108 



MARINE ANIMALS OF MASSACHUSETTS BAY. 



FIR. 140. 



bristles entirely removed, the lines of suture of the plates com- 

 posing it, and corresponding exactly to those of the spherical 

 Sea-urchin, may very readily be seen, (a and z, Fig. 139.) 



This flat Sea-urchin or Echinarachnius, as it is called, belongs 

 to a group of Sea-urchins known as Clypeastroids (shield-like 

 Sea-urchins). In a section (Fig. 140) exposing the internal 

 structure, one cannot but be reminded by its general aspect of 



an Aurelia. Could one solidify 

 an Aurelia it would present much 

 the same appearance ; another evi- 

 dence that all the Radiates are 

 biiilt on one plan, their differences 

 being only so many modes of ex- 

 pressing the same structural idea. 

 The teeth or jaws in this flat Sea- 

 urchin are not so complicated as in 

 the Toxopneustes, being simply flat 

 pieces, arranged around the mouth 

 (0, Fig. 140), without the apparatus of muscular bands by means 

 of which the teeth are moved in the other genus. It is a curious 

 fact, considered in relation to the general radiate structure of 

 these animals, that the teeth, instead of moving up and down like 

 the jaws in Vertebrates,, or from right to left like those of Articu- 

 lates, move concentrically, all converging towards the centre. 



STAR-FISHES. 



Star-fish. (Astracanthion berylinus AG.) 



Although there is the closest homology of parts between the 

 Star-fish and the Sea-urchin, the arrangement of these parts, and 

 the external appearance of the animals, as a whole, are entirely 

 different. The Star-fish has zones corresponding exactly to those 



Fig. 140. Transverse section of Echinarachnius ; o mouth, e e ambulacra, c m ambulacral ramifica- 

 tions, ww interamhulacra. (JJgassiz.) 



