116 



IXVEllTEBKATE ANIMALS. 



present. The water-vascular system consists of two long vessels 

 which run down each side of the body and communicate at each 

 articulation by a transverse vessel, the whole opening in the last 

 joint into a contractile vesicle. Each joint, or " proglottis," is sexu- 

 ally perfect, or hermaphrodite, containing both male and female 

 reproductive organs, which open on the surface by a small raised 

 aperture, the "generative pore" (fig. 73, h). Almost the whole of 

 each of the mature joints is filled up by a much-branched uterus. 

 As the head is the true animal, and the numerous joints are only 

 produced by budding, it follows that the entire organism is to be 

 regarded as a kind of colony, constituted by a single sexless zobid 



Fig. 73.— ^lorphology of 7^tvniada. a Head and a few following segments of Tamia 

 medioco nclUita ; b A few segments of the same furtlier removed from the head ; 

 c and d Segments p^llgre^,sn (.'ly further removed from the head, — all of the natural 

 size ; e Head of the same, enlarged ; h A single proglottis of the same, with its 

 branched uterus and lateral genital pore, enlarged two diameters ; / Emhryo of 

 Ti^'u.Ui baeillaris, with six honklets ; g C'ysticercu^ celtuln^a^, the "cystic" young of 

 Trnnla solium, with its lionlilet.s and suckers, its wrinkled neck, and its caudal 

 vesicle, enlarged. (After Leackart, Van Beneden, and Wcinland.) 



(ir " nurse " and numerous sexual zooids, produced by budding from 

 the former. 



The process of development — that is to s.ay. the process by which 

 this composite organism, commonly known as the tapeworm, is pro- 

 duced — is a very remarkable one, and is briefly as follows : Each 

 generative segment or joint, as already said, is hermaphrodite, and 

 contains innumerable ova. These eggs, however, cannot be de- 

 veloped within the body of the animal infested by the tapeworm 

 itself, but they are compelled to gain access to the body of some 

 different species of animal, if development is to proceed. To secure 

 this end, the mature joints of the colony break off', and are expelled 



