and Anatomy of the EcJnnodermata. 365 



in the Lower Silurian. The morphological data alone can 

 be appealed to here for the decision of the question. 



2. The Relationship bcttceen Asterida and Echinida, 



Having- described the organizational characters of the Echi- 

 nida, I may attempt in what follows to bring together the 

 reasons which give the greatest possible probability to the 

 proposition that the Asterida must be regarded as the pri- 

 mordial group most nearly approaching the stock-form of the 

 Echinodermata, and the Echinida to be derived from them, 

 as has already been supposed by Hackelj Gegenbaur, and 

 others. 



I know very well that with many tliis assumption passes 

 as an established proposition. For such what follows is 

 written only to a limited extent, so far as they, unlike myself, 

 are of opinion that this proposition is still unproven. I 

 would also further show that it is only possible to explain the 

 organizational characters of the Echinida if we derive them 

 from those of the Asterida, and that this assumption alone 

 admits of an unforced explanation of their structure. 



Palgeontology shows us that the Asterida are among the 

 most ancient of organisms, and that there is nothing to pre- 

 vent the Echinida, which are already represented in the 

 Lower Silurian, being derived from them. Of course in this 

 we have to consider only the regular Sea-urchins, but not 

 the irregular ones, such as the Spatangida3, which may with 

 great certainty be regarded as later formations. Hence, 

 when in what follows I speak of Echinida, it is especially 

 only the regular Sea-urchins that I refer to. 



In deriving the Echinid-organism from that of the Asterida, 

 the nervous system must be taken into consideration in the 

 first place. In the Starfishes the nervous system originates 

 in the ectoblast *, and retains its position in the ectoderm. 

 This applies to the central nervous system, the cerebral ring, 

 and five (or more) ambulacral nerve-trunks. The intestinal 

 nervous system I leave on one side, as not essential in our 

 comparison. 



In the Echinida, when the animal is mature, the nervous 

 system is no longer situated in the ectoderm ; it has come to 

 lie in the mesoderm ; and in them we find it connected with 

 the epithelium of the body only where sense-organs are 

 present. 



But are the elements which constitute the central nervous 



* See Ludwig, Asterina yibhosa. 



