110 ANIMAL PHYSIOLOGY. [LESSON 32. 



&c.), is very simple ; like the Hydra, it consists of a single sac, 

 with but one aperture. 



In others (Comatula, Encrinus)^ the digestive canal is more 

 lengthened, and curves upon itself, and has an outlet distinct from 

 the mouth. 



In the JEchinida (Sea-urchins) and Holothuridce (Sea-cucum- 

 bers) there is a long, narrow, convoluted intestine, passing through 

 the body, with very slight gastric enlargement. 



In the higher forms of Star-fishes (Holothuridae) the folds of the 

 long intestine are connected by means of a highly vascular mesen- 

 tery, which, whether injected artificially or otherwise, presents an ob- 

 ject of surpassing beauty. 



529. The mouth of the Asterias (Fig. 186) is placed in the cen- 

 tre of the inferior surface of the body, surrounded by long tubular 



FIG Ig6 tentacula, and pro- 



tected by fasciculi of 

 calcareous spines. 



By means of a 

 very short oesophagus, 

 it leads directly to a 

 wide and very dilata- 

 ble stomach (a), pro- 

 vided with a distinct 

 internal mucous lin- 

 ing, and an external 

 muscular coat. 



It occupies the 

 whole central part of 

 the body, from which 

 Nntrimental organs, Asterlus rnbens. the marginal divisions 



originate. From the stomach two long, tapering, ramified cceca, 

 which commence by a single trunk, are given off opposite the com- 

 mencement of each ray, and are distributed through it in a central line, 

 so that collectively there are ten pairs of the coecal appendages (b, b). 



530. Each of these cceca is attached to the integument along the 

 upper part of each ray, by a delicate vascular membrane. In addi- 

 tion to these appendages, the stomach is also provided with small, 

 short cceca (blind sacs) at its upper part within the disc, and at its 

 sides, between the great coecal trunks of the rays. 



531. The Echinida (Fig. 187) present a more elevated form of 

 the nutrimental organs ; the mouth is furnished with five large and 



