676 ASTERIDEA. 



perform the office of locomotion. The ovarium-holes are situ- 

 ated on the apex, and from these the eggs are extruded. The 

 forms of this order vary from that of the CAKE-URCHINS, ScuteUa, 

 (Lat. a salver,) slightly convex above, to the sub-globular 

 Echinus, (Plate XVIII. fig. 2.) This, with other genera having 

 large spines, is found on the bottom of the sea ; while the Cake- 

 Urchins, having short, bristly spines, burrow in sand. 



The HEART-URCHIN, Spalangus, (Gr. spatangos, a kind of sea- 

 urchin,) has a thin, delicate shell, of a lengthened, gibbous form, 

 with the vent posterior and placed upon the upper surface, (see 

 Chart.) The structure of the mouth in the Sea-urchin deserves 

 special notice. It is formed of ten series of hard plates, fur- 

 nished with teeth which are moved by very distinct muscles, and 

 put in motion by a complicated nervous system ; armed also 

 with five jaws so arranged as to correspond with the ten series of 

 plates. So powerful are these jaws that they are able to crush 

 shell-fishes and the hardest bodies. According to Agassiz, a 

 more complicated organization is scarcely to be found in the 

 Animal Kingdom. The mechanism of these five jaws, with 

 their singular array of arched teeth, Aristotle compared to a 

 lantern ; hence the Echinus has been called "Aristotle's Lan- 

 tern." 



Professor Forbes informs us that "in a moderate sized urchin, 

 there are sixty-two rows of pores in each of the ten avenues ; 

 and as there are three pairs of pores in each row, the total num- 

 ber of pores is 3720 ; but as each sucker occupies a pair of pores 

 in each row, the total number of 'suckers is I860." He says 

 also that "there are above three hundred plates of one kind, and 

 nearly as many of another, alJ dovetailing together with the 

 greatest nicety and regularity, bearing on their surface above 

 four thousand spines, each spine perfect in itself and of a com- 

 plicated structure, and having a free movement in its socket." 

 " Truly," he adds, " the skill of the Great Architect of Nature 

 is not less displayed in the construction of a sea-urchin, than in 

 the building of a world." 



THIRD ORDER. ASTERIDEA, (Gr. a<n%, aster, a star; eldos, 

 eidos, a form.) STAR-FISHES. 



This order o.f Radiates is distinguished by having the body- 

 more or less lobed, and the lobes chaneled beneath for cirrhi, 

 which act as suckers, and are organs of motion. In some 

 genera the arms, instead of having lateral cirrhi or filaments, 

 separate into branches. The Euryale or Gorgonocephalus, (Gr. 



