CHAP. VIII 



ECHINODERMATA 



283 



the portion of surface on which they are present — -in the starfish the oral 

 surface — is termed the ambulacral surface as opposed to the abambu- 

 lacral surface on which tube-feet are absent. 



The features mentioned so far may be observed in any species of 

 starfish — except that the number of rays though usually five is in a few 

 cases greater : they are in fact characteristic of the Asteroidea in general. 

 The other subdivisions of the Echinodermata have characteristic differ- 

 ences. Thus in the Ophiuroids (Fig. 1 16, B) and the Crinoids (Fig. 1 16, E) 

 the rays are more distinctly marked off from the disc : the Crinoids are 

 typically attached to the substratum by a long slender stalk (s) which, 

 projects from the centre of their apical surface : the Echinoids (Fig. 116, C) 

 and Holothurians (Fig. ii6, D) have lost their star shape and become 



Fig. 116. 



The chief types of Echinoderm. A, A Starfish ; B, a Brittle-star ; C, a Sea-urchin ; D, a Holo- 

 thuriau ; E, a Crinoid. u, Anus ; s, stalk ; t, oral tube-feet ; t.f, tube-feet. 



rounded or elongated — ^the rays being as it were withdrawn into the 

 disc — ^and the abambulacral surface has shrunk away almost to nothing, 

 the ambulacra stretching from the mouth right up to the neighbourhood 

 of the apical pole. 



A characteristic feature of the Echinoderm is its skeleton. In the 

 Starfish this is in the form of plates and bars (" ossicles ") of calcium 

 carbonate embedded in the body-wall. On the abambulacral surface 

 these form an irregular network : on the oral surface they are arranged 

 more regularly especially in the region of the ambulacral groove over 

 which they — the ambulacral ossicles — are arranged like the rafters of 

 a roof. Scattered over the surface — though not so well marked 

 in Asterias as in many other Echinoderms — are ossicles which project 

 outwards as spines — hence the name Echinodermata, i.e. spiny skinned. 

 Some of these spines have undergone an interesting modification, having 



