172 



PHYLUM ECHINODERMATA. 



V 



ra 



the other plates of the antambulacral surface. Sometimes they remain 

 throughout life as the sole plates of the antambulacral system (Cnemidaster 

 wyvillei], and sometimes only a few other plates are developed between 



them (Neomorphaster 

 talismani, Korethraster 

 setosiis, different species 

 of Marginaster}. 



The dorsolaterals 

 may be connected into 

 an irregular network, 

 and fit closely into one 

 another, and when the 

 marginals are not dis- 

 tinctly developed they 

 pass gradually into the 

 ventrolaterals at the 

 sides of the arms. 



The external skeletal 

 structures are spines, 

 spinelets, scales, gran- 

 ules and pedicellariae. 

 Like the plates they 

 are dermal structures 

 and are covered by a 

 layer of ectoderm. 

 They are processes of 

 or movably - attached 

 to the subjacent skele- 

 tal plates. 



The spines are elon- 

 gated pointed rods, 

 which project, usually 

 singly, from the sub- 

 jacent plates, to which 

 they are attached by 

 muscles and often by a 

 ligament. They are 

 found mainly on the 

 larger plates of the 

 abactinal surface and 

 on the marginal plates. 

 The spinelets are simply 

 small spines. They 

 often occur in tuft-like 



FIG. 122. Diagram of a starfish viewed from the abactinal 

 side to show the antambulacral skeleton (after Ludwig). 

 The arms are numbered i-v (see Fig. 83). A anus ; C 

 central plate ; dl dorso-lateral plates of arms ; ill, I ditto of 

 disc ; JR interradial plate of apical system ; JRA inter- 

 radius in which the anus is placed ; JRM iuterradius con- 

 taining the madreporite ; JR, Md madreporite and inter- 

 radial as one and the same plate ; M marginal ; Mil 

 madreporite separate from the interradial : Md' madre- 

 porite as fused with the interradial ; R radial ; Rbl. rtr. 

 secondary radials of arms ; Rdl, etc. secondary radials 

 of disc ; s supplementary plates ; T terminal plate. 



aggregations. Some- 

 times the plates carry- 

 ing such tufts project in a column-like manner : such projecting columns 

 with their tufts of spines are called paxilli * (Fig. 121). The spinelets 

 are found principally upon the adambulacral plates and upon the actinal 

 and abactinal intermediate plates ; they are attached to the subjacent 



* For a somewhat similar arrangement of spines in the Solasteridae and 

 Pterasteridae, see p. 191. 



