32iJ ORGANS OF DIGESTION. 



part of the body from which the marginal divisions ori- 

 ginate. In the ophiura and euryale the digestive sac, with its 

 ten small rudimentary cceca, are entirely confined to the central 

 disk, but in the asterias two long tapering ramified coeca, 

 like the biliary follicles of higher classes, commencing by a 

 single trunk, extend from the stomach to a very variable 

 length into each division of the body. Each of these rami- 

 fied coeca of the asterias is attached to the integument along 

 the upper part of the ray by a delicate vascular membrane, 

 and its lateral ramifications terminate in small vesicular en- 

 largements generally filled with digestive matter, or the 

 secretion of their own parietes. The stomach is also 

 furnished with small short coeca at its upper part within the 

 disk, and at its sides between the great coecal trunks of the 

 rays, which likewise vary much in their forms and dimen- 

 sions in different species. Above the stomach and towards 

 the side is situated the small glandular sac covered externally 

 with a solid calcareous plate and containing numerous 

 minute crystalline calcareous spicula. In the comatula there 

 is a distinct gastric cavity, and an alimentary canal long and 

 cylindrical, forming two convolutions round the stomach in 

 the central disk or abdomen, and open at its anal extremity. 

 The mouth forms a large circular aperture towards one side 

 of the centre of the inferior surface, and a small sub- 

 marginal anus is seen at the opposite side, not far from the 

 mouth, and opening at the end of a prominent papilla. The 

 same structure of the alimentary cavity with its two distinct 

 and approximated openings is seen in the pentacrinus, and it 

 appears to have been the common form of the digestive 

 canal in that great and almost extinct family of crinoid ani- 

 mals. 



The mouth becomes furnished with strong masticating and 

 salivary organs in the higher forms of echinida; and while it 

 preserves its central position on the lower or anterior surface 

 of the body, the anal orifice leaves that surface to assume a 

 diagonally opposite position in the centre of the upper or pos- 

 terior part, which prepares the structure for the lengthened 

 horizontal forms of the holothurida and the articulated classes 

 of animals where the axis of the trunk ceases to preserve the 

 vertical position so general in the radiata. In the sou- 

 tellum and the clypeaster both orifices still preserve the 



