ECHINOIDEA 269 



lacral plate with only a single, large primary tubercle, surrounded 

 by a more or less deep area, the " areole ". Primary tubercles 

 perforate, smooth or crenulate. The primary spines covered 

 by a special outer layer, the ostracum, often set with fine, hairy, 

 more or less anastomosing outgrowths. Only at the base of 

 the spine the outer layer is wanting, this part being termed the 

 " collar ". Secondary spines small, usually flat, more or less 

 strongly appressed ; this especially holds good of the secondary 

 spines surrounding the base of the primary spines, the so-called 

 " scrobicular " spines. Teeth without keel. Only globiferous 

 and tridentate pedicellarise present, the former usually in two 

 forms, the large and the small globiferous form. Sphseridia 



Fig. 149. — Cidaris cidaris ; naked test, oral side. Xat. size. 



are lacking. Si^icules of tube-feet small, irregular, spiny rods. 

 Large, bush-shaped outgrowths from the upper side of the 

 dental apparatus, the so-called Stewart's organs. 



Primary spines lacking the outer layer are often found on 

 the upper side of the test, round the apical system. These are 

 only spines which are not yet fully formed. Here and there 

 on the test also similar spines may be found ; these are 

 regenerating spines formed in place of spines that have been 

 lost. Old spines are very often covered by various foreign 

 organisms — Sponges, Bryozoa, mussels, etc. The primaries 

 round the peristome (the oral primaries) are usually much shorter 

 than those of the upper side of the test, and more or less 

 specialised, sometimes curved and strongly serrate. 



The globiferous pedicellarise of Cidarids are peculiar in having 

 the poison gland inside the valve, enclosed by the calcareous 



