196 MORPHOLOGICAL DEVELOPMENT. 



and now so rare, present a radial symmetry answering to 

 an incidence of forces that are equal on all sides. In the 

 general attitudes of their parts towards surrounding actions, 

 they are like uniaxial plants or like polypes; and show, as 

 those do, marked differences between the attached ends and 

 the free ends, along with even distributions of parts all round 

 their axes. In the Ophiuridea, and in the Star- 



fishes, we have radial symmetry co-existing with very differ- 

 ent habits; but habits which nevertheless account for the 

 maintenance of the form. Holding on to rocks and weeds 

 by its simple or branched arms, or by the suckers borne on 

 the under surface of its rays, one of these creatures moves 

 about not always with one side foremost, but with any side 

 foremost. Consequently, averaging its movements, its arms 

 or rays are equally affected, and therefore remain the same 

 on all sides. On watching the ways of the com- 



mon Sea-urchin, we are similarly furnished with an ex- 

 planation of its spherical, or rather its spheroidal, figure. 

 Here the habit is not to move over any one approximately- 

 flat surface; but the habit is to hold on by several surfaces 

 on different sides at the same time. Frequenting crevices 

 and the interstices among stones and weeds, the Sea-urchin 

 protrudes the suckers arranged in meridional bands over its 

 shell, laying hold of objects now on this side and now on that, 

 now above and now below: the result being that it does not 

 move in all directions over one plane but in all directions 

 through space. Hence the approach in general form towards 

 spherical symmetry — an approach which is, however, re- 

 strained by the relations of the parts to the mouth and vent : 

 the conditions not being exactly the same at the two poles as 

 at other parts of the surface. Still more significant is 



that deviation from this shape which occurs among such of 

 the Echinidea as have habitats of a different kind, and con- 

 sequently, different habits. The genera Echinocyamus, Spa- 

 tangus, Brissus, and Ampliidotus, diverge markedly towards 

 a bilateral structure. These creatures are found not on rocky 



