ORGANS OF DIGESTION. 



inferior surface, the anal aperture has acquired a sub-dorsal 

 aspect in the spatangus and many of the genera now extinct, 

 and in the cidaris and echinus the mouth and anus occupy 

 the opposite poles of a vertical axis. In the spatangus, 

 which burrows in the moist sands and passes that substance 

 constantly through its body in order to derive nourishment 

 from the innumerable minute animals contained in it, the 

 mouth, destitute of teeth and furnished with numerous long 

 tubular tentacula, is placed on the lower flat surface towards 

 the obliterated ambulacrum, and leads to a long convoluted 

 black-coloured delicate alimentary canal which performs two 

 revolutions in opposite directions, attached by a thin vas- 

 cular mesentery to the upper part of the shell, and terminates 

 at the marginal aperture on the posterior part of the body. 

 The slight gastric enlargement at the commencement of this 

 long intestine receives the opening of a single lengthened 

 hepatic follicle or coecum. The mouth of the echinus., which 

 subsists chiefly on young mollusca and Crustacea, is provided 

 with a strong dental apparatus (Fig. 8. 2. 3.) embracing 

 the commencement of the oesophagus, and is surrounded 

 with delicate fimbriated contractile lips and numerous long 

 tubular tentacula. The intestine, with a slight gastric dilata- 

 tion and of variable diameter in its course, forms a double 

 convolution in a waved direction round the axis, and is 

 attached by a short vascular mesentery, containing minute 

 tubercles like glands, to the interior of the shell. The anal 

 aperture, at the upper pole of the vertical axis, is surrounded 

 by a membranous expansion, sometimes with valvular 

 folds, and is provided, like the mouth, with circular and ra- 

 diating muscular bands for its contraction and dilatation. The 

 structure is nearly the same in the cidares which present the 

 most globular forms of the echinida, but the forms of the 

 slight saccular enlargements in the course of the alimentary 

 canal, and the zig-zag manner in which the intestine ascends 

 and descends in performing its revolutions round the axis, 

 vary much in the different species of cidaris and echinus. In 

 its general conformation the holothuria is like a lengthened 

 echinus deprived of its calcareous plates, and with the axis 

 of the trunk extended in a horizontal direction. The mouth 

 and anus are placed at opposite ends of the body, with a 

 long convoluted alimentary canal, almost destitute of gastric 



