PHYLUM ECH1NODERMATA 



into each arm, then divides into two branches, each of which 

 possesses a large number of lateral pouches; these branches are 

 called pyloric or hepatic cceca (Fig. 135, py). They are green 

 in color. Above the pyloric sac is the slender rectum (rect.), 

 which may open to the outside through the anus. Two branched 

 pouches, brown in color, arise from the rectum and are known 

 as rectal cceca (rect.cac.}. 



The food of the starfish consists of fish, oysters, mussels, 

 barnacles, clams, snails, worms, Crustacea, etc. When a mussel 



rect. 



mp 



br. 



rect caec. 



..-py 



OSS 



'. ax! \..' 



perih. \ peristome 



Tierv.circ 



w. v r. 



perih. 



miisc.tr. 00- 



FIG. 135. Diagrammatic longitudinal section of a starfish, ab., aboral 

 sinus; ax, axial sinus; ax.', inner perihasmal ring-canal; br., branchia or gill; 

 g.r, genital rachis; mp., madreporite; musc.tr., muscle uniting ambulacral 

 ossicles; nerv.circ, nerve-ring; n.r., radial nerve-cord; or., eye-pit; oss., ossicles 

 in skin; p.br., peribranchial sinus; p.c., pore canal; perih., (right) perihsemal 

 radial canal, (left) outer perihaemal ring-canal; py, pyloric caecum; red., rectum; 

 rect.ceec., rectal caeca; sp., spines; st.c., stone-canal; I, tentacle terminating 

 radial canal ; w.v.r., water-vascular radial canal. (From the Cambridge 

 Natural History.) 



is to be eaten, the animal seizes it with the tube-feet " and places 

 it directly under its mouth, folding its arms down over it in um- 

 brella fashion (Fig. 136). The muscles which run around the 

 arms and disc in the body-wall contract, and the pressure thus 

 brought to bear on the incompressible fluid contained in the 

 ccelom, forces out the thin membranous peristome and partially 

 turns the stomach inside out. The everted edge of the stomach 

 is wrapped round the prey. Soon the bivalve is forced to relax 



