634 



THE INVERTEBRATA 



Class ASTEROIDEA 



Star-shaped or pentagonal Echinodermata ; whose arms contain caeca 

 of the alimentary canal, and are usually not sharply marked off from 

 the disc ; which have an aboral madreporite ; open ambulacral grooves ; 

 and usually both suckers on the tube feet, and pedicellariae. 



The ossicles (Fig. 440) 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 infer o-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' 



,gill 



>oss 

 -amb.oss. 



Fig. 440. A diagram of a transverse section of the arm of a starfish. From 

 Borradaile. ab.m. muscle which straightens the arm; ad.oss. 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. 441. 



kinds, the most perfect of which is theforcipulate, found in Asterias j 

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

 interspaces between the ossicles arise delicate, hollow outgrowths, 

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



