CLASSIFICATION 



297 



ab 



or 



,or 



in some points of embryology, and in some of biochemistry, 

 they show resemblances to the chordates. The substitution of 

 radial for bilateral symmetry is perhaps connected with ancestors 

 which were all sessile, as some modern adults and many larvae 

 still are. There are five existing classes, 

 and some which are extinct. All exist- 

 ing echinoderms are marine, and vym^J^^mt^ 

 members of the phylum were common ^ or 



in palaeozoic times. 



CLASS I—ASTEROIDEA 



The starfishes are adequately 

 represented by Asterias. The shape 

 is characteristic — a five-pointed star, 

 in which the rays of the star are not 

 sharply marked off from the disc. The 

 diverticula of the gut extend into the 

 arms, the ambulacral grooves are 

 open, and the sucker-type tube-feet 

 and pedicellariae are usually present. 

 The madreporite is aboral. 



CLASS II—OPHIUROIDEA 



^or 



:4 



Fig. 223. — Diagrams to show the 



relative extent of the oral 



and aboral surfaces, and to 



compare the form of body, in 



the several classes of Echino- 



dermata. The diagrams are 



in the same morphological 



position. 



I, .\steroidea ; 2, Ophiuroidea ; 3, Echino- 

 idea ; 4, Holothuroidea ; 5, Crinoidea. 



ab., aboral surface ; or., oral surface. 



In these, the brittle stars, the arms 

 are clearly marked off from the disk, 

 so that the star-shape is lost. The 

 other characters are also the opposite of those of the starfish ; the 

 gut diverticula do not extend into the arms, the ambulacral groove 

 is covered, the tube-feet have no suckers, and there are no pedi- 

 cellari^. The madreporite is oval. Ophiothrix is one of the com- 

 moner British genera. 



CLASS III—ECHINOIDEA 



The sea urchins are more or less globular, without arms. The 

 adambulacral area is greatly extended to form the sides as well 

 as the base of the globe ; the abambulacral area is confined 

 to a small polar region round the madreporite. The ambulacral 

 grooves are covered, the tube-feet have suckers, pedicellariae are 



