ALIMENTARY CANAL 



563 



Int.l. 



ised by its strong musculature. Next follows (3) the intestine, 

 a thin-walled tube comprising the middle limb and most of the 

 descending and ascending limbs. This finally passes into (4) the 

 wide terminal " rectum," or " cloaca," which is connected to the 

 body-wall by muscular bands which 

 traverse tlie coelom (Fig. 256, 10). 



The cells lining the oesophagus 

 resemble ectodermal cells ; those 

 lining the stomach are nearly all 

 gland -cells, and obviously secrete 

 the digestive juice. The powerful 

 muscles of this portion of the gut 

 produce a strong peristalsis which 

 thoroughly mixes the juice with 

 the food, and in the thin-walled 

 intestine absorption of the digested 

 material takes place. The extreme 

 thinness of the intestinal wall is 

 common to many animals (e.g. 

 Sipunculus, Vol. II. p. 412) which 

 swallow mud and sand for the 

 sake of the organic matter which 

 they contain. 



The rectum, or cloaca, is one of 

 the most characteristic features in 

 this and most other Sea-cucumbers. 

 In addition to the passing of faeces, 

 it is used to pump water in and 

 out, and it thus serves as a 

 breathing organ. This pumping 

 is effected by alternate contrac- 

 tions of the radiating muscles 

 attaching the cloaca to the body- 

 wall, and of the circular muscles 



which immediately surround it. Two long branched tubes termed 

 Respiratory trees (Fig. 256, 11) open into the cloaca, and into 

 these the inspired water penetrates. The finer branches of these 

 gills end in rounded thin-walled swellings termed " ampullae " ; 

 and when water is forced into these they become tense, and 

 a considerable quantity diffuses through their walls, carrying 



Int.3. 



Fig. 255. — A, diagrammatic cross-sec- 

 tion of a Holotliurian ; B, diagram- 

 matic longitudinal section. I-V, 

 radii ; Int. \-Int. 3, the three limbs 

 of the alimentary canal ; M'^-M^, 

 the three mesenteries attaching the 

 same to the body-wall. (After 

 Ludwig. ) 



