ECHINODERMATA. 141 



in these, from the increased size of the body, the rays are united at 

 their origin, and become so much dilated as to contain prolongations 

 of the viscera lodged in their interior ; an arrangement not met with 

 in Ophiuri and other slender-rayed Asteridse. The dilatation of 

 the central part proceeds, and in the same proportion the rays be- 

 come obliterated ; so that at length, the asteroid shape becoming 

 totally lost by the progressive filling up of the interspaces between 

 the rays, we arrive ultimately at completely pentagonal forms, the 

 sides of the pentagon being perfectly straight lines. 



(182.) It is extremely interesting to remark the changes which 

 occur in the nature of the locomotive organs during these diversi- 

 fications of external figure. We have seen that, in the lower 

 Echinodermata possessing long and flexible rays, such organs were 

 fully adequate to perform all movements needful for progression ; 

 but as the mobility of these parts is diminished by their gradual 

 curtailment, and the filling up of the spaces between them, some 

 compensating contrivance becomes indispensably necessary, and 

 accordingly we find an apparatus gradually developed, well cal- 

 culated to meet the exigencies of the case. In Ophiurus 

 we have already mentioned the existence of protrusible suckers 

 around the opening of the mouth, well adapted, from their posi- 

 tion, to take firm hold of food seized by the animal ; and it is by 

 increasing the number of such organs that ample compensation 

 is made for the loss of motion in the rays themselves. On ex- 

 amining the lower surface of an Aster ias, even in those forms 

 which most approximate a right-lined pentagon in their marginal 

 contour, the number of rays will still be found to be distinctly in- 

 dicated by as many furrows radiating from the mouth, and indicat- 

 ing the centre of each division of the body. These ambulacral 

 furrows, as they are termed, exhibit, when examined in a dried 

 specimen, innumerable orifices arranged in parallel rows, through 

 each of which, when alive, the animal could protrude a prehensile 

 sucker, capable of being securely attached to any smooth surface. 



No verbal description can at all do justice to this wonderful 

 mechanism, even leaving out of the question the means by which 

 each individual sucker is wielded, for of this we shall speak here- 

 after ; but let any of our readers, when opportunity offers, pick 

 up from the beach one of these animals, the common star-fish of 

 our coast, which, as it lies upon the sand left by the retiring 

 waves, appears so incapable of movement, so utterly helpless and 

 inanimate ; let him place it in a large glass jar filled with its 



