ECHINUS. IfH 



these passing the one into the other by the most 

 gradual transitions? Setting out from the star with 

 five slender rays, which is the standard form of the 

 Asterias ; we find the rays, in succeeding species, 

 assuming gradually a greater breadth at their base, 

 and their sides joining at more obtuse angles: the 

 star-like form is gradually effaced, and the outline 

 is rather a pentagon, with its sides curved inwards 

 (Fig. 89). We soon perceive this curvature giving 

 place to a straight line, so that the shape becomes 

 an exact pentagon. The next change effected is 

 in the angles of this pentagon, which by degrees 

 are lost in a general rounded outline; still, how- 

 ever, preserving its flatness. This stage is attained 

 in the Scutella, and the Clypeaster. (Fig. 90.) We 

 next find that, in the Spatangus, the thickness in- 

 creases ; though at first with an oval outline, and 

 with several changes in the situation of the mouth 

 of the animal. At length, after passing through 

 many intermediate steps, we arrive at the perfectly 

 circular and spheroidal ^c//m?«5. (Fig. 91.) If we 

 might be permitted to conjecture the objects of all 

 these changes, which occur in this continuous gra- 

 dation, we might not unreasonably suppose them 

 to be the concentration of the internal organs into 

 one compact mass, and the retrenchment of all the 

 external appendages. It is also curious to observe, 

 how, amidst all these modifications, the double rows 

 of perforations, which constitute the ambulacra, re- 

 tain their situations, diverging in five equidistant 

 lines from one of the extremities of the axis, and 

 winding round to the other. 



Returning to the Asterias, we can trace changes 

 equally gradual, though in an opposite sense, in 

 another series, which presents a striking contrast 



