370 ZOOLOGY SECT. 



There are 110 definite calcareous plates ; but the integument is 

 tolerably hard, owing to the presence in its substance of innumer- 

 able microscopic calcareous spiculcs, very variable in shape in 

 different species of Cucumaria, and in Colochirus having the form 

 of sieve-like or lattice-like plates, some of which are to be found 

 even in the Avails of the tube-feet. The tube-feet are, like those 

 of the Starfish, used in locomotion, progression being effected by 

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

 Sea-cucumber living undisturbed under natural conditions there 

 will be found protruded through the mouth a circlet of ten 

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

 specially modified tube-feet. 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 observed to correspond to each of 

 the ambulacra! areas. The pair situated opposite the middle 

 ambulacra! 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 Avhen 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- 

 ,//< itfur)/ layer, of two layers of muscle in addition to a thin layer 

 of cells, the peritone/int or ccchm/ic epithelium, lining the ccelome. 

 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 ; 

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

 of five flattened bands which run longitudinally from the oral to 

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

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

 runs a /-"i/i'ij inl//lncrl vessel (Fig 300, n.nl. ml>.) together with a 



Ambulacral System. Just behind the bases of the tentacles, 

 and surrounding the beginning of the oesophagus is a circular 

 ambulacral ressel {ring, res.} which giA*es off the five rat/la! vessels; 

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

 and then run backwards, passing along the ambulacral areas and 

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

 with its in i>nUa. From the ring-vessel is also given off a large 

 pear-shaped Polimi vesicle (/>/>!. res.), 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 the Sea-urchin, 

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



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

 nerve*. A vascular ring ( ri. IL res.) lies close to the nerve-ring 



