ORGANS OF DIGESTION. 327 



mantle; these four radiating alimentary canals are conti- 

 nued down through the four long marginal tentacula which 

 extend 'from the edge of the disk, and the parietes of the 

 stomach appear to be already provided with ramified biliary 

 follicles which pour their secretion into its cavity. From the 

 remains of minute rotifera, Crustacea, and mollusca, found 

 in the alimentary cavities of the rhizostome forms of acale- 

 pha they appear to subsist on animal matter more highly 

 organised than' themselves, and already divided into very mi- 

 nute ^parts, so that they require neither masticating nor 

 glandular organs to assist in digestion ; but in the monosto- 

 matous species adapted for larger food, the cartilaginous 

 parietes of the mouth may compress or divide the prey, 

 and the biliary follicles aid in its assimilation. 



V. Echinoderma. The structure of the digestive organs 

 in these fixed or slow-moving, thick-skinned, predaceous 

 animals is as various as their outward form and their liv- 

 ing habits, and presents the links of transition from the 

 broad and radiated alimentary cavity of the acalepha to 

 the long cylindrical narrow intestinal canal common to the 

 articulated classes. In many of the stellated echinoderma, 

 as the euryale, the ophiura, and the asterias, we observe a 

 simple sac with one orifice, as in the hydra and the simplest 

 polypi of zoophytes ; in others, as the comatula and encrinus, 

 the digestive canal is more lengthened and curves upon 

 itself, as in alcyonella smdflustra, and has an anal opening 

 distinct from the mouth ; and in the echinida and holothur- 

 ida there is a long narrow convoluted intestine passing 

 through the body, with as little gastric enlargement as in the 

 long straight intestine of a worm. But in these various forms 

 of echinoderma the digestive cavity is always bounded by 

 parietes distinct from the common integument of the body, 

 as in all the higher classes of animals, and is generally 

 connected with them by means of a highly vascular mesen- 

 tery. The mouth of the asterias, surrounded with long tu- 

 bular tentacula and protected by fasciculi of calcareous 

 spines, is situate, as in most cyclo-neurose animals, in the 

 centre of the inferior surface of the body, and by a short 

 oesophagus leads to a wide and most dilatable stomach 

 provided with a distinct internal mucous lining and an 

 exterior muscular tunic, and occupying the whole central 



