34:4: ECHINODERMA. 



distinguished from the interambulacral by the ambulacral pores 

 by which the ambulacra on the surface are connected with the 

 internal ampullae. In most sea urchins the paired grouping of 

 the pores results from the fact that a double canal extends from 

 ampulla to ambulacrum. 



In the arrangement of the ambulacra two modifications, the band form 

 and the petaloid, occur. In the first the ambulacra are equally developed 

 from peristome to periproct (fig. 328). In. the second oral and aboral 

 regions may be distinguished (fig. 329). In the oral region alone are loco- 

 motor feet always present, but these are so irregularly arranged that no 

 striking figure results. In the aboral area the ambulacra are tentacular 

 in character and are regularly arranged, their pores bounding five petal- 

 like figures around the periproct, very distinct after removal of the spines. 

 In the Echinoids, the Cidarids excepted, the interambulacral plates around 

 the peristome show five pairs of notches for the gills, five pairs of thin- 

 walled branching extensions of the body cavity. 



Ambulacral and interambulacral fields both end at the periproct 

 with an unpaired plate, the five ambulacral plates (radialia of mor- 

 phology) being called ocular plates, since they often bear pigment 

 spots formerly regarded as eyes. They are perforated by the end 

 of the radial canal and nerve, the latter here uniting with the epi- 

 thelium of the skin. The five interambulacral plates are called 

 genital plates, since they usually contain the openings of the genital 

 ducts. One is often madreporite as well. 



The interior of the body is occupied by a spacious coelom, to 



nd d 



FIG. 331. Sea urchin opened around the equator. A, ambulacral area; J, interam- 

 bulacral area; L, lantern; d, intestine; ed, anal end of intestine; g, gonads; nd, 

 siphon; oe, oasophagus; p, p', ring canal and Polian vesicles; st, stone canal. 



the walls of which the thin-walled alimentary tract is fastened by 

 a mesentery. In the Clypeastroids this tract forms a simple spiral, 



