FORM OF THE BODY OF VERMES. 



131 



Fig. 52. Yonng 

 Toenia, with head 

 pushed in. a Head. 

 b Envelope, c The 

 six embryonic 

 hooks, remaining 

 at one point of the 

 envelope (after V. 

 Siebold). 



Fig. 53. The 

 same Tajnia, with 

 head protruded , 

 Letters as in Fig. 

 52 (after V. Sie- 

 bold). 



is based on tlie notion tliat certain abnormal external conditions of 



life gradually became normal, in consequence of the adaptation of 



the organism to them, and that it did not arise by a simple 



antagonism to the worm^s primitive ontogenesis, which now includes 



this cystic form as a normal part of 



its cycle. What has happened is 



this — that the process of adaptation 



has seized upon and exaggerated 



a normal inherited phase of the 



worm's ontogeny, and in virtue 



of the continuation of conditions 



favourable to the appearance of this 



exaggerated phase, its appearance 



has become a normal phenomenon. 



The variations of the cystic form 



are all readily deducible from the 



first developmental stage of the 



Cestoda. The embryo is generally 



provided with three pairs of hooks, 



and a cestoid head may be observed 



to be differentiated within it (Fig. 52, a) ; when fully developed 



this is pushed out, so that the envelope, which at first was external, 



becomes the portion of the body below the head (Fig. 53, h). In 



the Cysticercus-form the embryo grows into a vesicle filled with 



fluid, from the walls of which the head is budded 



out. When the head is protracted, the vesicle 



forms a terminal appendage of the body (Fig. 54). 



When a number of buds are formed on the wall 

 of the vesicle, in which protractile heads are dif- 

 ferentiated, we have the Coenurus-form. When the 

 buds break off into the interior of the vesicle, and 

 there form new vesicles, on the walls of which the 

 same budding process goes on, leading to the for- 

 mation of systems of vesicles, placed one within the 

 other, and when the youngest of these can again 

 bud off tsenia-heads on its inner wall, we get the 

 Echinococcus-form. 



Notwithstanding the various characters of their 

 final products, these processes of gemmation may 

 be derived from a common ground-form. They are 

 by no means unparalleled among thePlatyhelminthes, 

 for in not a few an asexual multiplication occurs, which is 

 very similar to these in many points. It is very common among the 

 Trematoda, where the embi-yo gives rise to an asexual stage known 

 as the " sporocyst." The body-parenchyma of this sporocyst 

 becomes differentiated, generally into similar structures, in which in 

 their turn the larv£e known as " Cercarias " are produced, and these 

 are developed into the sexually mature form. The variation in the 

 forms of the separate generations seems to be due, in a general 



K 2 



Fig. 51. Cysti- 

 cercus cellu- 

 loses ; head pro- 

 truded (nat.size). 

 a The caudal ves- 

 icle, filled with 

 fluid, c The an- 

 terior part of the 

 body, d The head 

 (after V. Siebold). 



