VIII 



ECHINODERMATA 



28s 



straight from mouth to anus, but it shows an unusual degree of differentia- 

 tion. The mouth leads into a large baggy portion (Fig. 117, s), usually 

 called the stomach, which is capable of being pushed out at the 

 mouth and wrapped round the mollusc which serves the starfish as 

 food. 



The stomach leads into a flattened pentagonal pyloric sac (Fig. iij, p.s) 

 each angle of which is continued as a tubular prolongation into the arm 

 and bifurcates to form two large glandular caeca (Figs. 117 and 118, p.c), 

 extending nearly to the tip of the arm. These secrete digestive ferment 

 which passes into the pyloric sac. From the pyloric sac there passes to 

 the anus the very short intestine (Fig. 117, tni) and the wall of this 

 bulges outwards to form two irregular rectal caeca (r) — apparently also 



P? 



a. 



mi. 



car 



Fig. 117. 



Vertical section through a stariish. a, Anus ; co.r, circnm-oral ring of hydrocoele ; e, eye ; 

 int, intestine ; M, mouth ; m, madreporite ; n.r, circum-oral nerve ring ; p.c, pyloric caecum ; 

 p.s, pyloric sac ; r, rectal caecum ; r.c, radial canal of hydrocoele ; r.n, radial nerve ; s, stomach ; 

 S.C, stone canal j t.f, tube-feet ; t.t.f, terminal tube-foot bearing eye at its base. 



glandular in function. In Asterias, as already indicated, the anal 

 opening (a) is very small : in some other starfishes it has disappeared alto- 

 gether, and in the group of Ophiuroids this has become the normal con- 

 dition — the alimentary canal being reduced to the stomach into which 

 the mouth opening leads. In the other groups of Echinoderms the 

 alimentary canal, while showing less marked division into different parts 

 than in the Asteroids, has become elongated and tubular in form. In 

 the typical Echinoids a new complication has made its appearance in 

 the development of five powerful teeth, meeting in the centre of the 

 mouth and provided with a very complicated arrangement of skeletal 

 structures described long ago by Aristotle and called by him the " lantern " 

 of the Sea-urchin. It is still commonly spoken of as " Aristotle's 

 lantern." 



