116 



IXVEIiTEBRATE AXIMALS. 



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 oi)ening in the last 

 joint into a cimtractile vesicle. Each joint, or " ]jroglotti.s," is sexu- 

 ally 2>erfect, or hernia]ihriidite, containing l.ii.th male and female 

 I'eproductive organ.s, which open on the surface by a small raised 

 aperture, the "generative jioro" (tig. 7.'5, /()■ Almost the whole of 

 each of the niatm-e 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 zooid 



;. ";:!.— :M(.r|>holr.ay of T^enindn. a Head and a few following segments of Timia 

 'ineilloc'iii.ltnlii ; }i A lew segnientn of the same fnrtlier rnuoved from the head; 

 c and (( Segments ijrogressixely fuilher remoi-eil froiri the head,— all of the natural 

 siw ; e I-Iejid of the same, enlarged ; /t A single itii.ghiltis of the same, with its 

 liranched uterus and lateral genital jiiire, enlarged two diameters; /Embryo of 

 J'.r/i;,. bi'fiiUfiris. witli six hofiklets ; ;; rysli.rrais nlhiluy,,-, the "cystic" young of 

 J'.e/ioi si:lhivi, with ils hoiikh-ts and surliers, i|-,s wrinkled neck, and its caudal 

 vesii-le, enlaiged. (.\tter Leuckart, Van Beneden, and AVeinland.) 



or " nurse " and numerous sexual zooids, produced by budding from 

 the former. 



Tlie process of development— that is to say. the process by which 

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

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

 generative segment or joint, as already .said, is herm;i,plirodite, and 

 contains innumerable ov;i. Tliese eggs, however, c.-mnot be de- 

 velo]>ed within the bcdy of the animal infested by the tapeworm 

 itself, b\tt they are conjjielled to g.'un access to the body of some 

 ditfcrent species of annn:d, if develojjment is to jd-oceed. To secure 

 this end, the nriture joints of the colony break oti', and are e.xpelled 



