114 ANATOMY AND DEVELOPMENT OF THE ECHINODERMS 



either of the first three forms into the Echinoderm, or may run through 

 them all." (Larven u. Metam. d. Holoth. u. Asterien, p. 33.) 



Furthermore it may be stated that the nature of the change here 

 called development of the Echinoderm, is, that a process of the integu- 

 ment of the larva grows inwards and lays the foundation of the future 

 water-vascular system, on which the other organs of the Echino- 

 derm, whether nervous, vascular or tegumentary, are in a manner 

 modelled.! 



It is of very great importance to remember this fact in considering 

 the homologies of the parts of the Echinoderms. 



If the larva of the Echinoderm pursued its normal course of de- 

 velopment, it is obvious that its nervous .system, for instance, would be 

 homologous in form and position with that of other Annulose forms. 

 There would be a ring with cerebral ganglia round the oesophagus and 

 a chain of ganglia proceeding therefrom, if the nervous system were of 

 the type of the Annelids. Or if it resembled that of the Trematoda, 

 there would be an oesophageal ring with two opposite ganglia, from 

 which a cord would proceed on each side of the body. But the ner- 

 vous system of the adult Echinoderm can be reduced to neither of 

 these types ; it consists invariably in the Ophiuridae, Asterid^, 

 Echinidje, and Holothuriadje, of a circular or pentagonal cord sur- 

 rounding the oesophagus (of the Ecliinoderiii) without distinct ganglia. 

 From this five cords proceed, in a perfectly radiate manner, following 

 the course of the wAter-canals. 



The study of development renders the reason of this discrepancy 

 obvious. The oesophagus of the Echinoderm is not homologous with 

 the oesophagus of the larva, nor with the oesophagus of an Annelid, 

 and therefore the nervous ring of the Echinoderm is not homologous 

 with the nervous ring of the Annelid. Indeed, since the mouth of the 

 Echinoderm answers homologically to an aperture in the dorsal wall 

 of the stomach of the larva, and since the nervous system of the 

 Echinoderm follows exactly in its form the form of the water-vascular 

 system of the Echinoderm, which is essentially a process of the dorsal 

 integument of the larva, we might be tempted to conclude that the 

 nervous system of the Echinoderm is homologous, not with the 

 •ordinary ganglionic chain of an Annelid, but with that elaborate 

 system of dorsal-proboscidean nerves which M. Ouatrefages has 

 detected and described in the latter. 



The fact that these nerves supply eye-spots would indeed present 

 some difficulties in the way of this hypothesis, if this system of nerves 



' Hitherto we have chiefly quoted I'rof. Miiller, but for what follows we must be con- 

 sidered alone responsible, unless direct mention be made to the contrary. 



