CHAP. VIII 



KCIIIXODKK.MATA 



283 



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

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

 lacral surface on which tuhr-fret ;uv 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 lew 

 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. 116, B) and the Crinoids (Fig. 116, 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. 116, 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- 

 thurian ; E, a Crinoid. a, 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rounded or elongated the rays being as it were withdrawn into the 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 



