3276 



ECHINODERMS 



JAWS OF STONE X7RCHIN. 

 (Natural size.) 



at the end. These latter zones are, therefore, the ambulacral zones; one of 

 them being seen in the middle of the illustration. The other zones are called 

 interambulacral, and one of them is shown on the left of the same illustration. All 

 the zones converge toward the summit of the test, where the vent is situated 

 in a circular space covered with membrane. This membrane contains a few 

 irregular granules, and is surrounded by five large interradially placed plates, 

 pierced by the ducts of the generative glands. One is also pierced by a large 

 number of small water pores, and is called the madreporite. Outside these five 

 plates, and alternating with them, are five other plates, each situated at the top of 



an ambulacral zone, and pierced by the 

 unpaired tentacles, which terminate the 

 water canals, and represent the unpaired 

 tentacles near the eye at the ends of the 

 arms of a starfish. At the other pole of 

 the body is another membrane, surround- 

 ing the mouth opening, through which 

 may be seen five pointed teeth. These 

 belong to a very elaborate masticating ap- 

 paratus, shown in the illustration, and 



found in all the regular urchins, as also in the Clypeastrida among the irreg- 

 ular urchins. This consists of twenty principal pieces arranged into a five-sided 

 conical mass, compared by Aristotle to a lantern (a). In the centre of the whole 

 are five teeth (b, c~), working in bony sockets, or pyramids, connected by muscles 

 with one another, with the interior of the test, and with the arched processes, 

 known as auricles (</), that surround the mouth opening. There are yet other 

 calcareous pieces connecting the pyramids together, and serving as attachments for 

 yet other muscles. Such a sea urchin as that described, preserves as much as any 

 echinoderm the five-rayed symmetry of 

 the group; but in many forms the five- 

 rayed type is not so obvious, for the 

 animal has become elongated along one 

 of the axes, so as to have a superficial 

 two-sided symmetry. This is naturally 

 connected with constant movement in 

 one direction, as though the animal 

 had a head and tail; and such modifi- 

 cation is found among those urchins 

 that live on muddy bottoms, and espe- 

 cially in those from considerable depths. 

 Not only is the test elongated, but 

 the mouth moves forward to the front 

 margin, and the vent downward to the 

 hinder margin, so as eventually to lie 



on the under instead of on the upper SHIELD URCHIN, FROM ABOVE. 



surface of the test. An earlier stage in (Natural size. ) 



