162 KEPRESENTATION OF PROTOMORPHIC ALTERNATION 



much in the same ^Yay 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 Bothrioceplialus, 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 anyAvhere 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 Coenurus be, as Siebolcl contends, a 

 mere variety of Cysticercus. Even in admitted forms of 

 Cysticercus, however, it is probable such multiple gemma- 

 tion of T^enia-heads may at times occur, for when the cyst 

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

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

 seem, may bear T^enia-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 ;"j* while 

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

 creases pari passu with the decline of the matrix, has 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 tlie Ear So- 

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



t " Pluteus," in Echinus and OpMura. "Bipinnaria" in the Starfish. 



