SKELETAL SYSTEM. 121 



of networks or spongeworks of calcareous matter in the con- 

 nective tissue. In many of the classes (Asteroids, Ophiuroids 

 and Echinoids) these plates carry spines and processes which 

 project on the surface, and in the young state at least are covered 

 by the epidermis. The majority of the spines are movably 

 articulated with the plates, and in Asteroids and Echinoids 

 some of them are specially modified as snapping organs the 

 pedicellariae. The function of the spines is probably mainly 

 protective, but in the case of the long spines of Echinoids it is 

 locomotory. The pedicellariae are protective, seizing foreign 

 organisms (p. 224). They are said not to bite animals of the 

 same species (autodermophily, Uexkiill). 



The epidermis is generally ciliated, but in Ophiuroids, and on 

 the aboral surface of Crinoids it is difficult to distinguish it as 

 a layer distinct from the cutis. 



The description of the skeletal system will be best dealt with 

 in connexion with the different classes of the phylum. It will 

 be well however to call attention here to certain plates which are 

 supposed to be homologous throughout the group : these are 

 the plates of the oral and apical systems. The oral plates are 

 five in number and are placed interradially round the mouth on 

 the oral surface. They can be distinguished in many Crinoids 

 and in Ophiuroids, but are not clearly distinguishable in any 

 other class. 



The plates of the apical system are placed at the aboral pole, 

 and when present are usually discernible at a comparatively 

 early stage of development. In typical cases (Ophiuroids, 

 Fig. 84, many Crinoids, Fig. 85) they consist of a central plate 

 surrounded by ten plates, five of which are radial in position 

 and five interradial : the former are called infrabasals, the latter 

 basals. Beyond this circle there are five radially placed plates 

 called the radials. Of these plates the infrabasals, or under- 

 basals as they are sometimes called, are frequently absent. In 

 Echinoids the central plate is not pierced by the anus which is 

 placed on one side of it ; moreover it is frequently very difficult 

 to distinguish it, owing to the presence of a number of small 

 plates, called the periproct plates, at the apical pole. In Crinoids 

 the position of the central plate is not certainly known, but it is 

 supposed to be represented by the so-called dorso-central which is 

 found at the peripheral end of the stalk. In Asteroids a complete 



