CHAPTER VIII. 



THE ECHINODERMA. 1 



PHYLUM ECHINODERMA. 



GRADE A. PELMATOZOA. 



CLASS I. CYSTIDEA. 



II. BLASTOIDEA. 



III. CUINOIDEA. 



IV. EDRIOASTEROIDEA. 



GRADE B. ELEUTHEROZOA. 

 CLASS I. HOLOTHURIOIDEA. 



11 



II. STELLEROIDEA. 



III. ECHINOIDEA. 



General Features. This is one of the best characterised and 

 most distinct Phyla of the Animal Kingdom. Nearly all the living 

 animals included in it, such as the sea-urchin (Echinoid), starfish 

 (Asteroid), brittle-star (Ophiuroid), sea-cucumber (Holothurian), 

 sea-lily (stalked Crinoid), or feather-star (free Crinoid), can readily 

 be distinguished through their possession of a radial symmetry, in 

 which the number five is dominant, of a sub-epidermic skeleton com- 

 posed of calcium carbonate, with a characteristic micro-structure 

 resembling trellis-work, and of a system of sacs, canals, and tubes 

 that carry water through the body, especially by means of five 

 radial canals from which small branches called podia are given off to 

 the exterior. The extinct forms known as Blastoidea and Edrio- 

 asteroidea appear to have had a similar organisation ; and the same 

 statement maybe made of most of the Cystidea, another extinct class. 



It is true that there are recent forms in which the quinqueradial 

 symmetry or pentamerism is obscure ; but, on the whole, it is so 

 marked a feature that the early zoologists, and notably Cuvier, 

 placed the Echinoderma together with the Coelentera in a sub- 



1 By F. A. Bather, M.A. 

 I 



