus), are without a rostellum* The segmen- 

 tation is indistinct, and sometimes altogether absent, in which case the 

 body has a very simple structure. The joints are always broader than 

 long, and in their mature state so firmly united, that they are not detached 

 singly, but in numbers. The uterus is a simple, more or less winding 

 canal, with a ventrally situated opening, through which the eggs issue after 

 the completion of the embryonic development. The reproductive organs 

 persist throughout life, and, constantly emit new products. The yolk-glands, 

 which are of considerable size, are symmetrically divided between the two 

 sides, and often distributed over the whole cuticular layer. The cirrhus 

 and vagina generally open externally, not far from the uterus, and more 

 rarely (as in Tsenia) on the border. The eggs contain a considerable 

 quantity of yolk, and possess from the first a firm shell, with an operculum 

 at one end, so that the embryo can readily find an exit. The embryo is 

 usually covered with a distinct ciliated mantle, by means of which it can 

 swim about for some time. 



Except in the case of Archigetes, the development of the larva takes 

 place in an intermediate host, but in a somewhat direct way, for the 

 embryo merely elongates, and forms the subsequent suctorial grooves. 

 Sometimes it grows into a jointed tape-worm, even in its intermediate 

 host ; but as a rule this takes place only later, after the parasite has 

 migrated from its original abode into the intestine of the definitive host. 

 A true bladder-worm stage seems always wanting, but in some cases there 

 is an appendage on the posterior border of the larval body, which in 

 genetic significance corresponds to the caudal bladder of the Cysticerci. 



If we are right in ascribing the highest place among the Cestodes 

 to the Tseniadse, and especially to the cystic tape-worms, the family 

 Bothriocephalidae (in the sense here defined) must be regarded, on the 

 contrary, as the lowest group. This is evident, however, not so much 

 from the anatomical structure of the several organs, as from the fact 

 that the individual! sation of the different parts composing the body 

 is upon the whole much less pronounced than in the other tape-worms, 

 and in many cases is even completely wanting. The head, which is 

 elsewhere a structure of great independence and characteristic structure, 

 is small and simple, and often hardly other than the anterior end of 

 the body provided with the attaching apparatus. Similarly, the 

 segments are usually imperfectly separated, and sometimes not marked 

 off in any way from each other, so that the segmentation is only 

 shovel by the regularly recurring sexual organs (Ligula, Triwnophorus). 

 But when, as in some cases (Caryophyllceus, Archigetes), the number of 

 these structures decreases to one, the appearance presented is entirely 

 different. In place of the polyzootic tape-worm there is a simple 



