548 INVERTEBRATE MORPHOLOGY. 



quent stages additional stone-canals develop from the oral 

 ring, and at the same time additional pores develop in the 

 walls of the body, forming what are called the calyx-pores 

 (u-p). These may reach a considerable number, it being esti- 

 mated that in Antedon there are no less than fifteen hundred 

 of them scattered over the disk ; in lihizocrinus, Ilyocrinus, 

 and Holopus, however, there are only five pores, one piercing 

 each of the oral plates present in these forms. 



The schizocoelic system consists of five radial sinuses (Fig. 

 251, rh) lying between the radial hydrocoel vessels and the 

 more superficial radial nerve, together with, according to some 

 authors, a circular sinus surrounding the mouth into which 

 the radial sinuses open. A plexus of lacunae occurs in the 

 walls of the intestine, and another surrounds the oesopha- 

 gus, this latter in part aggregating itself into a structure re- 

 sembling a lymphatic gland and known as the spongy body ; 

 the dorsal organ likewise contains a dense network of tubes 

 lined with epithelium. Along the sides of the hydrocoel- 

 canals, in the disk, arms, and pinnules, alternating in the two 

 last with the triads of tentacles, in the walls of the intestine, 

 and occasionally elsewhere, there are imbedded in the con- 

 nective tissue yellowish spherical bodies known as the sacculL 

 The interior of each sacculus is lined with cells, and contains 

 a number of pyriform masses formed of small highly-refrac- 

 tive spherules, apparently of an albuminoid substance. The 

 function of these bodies is very obscure ; they have been re- 

 garded as organs for secreting carbonate of lime, as excretory 

 organs, as parasites, as mucous glands, and lately as organs 

 of reserve in which proteid matter may be stored up for 

 future use. At present, however, the question is an open 

 one, and a function cannot with certainty be assigned to them. 



The mouth (Fig. 251, m) is usually situated at the centre 

 of the oral disk, and opens into a simple tubular intestine 

 which coils once round the coelomic cavity in the direction of 

 the hands of a watch and then, bending upon itself, turns 

 orally to open in the interradius CD upon the disk. In Ac- 

 finometra, a genus closely related to Antedon, the intestine 

 lies in four coils, but there is as a rule little variation from the 



