ECHINUS. 155 



general rounded outline; still, however, preserving its flat- 

 ness. This stage is attained in the Scutella, and the C/y- 

 jnaster. (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 

 animal. At length, after passing through many intermediate 

 steps, we arrive at the perfectly circular and spheroidal 

 Echinus. (Fig. 91.) If we might he permitted to conjec- 

 ture the objects of all these changes, which occur in this con- 

 tinuous gradation, we might not unreasonably suppose them 

 to be the concentration of the internal organs into one com- 

 pact mass, and the retrenchment of all the external appen- 

 dages. It is also curious to observe, how, amidst all these 

 modifications, the double rows of perforations, which con- 

 stitute the ambulacra, retain 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 with the former. Here, instead 

 of the retrenchment of the appendages, we find them great- 

 ly developed, and amplified in every possible degree. The 

 rays of the Asterias become narrower, while their length is 

 at the same time increased; the vital organs, and also the 

 tubular feet, are gradually withdrawn from them, and retire 

 within a central disk, to which the slender rays, now bereft 

 of feet, become mere appendages. Such is the condition of 

 the Ophiura. (Fig. 92.) By the prolongation and taper- 

 ing of these rays to slender filaments, they acquire a greater 

 prehensile power, and twine with ease round their prey. 

 We next find their number augmented; it is at first dou- 

 bled, then tripled, and at lengtli indefinitely augmented. 

 They also become branched, subdividing by simple bifurca- 

 tions, as in the Euryale palmiferiim (Fig. 93;) next into 

 minuter ramifications, as in the Caput Medusic, where the 

 thousands of filaments have the appearance of a tangled web, 

 which defies all attempts at unravelling. 



The steps are but short from the Comatula to the Crinoi- 



