oOC 



ECHIXODERMA 



Order III. Clypeastroidea. 



Flattened echinoids with central mouth and teeth; anus in tlic posterior inter- 

 radius, sometimes mar^'inal; fn e ]K'laloid anibulaerai areas. c7v,''ii;,v/('r (,lig. 

 30J). iLcliiiuraL-hiiiiii-" ^sand dollar, hi;, ^^oo"). J/(//;7ci.''- with holes through the 

 test. 



Order IV. Spatangoidea. 



Bilateral tlattened forms more or less heart-shaped; mouth and anus cx- 

 centrie, no leedi; usuallv fi\e ]>elaloid anilnilaeral areas and four genital plates. 

 From the forward posiiion of ihe mouth it follows ili.ii onl\- iwo amhulacral 

 areas (lii\ium, ]i. joO ai-e upon the lower siui'aee. Warmer seas. Sf'otaiii^iis* 

 (tig. 307], EchuiocardiiDn, Biissns. 



Fig. J06. 



Fio. jo;. 



Fig. ^qo6. — Oral (.Fl and aboral [B) surfaces of the sand dollar. Fxhintirachnhis 

 faniui. d, anus; l;, genital pv^res; /, anibulaerai areas; in, ni.idreporilc; e, moiuh. 



Fig. J07. — Young Sf'aUiiixiis [>ur[mr,-i(S lalUT .Xgassi.."!, the spines removed, 

 oral surface. In front, the slit-like mouth; behind, the anus. The bi\iuni without 

 tubercles. 



Class V. Holothuroidea. 



The sea cucumlicrs are most removed of any group from the typieal 

 echinoderm appearanee. At the lirst ghince, except in Psolits, the skin 

 appears naked and the characteristic pUites absent. Yet these are im- 

 beddeil in the skin in the shape of pkites, wheels, and anchors. 'Fhe 

 integument is leathery and muscular, with longitudinal and circular hbres. 

 The saccular body gives these forms a worm-like appearance, sti-englh- 

 ened by its elongation nt the main axis, and with the mouth and anus at 

 the poles. Unlike other echinoderms these move with the main axis 

 parallel to the ground, a condition which, to a greater or less extent, leads 

 to a replacement of r:idial by liilaleral symmetry. F)nc surface (tri\ium) 

 becomes ventral, the bivium dorsal, atul in niany the trivial ambulacra 

 alone are locomotor, those of the bixium being tactile or wholly absent. 



The alimentary c;mal (hg. 30S) (except in Sviiapia) is coiled in a 

 uniform manner, although m;iny minor con\olutions may obscure this. 



