Il8 ANATOMY AND DEVELOPMENT OF THE ECHINODERMS 



so-called "'compound animals" must be revised, and their termin- 

 ology altered. A whole tree of Sertularia, a Pennatula, a PjTosoma,. 

 a mass of Botrylli, must no longer be considered as an aggregation 

 of individuals, but as an individual developed into many zooids. 



And if the term " compound animal " is to be retained in its old 

 meaning, we know of only one creature which is entitled to the name,, 

 viz. the Diplozoon paradoxuin, which Von Siebold has just shown to 

 be really formed by the fusion of two previously distinct individuals. 



\\'e hope that the reader will pardon this long digression into the 

 I'egions of abstract thought. Whether he adopt our view or not, we 

 trust that at any rate, we have pointed out where the real battle of the 

 alternation theory lies. 



The onus of giving a new meaning to the A\-ord " individuality " 

 must rest with the advocates of the alternation theor}- ; we have en- 

 deavoured merel}' to make a consistent extension of the old meaning 

 to embrace new facts. 



The Echinoderms have been included under the " Alternatioa 

 theory " ; but, if the reasoning above be correct, unjusth", as is indeed 

 plainly pointed out on other grounds by Prof Muller in his second 

 memoir. He justl}- observes that the process of development of the 

 Pxhinoderm partakes as much of the nature of metamorphosis as of 

 " alternation." The larva and the F.chinoderm cannot be said to be 

 two indi\-iduals, when they possess the same intestine. 



Xor, as to the question of fact, does the development of the 

 Echinoderm appear to be a case of " Parthenogenesis." 



The structure of the integument of the larva, at the place ^\•herethe 

 tubular rudiment of the Echinoderm is subsequently formed, is quite 

 undistinguishable from that of any other spot. There are here no 

 descendants of the embrj'o-cells special!)' set aside to become developed 

 into the new structure.^ 



The development of the Echinoderm is then neither a process of 

 " alternation of generations " nor of " Parthenogenesis," but the indi- 

 vidual consists of two zooids — a larva-zooid and an Echinoderm-zooid, 

 the latter of which is developed from the former hy a process of 

 internal gemmation." 



•* The elongated cellular masses which exist on each side of the digestive canal in the 

 larvcc, are very possibly the immediate descendants of the embryo-cells. But Prof, iliiller 

 leaves it very doubtful, whether these masses have anything to do with the development of 

 the Echinoderm. Certainly they are not concerned in the development of o "le most impor- 

 tant part of it — the water-vascular system. See Mull. Arch. 1850, p. 466. Ibid. 1851, p. 4. 



'^ According to Prof Muller (Archiv, 1S51, p. 18) the development of the Echinoderm- 



