XIV] 



ECHIN01DEA 



345 



might be described as a star-fish in which the upper surface had 

 shrunk to insignificant proportions, being represented by a small 

 patch of leathery skin at the upper pole : or, if we regard the whole 

 radial tube with its tube-feet as one immense branched tentacle and 

 the arm as its support, we should say that the arm had been again 

 merged in the body so that the radial tube was bent back in a 

 curved course. As a matter of fact the end of the radial tube 



-- 5 



--6 



FIG. 160. Diagram of an aboral view of the dried shell of an Echinoid x 1. 

 The spines, pedicellariae and tube- feet have been removed. 



1. Anus. 2. Leathery skin round anus, periproct. 3. Madreporic plate. 

 4. Genital plate with genital pore. 5. Ocular plate with eye. 



6. Line of junction of ambulacral and interambulacral plates. 7. Ambu- 

 lacrum. 8. Pores through which tube-feet protrude. 9. Bosses 

 which bear the spines. 



projects very slightly beyond the general surface and bears at its tip 

 a mass of pigment which corresponds with the eye of the star-fish, 

 though no eye-structure has been detected in it. This is situated 

 near the upper end of the body, just outside the small area of 

 leathery skin mentioned above. 



The skeleton of the Sea-urchin is a cuirass of plates fitting edge 

 to edge, with two openings. Of these the upper (already referred 



