134 . Echmoderma. 



body ; large c^ca are wanting, there are only short pouclieSj whicli 

 do not extend into the arms. There is no anus. The opening or 

 openings of the stone canal are in the madreporite, which lies close 

 to the mouth. On the lower side of the disc, close to the bases 

 of the arms, there are ten narrow slits, leading into the same number 

 of sacs,* which have a respiratory significance {bursse) . On 

 their walls are the genital glands; ova and spermatozoa 

 escape into the sacs, and leave the body through the slits. 

 Eyes, pedicellariee, and gills are absent, but longer or shorter 

 spines, which are important in locomotion, occur especially along 

 the edges of the arms. 



Those Ophiurids which are unable to evert the stomach, feed upon 

 dead animals, or upon such as are not capable of resistance ; they 

 gnaw their food with the denticles mentioned above. 



1. True Brittle-stars (genus Ophiwra, etc.). With five (rarely a 

 larger number) of simple arms ; occun-ing in all seas, and represented in those of 

 northern Europe by a number of nearly allied species. Some are spiny, others 

 smooth. They may often be found climbing over foreign objects with the aid of 

 their arms. 



2. Astrophyton, distinguished from the true Ophim-ids by the fact that the 

 five ai-ms, which pan be rolled up towards the mouth, are much branched. 

 The dermal skeleton is somewhat less developed than in the true Ophiurids, and 

 they can swim like the Antedons. They attain a considerable size. Species of 

 this genus ocom- in northei'n seas, but are much less abundant than the former. 



Class 3. Echinoidea {8ea-urcUns). 



In some Sea-urchins the body is almost spherical, but in most, 

 on account of the shorter main axis, it is flattened or occasionally 

 discoid; arms are completely absent. The greater portion of the 

 body-wall is furnished with immovably connected calcareous plates. 

 In the so-called Regular-urchins, with spherical body 

 (the transverse axes being of about equal lengths), there are 

 twenty rows of these plates extending meridionally from one end 

 of the principal axis to the other. Ten of the rows bear fine pores, 

 each plate having one or several pairs ;t each pair of pores corre- 

 sponds with a tube-foot. In each radius there are two series of 

 these, pore or ambulacral plates, whilst in each inter- 

 radius there are two interambulacral plates. The latter are 

 often broader than the pore plates, and like these are covered with 

 larger or smaller, nearly hemispherical knobs, each of which bears 



* Sometimes twice this number of slits is present, each of the original ones being 

 divided by a transverse bridge ; but the number of sacs remains the same. 



t Each pore plate bears primitively only a single pair of pores ; in consequence 

 of the fusion of several plates, however, there are several pairs in most of the Eegtdar- 

 urchins. 



