394 ZOOLOGY 



SECT. 



creeping with the ventral surface applied to the ground. In a 

 Sea-cucumber living undisturbed under natural conditions there 

 will be found protruding through the mouth a circlet of ten 

 tentacles, which are to be looked upon as greatly developed and 

 specially modified podia. These are tree-like in shape a central 

 stem giving off a number of short branches, which may in 

 turn be branched and they are highly sensitive and contractile. 

 Two of these tentacles will be seen to correspond to each of 

 the ambulacral areas. The pair situated opposite the middle 

 ambulacral area of the ventral surface are very much smaller than 

 the others, and will be observed to perform the special function of 

 pushing the food-particles into the mouth. All the tentacles are 

 drawn completely back within the mouth when the animal is 

 disturbed. 



Structure of Body-wall. When the wall of the body is 

 divided, it is found to consist, in addition to the hardened integu- 

 mentary layer, of two layers of muscle in addition to a thin layer 

 of cells, the peritoneum or ccelomic epithelium, lining the coalome. 

 The outer layer of muscle is a complete, continuous layer of 

 muscular fibres which have a circular arrangement, i.e. are 

 arranged in a ring-like manner around the long axis of the body ; 

 while the inner layer is not continuous, consisting, in fact, merely 

 of five flattened bands which run longitudinally from the oral to 

 the anal extremities, each underlying one of the ambulacral areas. 

 In close contact with each of these bands, on its inner surface, 

 runs a radial ambulacral vessel (Fig. 334, rad. amb) together with a 

 radial nerve. 



Ambulacral System. Just behind the bases of the tentacles, 

 and surrounding the beginning of the oesophagus, is a circular 

 ambulacral vessel (ring, ves) which gives off the five radial vessels ; 

 these first run forwards and give off branches to the tentacles, 

 and then backwards, passing along the ambulacral areas and 

 giving off branches to the tube-feet, each of which is provided 

 with its ampulla. From the ring-vessel also arises a large pear- 

 shaped Polian vesicle (pol. ves}, and a short sinuous canal, the 

 madreporic canal (mad. can), which ends in a perforated extremity 

 not situated, like the madreporite of the Starfish or Sea-urchin, 

 on the outer surface of the body, but in the interior of the crelome. 



A nerve-ring surrounds the mouth and gives off the five radial 

 nerves. 



Both perihsemal and haemal .systems are well developed. 

 The latter comprises a ring-like strand (ri. bl. ves) situated close 

 to the nerve-ring and sending off five radial strands, as well as 

 dorsal and ventral strands (int. ves) accompanying the enteric 

 canal, and a plexus surrounding the left respiratory tree (p. 396). 



The coalome contains a fluid in which float numerous r.mcebo- 

 cytes, similar to those of the Starfish, and also a number of 



