ECHINODERMS: 



135 



Branch IV. — ECHINODERMATA. 



The niiiiuals (.'iiibraceil in this group were included by Ciivior in his great division 

 •of Kadiata, along with the- cadenterates. More complete knowledge of the anatomy 

 and especially the embryology has shown that the two groups have nothing in com- 

 mon, excejit those features which are common to all Metazoa and the absence of a 

 segmentation of the body. The radiate arrangement, I'eally a feature of minor 

 importance, is here very strongly marked in most forms, for crinoids, star-tishes, and 

 serjient-stars have a central -disc which branches into five or more arms, which radiate 

 from it like the spokes of a wheel from the hub. In the sea-urchins and holothurians 

 this radial symmetry is less marked, but it may readily be traced, although the radiat- 

 ing arms are apparently lacking. 



Though at first sight widely different, the similar structure of a star-fish and a sea- 

 urchin can readily be traced. Let us first examine that of the first-named form. We 

 have a central disc with five radiating arms ; in the centre of the lower surface of the 

 disc we find a mouth, hence this is called the oral surface. On the upper or abora) 

 surface we find no opening (or only a very minute one), but a little one side of the 

 middle, between the bases of two of the arms, is a round plate, wliieli, from its pecu- 

 liarly ornamented appeai'ance, has received the name of madrej)oric body, in allusion 

 to its resemblance' to , 



some of the corals. On 

 the tinder surface of 

 each arm will be found 

 a series of plates, be- 

 tween which project 

 little tubul.ar suckers. 

 As these sucking tidies 

 are used in walking, 

 they have received the 

 name ambulacra, while 

 each of the series of 

 plates between which 

 they project is known 

 as an ambulacral area. 



Turning now to the 

 sea-urchin, we find a 

 nearly spherical body. 

 On the under side we 

 have a mouth, from 

 which radiate a series 

 of plates, between 

 which project ambu- 

 lacra very similar to 

 but longer than those of the star-fish. These rows of plates continue arotmd the 

 sides of the sphere, and terminate near the centre of the upper surface, where we 



Fig. 124. — Aboral surface of sea-urchin {Arbacia); a, anal plates; c, ocular i)lates, 

 g, ambulacral area; o, madreporio body; 1, 2, 3, 4, 5, the five rays. 



