io66 LIFE : OUTLINES OF GENERAL BIOLOGY 



So the shells of whelk and cockle, the exoskeleton of crab and 

 lobster, the test of the sea-urchin, and scores of other external 

 encasements are protective static adaptations. Yet how rare it is 

 to find anything quite static in the world of organisms, for the shell 

 of the cockle, like the tortoise's box, is in part of importance in 

 serving for the insertion of the muscles of their respective locomo- 

 tion. Part of the adaptiveness of the sea-urchin's rigid globe lies in 

 its capacity for growth. New plates are formed around the apical 

 disc, and each of the hundreds of plates is increased in size by 

 contributions from hardly detectable connective-tissue cells between 

 them. 



There may be static adaptations that have nothing to do with 

 hard parts, as we see in fixed colouration when that harmonises with 



ECT' 



Fig. 185. 



A Typical Hydrozoon Polyp, in vertical median section. M, the mouth; T, 

 a hollow tentacle, one of the many around the mouth; G, the coelenteron 

 or gastral cavity, the food-canal in short, which is in Coelentera the only 

 cavity in the body. The symmetry is radial. The body- wall is two-layered 

 (diploblastic), with an external ectoderm (ECT) and an internal endo- 

 derm (END), Between these there may be a structureless lamina as in 

 the common Hydra, representing the mesogloea of sea-anemones and 

 other higher (Actinozoon) polyps. 



the habitual background. There is surely some adaptiveness in the 

 sandy colour of desert animals, in the green colour of some arboreal 

 animals, and in the white colour of various polar animals; but we 

 have already noticed that the colouration adaptiveness need not be 

 restricted to making the animal inconspicuous. 



A second group of structural adaptations includes shapes; and 

 in these again the emphasis is partly on the passive side. Compact- 

 ness of body, as in the sea-urchin, is adapted to creeping about 

 among the rocks, for the spherical shape lessens the grip that tidal 

 currents take of the body, and it also allows the locomotor tube-feet 

 to be disposed uniformly in all directions. In the burrowing heart- 

 urchins there is an interesting departure from the spherical and 

 radial towards the cordate and bilateral, thus of evident advantage 

 in such a difficult kind of locomotion. 



The passage from water to dry land, effected by so many types, is 



