ECEHNODERMATA. 



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echinoderms the food is extremely varied. The starfish are carniv- 

 orous and eat barnacles, oysters, mussels, and other animals which 

 are not good movers. The sea-urchins are vegetable feeders, in 

 part at least, and have five well-developed teeth by which the biting 

 occurs. The sea-cucumbers eat very much as the earthworm does, 

 by taking into itself sand and mud in which are organic objects. 

 These are digested out and the sand is voided at the end of the process. 

 These differences accompany great differences in the structure of 

 the whole digestive tract. 



FIG. 63. Vertical (sagittal) section through the disc and arm of a starfish (dia- 

 grammatic). a, anus; amp, ampulla; c.b., circular blood space; c.w., circular water 

 canal; co, ccelom; co.e., ccelomic epithelium; d.b., dermal branchiae; e, position of the 

 eyespot; ect, ectoderm; ent, entoderm; /, ambulacral feet; g, ambulacral groove; mo, 

 mouth; m.p., madreporic body; n.r., nerve ring; os, bones in the mesoderm; r.n., radial 

 nerve band; r.b'., radial blood vessel; r.p., reproductive pore; r.w., radial water canal; 

 s.c., stone canal; sp, spines; z, blood spaces in mesoderm. 



In the starfish the mouth is simple and opens directly into an 

 enlargement called the cardiac stomach. There may be pairs of 

 specially developed spines about the mouth, which may assist in 

 holding the food. This cardiac stomach is thin-walled and large, 

 and may be turned out over prey that is attached or is too large to be 

 swallowed, as an oyster. In such cases the digestion takes place 

 outside the body, and when the food has been absorbed the stomach 

 is withdrawn. It narrows into a smaller pouch the pyloric divi- 

 sion of the stomach into which pours the secretion of a pair of 

 large "liver" glands in each arm. A very short, narrow intestine, 

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