12 Prof. Muller on the Anatomy and Development 



here called development of the Echinoderm, is, that a process of 

 the integument of the larva grows inwards and lays the foun- 

 dation of the future water-vascular system, on which the other 

 organs of the Echinoderm, whether nervous, vascular or tegu- 

 mentary, are in a manner modelled*. 



It is of very great importance to remember this fact in consi- 

 dering the homologies of the parts of the Echinoderms. 



If the larva of the Echinoderm pursued its normal course of 

 development, it is obvious that its nervous system, fur 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 there- 

 from, 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 nervous system 

 of the adult Echinoderm can be reduced to neither of these types ; 

 it consists invariably in the Ophiuridse, Asteridse, Echinidse, and 

 Holothuriadse, of a circular or pentagonal cord surrounding the 

 oesophagus (of the Echinoderm) 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 discre- 

 pancy obvious. The oesophagus of the Echinoderm is not ho- 

 mologous 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 gan- 

 glionic chain of an Annelid, but with that elaborate system of 

 dorsal-proboscidean nerves which M. Quatrefages has detected 

 and described in the latter. 



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

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

 of nerves in the Annelida is truly stomatogastric. But in the 

 first place it has not been shown so to be ; and in the second 

 place, the existence of well-organized eyes supplied by nerves 

 from the ordinary ventral ganglia in each segment of Poly- 



* Hitherto we have chiefly quoted Prof. Midler, but for what follows we 

 must be considered alone responsible, unless direct mention be made to the 

 contrary. 



