16 



gradually thins out as the oesophagus passes into the 

 intestine, in the walls of which it is difficult to trace. 



The epithelium and subjacent nerve layer rest upon 

 a delicate basement membrane of connective tissue, which 

 is, however, enormously thickened in the rectal portion of 

 the intestine (PI. II., fig. 27, en. ts.). Thin but definite 

 layers of muscular fibre form sphincters around the 

 oesophagus and rectum (figs. 25 and 27, sj^h. fh.), but 

 the musculature is elsewhere feebly developed and less 

 definite. 



The anal funnel has been observed to contract 

 rhythmically, doubtless in connection with the ejection of 

 water and excreta. The food of Antedon consists of 

 diatoms, radiolarians, foraminifera, and other small 

 organisms, which are captured by the tentacle-fringed 

 arms and pinnules and swept down the ambulacral grooves 

 to the mouth by the action of the cilia which line them. 



The Blood-vascitlar or Lacunar System. 



This system is highly characteristic of the majority 

 of the Echinodernia, and consists, in Antedon, of 

 a number of lacunar spaces, bounded by walls of 

 connective tissue scarcely distinguishable from that 

 which forms the numerous strands and trabecula? 

 of the body cavity (PI. Y., fig. 52). These lacunije 

 traverse the body cavity in all directions, and are 

 generally recognisable in sections by their albumi- 

 noid contents, Avhich are coagulated by the fixing 

 reagents and may even stain faintly. As will be presently 

 seen, some of the lacume simulate true vessels by the 

 assumption of a tubular form and considerable thickening 

 of their walls. The largest and most easily recognisable 

 lacuna is one to -svhich the name 'circumoral blood-vascular 



