304 



ECIIIXODKRMA 



are situated the spines, either long ami pointed or swolleit to splierieal 

 phites. These spines are nio\ed lo' inuseles so that the\- ser\"e Koth as 

 proteeting and loeomotor stritctnres. Tire ambulacra! plates are distin- 

 gtiislied from the iirteramliuhieral li)' the ainlHihteral pores by which the 

 ambulacra on the surface are coniu>cted with the internal radial canals atid 

 ampulke. lu most sea urchins the paired grouping of the pores results 

 from the fact that a double canal extends from ampulla to ambulacrum. 



Fie. ,;oj. l''ic. 303. 



Fie,. ;o;. — i'lvfJiMstcr suh,!i-i'yi-ss-i(S (after .\,uas?i.:V .\lioral view, sIiowIhl; the 

 [ict,iloiil en Is of llio ainliul;UT,il areas. 



Fig. ^o^ — Dia^rammalie loiiL;Uiulinal section through a sea urchin. 



Tn the arrangement of llie ambulacra two niodihe itions, the band form 

 and the petaloid, occur. In the first (Regularia) the ambulacra are eqiially 

 de\"eloped from peristome to ])eriproel i^lii;. 301). In the second oral ,ind aboral 

 regions may be distinguished (fiL;, 30::). In the oral region alone ai'c loeo- 

 motor feet always present, but these are irregularly arranged. In die aboral 

 area the ambulacra are branchial or tenlaeular and are regularly aiTanged, their 

 pores bounding li\e petal-like hgures arouiul the periproei, \erv dislinci aller 

 renioxal of the spines (fii;. ^oob In the Regularia, the tddarida- excepted, the 

 interambulacral plates around the peristome show Twe pairs of notches for the 

 gills, live pairs of thin-walled branching extensions of the body ca\ity. 



Ambulacral and interambulacral fields both end at the periproct with 

 an unpairetl plate, the live ambulacral plates (U-niiiiuilid of morphology) 

 being called oailar plalcs, since they often hear pigment spots formerly 

 regarded as eyes. Ivich is jierforaled by the end of the radial canal and 

 nerve. The live interarnhulacral plates (Ihisiilia) are called i^fiiilal 

 philes, since they usually conlaiir the openings of the genital duels. 

 One is often nradreporite as well. 



Inside of Ihe body is a spacious cadom, to the\yalls of which the thin- 

 ^vaIled alimentary tract is fastened by a mesentery. In the (dypeastroids 

 this tract forms a simple siiiral, but elsewhere it asceiuls from the motilli, 

 turning once, and then, lieiuling on itself, coils in the reverse direction to 



