NERVOUS SYSTEM. 123 



that the very remarkable similarity in their arrangement may be 

 clue to the similarity in the general structural conditions, viz. the 

 presence of a central point round which the plates are regularly 

 arranged. This view derives some support from a consideration of the 

 fact that when the matter is minutely examined there is a considerable 

 amount of variation in the arrangement of the apical plates. For in- 

 stance the apical pole may be occupied by a central plate (many Echin- 

 oids, larva of Antedon, many Asteroids and Ophiuroids), or by five 

 interradially placed plates (basals in many irregular Echinoids), or by 

 five radially placed plates (infrabasals of some Crinoids). Again the 

 number of circles of these plates varies considerably. In many Crinoids 

 there are three, viz. infrabasals, basals, radials. The same arrangement 

 occurs in many Asteroids and Ophiuroids, but in these classes there is 

 the greatest variation in the arrangement of the plates near the apical 

 pole, and in many of them the plates in this region are small and numerous 

 and the typical apical plates cannot be recognized. In Echinoids, on the 

 other hand, there is never more than one circle, the interradially placed 

 basals. 



The alimentary canal. The variations in the position of the 

 mouth and anus have already been indicated (p. 117). The 

 alimentary canal passes between the two, and is chiefly remarkable 

 for the very general absence of separate glandular appendages. 

 The anus is absent in the adult of Ophiuroids and of a few 

 Asteroids. For details the reader is referred to the account of 

 the different classes. 



The central nervous system consists of three parts which are 

 variously developed in the different classes. (1) The ventral, 

 (2) the deep oral, (3) the apical. 



The ventral system consists of a concentration of a diffuse 

 subepithelial plexus, found in most parts of the body in both 

 ectoderm and endoderm. It is mainly a sensory system and 

 supplies the skin, the feet and the gut. The ectodermal part 

 of this plexus, which is continuous with the endodermal and 

 may be called the ectoneural, is especially concentrated in an 

 annular tract round the mouth (circumoral nerve ring) and in 

 prolongations of this along the whole length of the radii (radial 

 or ambulacral nerves). These concentrations constitute what 

 we have called the ambulacral central nervous system. 



The general disposition of the ectoneural plexus is well shown 

 in Fig. 86 : it extends into the tube-feet, in some of which, 

 especially in the pointed variety, it is well developed, as well as 

 into the spines, papulae and pedicellariae. The endodermal 

 part of this plexus, which is called the endoneural and presents 

 in Asteroids special concentrations round the edge of the mouth 



