162 REPRESENTATION OF PROTOMORPHIC ALTERNATION 



much in the same way on the investing or blastodermic 

 layer of the germ-mass, but it gradually becomes invagi- 

 nated within the amniotic folds of that membrane, some- 

 what as the head and neck of the Cysticercus are within the 

 primary cyst, in the early stage of its development. In 

 such cestoid forms as the Bothriocephalus, the progressive 

 development is, if possible, of a still more continuous kind, 

 the Tsenia-head appearing to be formed from the original 

 contents of the ovum, by as direct a process of organiza- 

 tion as is anywhere met with in animal embryogeny, and 

 the act of gemmation not being obviously represented by 

 any one particular event in the course of evolution. 



We have, then, in this single order, a series of cases 

 establishing a transition from a well-marked " alternation," 

 attended with multiplication and detachment of gemmae, to 

 ordinary continuous embryogeny. This argument is of all 

 the greater cogency if Ccenurus be, as Siebold contends, a 

 mere variety of Cysticercus. Even in admitted forms of 

 CysticercuSy however, it is probable such multiple gemma- 

 tion of Tsenia-heads may at times occur, for when the cyst 

 developes itself in the brain (the usual seat of Ccenurus,) it 

 occasionally buds off secondary cysts, all of which, it would 

 seem, may bear Tsenia-heads.* 



6. The Echinodermata afford another interesting illus- 

 tration of such a transition series. In some species of the 

 class, the primary germ-mass has so much the character of a 

 complete animal as to have been mistaken for one of a 

 totally different family, and named accordingly ;f while 

 the new axis of growth, which appears at one point, and in- 

 creases pari passu with the decline of the matrix, lias all 

 along such a distinct character, and so much individuality 

 of its own, that nothing but actual observation could im- 



* Siebold's Memoir is translated in the second volume of the Kay So- 

 ciety's Edition of Kiichenmeister's work on Parasites. 



f " Pluteus," in Echinus and Ophiura. "Bipinnaria" in the Starfish. 



