304 



ECHINODERMA 



are situated the spines, either long and pointed or swollen to spherical 

 plates. These spines are moved by muscles so that they serve both as 

 protecting and locomotor structures. The ambulacral plates are distin- 

 guished from the interambulacral by the ambulacral pores by which the 

 ambulacra on the surface are connected with the internal radial canals and 

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

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



FIG. 



FIG. 303. 

 Aboral view, showing the 



FIG. 302. Clypeaster subdepressus (after Agassiz). 

 petaloid ends of the ambulacral areas. 



FIG. 303. Diagrammatic longitudinal section through a sea urchin. 



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

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

 developed from peristome to periproct (fig. 301). In the second oral and aboral 

 regions may be distinguished (fig. 302). In the oral region alone are loco- 

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

 area the ambulacra are branchial or tentacular and are regularly arranged, their 

 pores bounding five petal-like figures around the periproct, very distinct after 

 removal of the spines (fig. 306). In the Regularia, the Cidaridae 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 (terminalia of morphology) 

 being called ocular plates, since they often bear pigment spots formerly 

 regarded as eyes. Each is perforated by the end of the radial canal and 

 nerve. The five interambulacral plates (basalia*) are called genital 

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

 One is often madreporite as well. 



Inside of the body is a spacious ccelom, to the walls of which the thin- 

 walled alimentary tract is fastened by a mesentery. In the Clypeastroids 

 this tract forms a simple spiral, but elsewhere it ascends from the mouth, 

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



