4 COMPARATIVE ANATOMY. 



to supporting the innermost row of slender spines described in 

 the preceding preparation, also marks the junction of the 

 ambulacral and ant-ambulacral areas. The ad-ambulacral ossi- 

 cle is best seen in the transverse section. Externally to the 

 ad-ambulacral ossicles is a series of stout, rib-like rods, fewer in 

 number than the transverse processes, as there is but one of the 

 former to three of the latter. These form the lateral boundaries 

 of the ad-ambulacral region. These 'ribs' are connected with 

 each other by at least two well-marked rows of cross-bars on 

 each side; extending from one 'rib' to the other. The 'rib' is 

 not a single ossicle, but composed of several pieces joined to- 

 gether by oblique surfaces: so as to admit of some amount of 

 sliding motion upon one another, in order to allow the consequent 

 dilatation or contraction of the cavity, the walls of which they 

 contribute to form. Towards the dorsal region, or roof of the 

 ray, the 'rib' breaks up, and the pieces cross in all directions, 

 forming an irregular network of ossicles imbedded in the thick 

 integument. The first axial ossicle in each ray is much larger than 

 the rest : and by their abutment upon each other a strong cal- 

 cified ring is formed, encircling the mouth. The skeleton of 

 the disk in no way differs from the structure of the rays : as 

 it is in fact formed by the confluence of the ant-ambulacral 

 integument of the five rays: and the axial ossicles themselves 

 with their transverse processes are continued to the margin of 

 the mouth, and only then abut upon each other. In the fresh 

 state the radial ambulacral canal would lie in the ambulacral 

 groove immediately beneath the axial series of ossicles, and 

 still more superficially the radial nerve-cord: while in the 

 cavity of the ray the digestive and generative systems are 

 contained. In the peristomial depression, and external to the 

 calcified oral ring, the pentagonal nerve-collar would lie. It 

 will thus be seen that from its relation to the nervous system 

 and the radial ambulacral canals, the skeleton is really an in- 

 ternal one, and does not correspond to the ambulacral and 

 inter-ambulacral plates of the Echinus, which form a true exo- 

 skeleton. 



6. A Common Cross-Fish (Asterias rubens], dissected to 

 shew its digestive system. 



The whole of the integument has been removed from the 



