CATALOGUE 



OF 



BRITISH ECHINODERMA. 



INTRODUCTORY REMARKS. 



The Position of Echinoderms in the Animal Kingdom. — The 

 Echixoberjia belong to that division of the Animal Kingdom in 

 which the organism consists of more than one cell, and is always, 

 at some period or other of its life -cycle, derived from a single 

 egg-cell which has been fertilized by a sperm-cell. In other words, 

 it is one of the Metazoa. In the course of development from the 

 egg there appears, in addition to the gastric cavity, another space 

 which is known as the body-cavity or ecelom. 



Among the Metazoa Ccelomata the Echinoderma occupy a very 

 isolated position, for we know of no link which unites them to 

 any other " phylum *' or great division of this series. 



The Symmetry of Echinoderms. — It would appear that, at a very 

 earl}- stage in the evolution of the Echinoderma, the adult tended 

 to acquire a marked radial symmetry ; but, at first, the number 

 of rays was by no means constantly live : now the symmetry is so 

 generally quinqueradial that wonder is nearly always expressed 

 when the number five is diminished or increased. 



The first point, then, which we have to notice is the possession 

 of rays; the " rays " and the "body" generally may appear to be 

 conterminous, as in the Holothurian or the Echinus, or the " body " 

 may seem to gradually pass into the " rays," as in the common 

 Starfish, or may be quite distinct from it, as in the Brittle-star and 

 Sand-star. AYhere a marked distinction can be made between the 

 body and the rays, the former is often called the disk and the 

 latter the arms. 



The radial symmetry which is indicated externally by the rays 

 is to be observed in some at any rate of the internal systems of 

 organs. It is characteristic of Echinoderms to have a special system 

 of tubes, not represented in other types of animal life, some of the 



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