THE GENERAL SHAPES OF ANIMALS. 179 



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 ere vices 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, restrained by the relations o* 

 the parts to the mouth and vent : the conditions not beino 

 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, consequently, dif- 

 ferent habits. The genera Echinocyamus, Spatangus, Bris- 

 sus, and Amphidotus, diverge markedly towards a bilateral 

 structure. These creatures are found not on rocky shores 

 but on flat sea-bottoms, and some of them only on bottoms 

 of sand or mud. Here, there is none of that distribution of 

 surfaces on all sides which makes the spheroidal form con- 

 gruous wiih the conditions. Having to move about over an 

 approximately-horizontal plane, any deviation of structure 

 which leads to one side being kept always foremost, will be 

 an advantage : greater fitness to function becoming possible 

 in proportion as function becomes fixed. Survival of the 

 fittest will therefore tend to establish, under such conditions, 

 a form that keeps the same part in advance a form in 

 which, consequently, the original radial symmetry diverges 

 more and more towards bilateral symmetry. It may 



be well to add that the validity of these interpretations does 

 not depend on the view taken of the alliances of the Echino- 

 derms, and their primitive type of symmetry. If, as Pro 

 fessor Huxley contends, the Echinoderma, having bilateral 

 larvae, cannot be held akin to those lower types in which the- 



