GRADATION IN THE ECHINODERMATA. 211 



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, however, preserving its flatness. This 

 stage is attained in the Scutella, and the Cly- 

 peaster. (Fig. 90.) We next find that, in the 

 Spatangus, the thickness increases ; though at 

 first with an oval outline, and with several 

 changes in the situation of the mouth of the ani- 

 mal. At length, after passing through many in- 

 termediate steps, we arrive at the perfectly cir- 

 cular and spheroidal Echinus. (Fig. 91.) If we 

 might be permitted to conjecture the objects of 

 all these changes, which occur in this continuous 

 gradation, we might not unreasonably suppose 

 them to be the concentration of the internal 

 organs into one compact mass, and the retrench- 

 ment of all the external appendages. It is also 

 curious to observe, how, amidst all these modifi- 

 cations, the double rows of perforations, which 

 constitute the ambulacra, retain their situations, 

 diverging in five equidistant lines from one of 

 the extremities of the axis, and winding round to 

 the other. 



