558 THE INVERTEBRATA 



The ossicles (Fig. 415) of the body wall of a starfish may, as in 

 the familiar Asterias, constitute a toughening meshwork, or may have 

 the form of more closely set plates, but are not united to form a con- 

 tinuous shell. Along the sides of the arms run two rows of strong 

 pieces, the supero- and infero-marginal ossicles^ which are hidden in 

 Asterias but in many genera appear on the surface. The ossicles bear 

 spines, which vary much in size and shape and arrangement,^ being 

 often longer than the stumpy structures on the back of Asterias. 

 Around and between the spines are usually pedicellariae of various 



sp 'ped' 



Fig. 415. A diagram of a transverse section of the arm of a starfish. From 

 Borradaile. «&.m. muscle which straightens the arm; at/.05^. adambulacral 

 ossicle; ad.sp. adambulacral spine; amb.oss. ambulacral ossicle; amp. am- 

 pulla of tube foot; m.' muscle which opens the ambulacral groove; ped.' 

 one of the small pedicellariae with crossed jaw ossicles; ped." one of the 

 large pedicellariae whose jaw ossicles are not crossed; r.b.v. "radial blood 

 vessel"; t.f. tube foot. Other letters as in Fig. 416. 



kinds, the most perfect of which is tliQ forcipulate , found in Asterias, 

 which has a basal ossicle: its jaws may be straight or crossed. Over 

 interspaces between the ossicles arise delicate, hollow outgrowths, the 

 gills, into which the perivisceral cavity is prolonged. Above each 

 ambulacral groove runs a double row of large, transversely placed, 

 ambulacral ossicles, movable upon one another by muscles. Each has 

 a smaller adambulacral ossicle at its outer end. Adambulacral spines 

 stand on the adambulacral ossicles. In Asterias they are long, and 

 bear groups of large pedicellariae of the kind with uncrossed jaws. 

 They can be turned inwards to protect the ambulacral grooves. 



