136 Dynamic Theory. 



being washed about by currents, as the Crinoids, the Lingula, and other 

 Brachiopod Mollusks, the Tunicates, &c. In many cases it is known 

 that such clinging organs have been modified from organs of active loco- 

 motion. This is the case with the Anatif a and the Barnacle, as mentioned 

 further on ; the clinging stem of which is modified from the fore limbs, 

 with which it swims while in the nauplius or early infant stage. 



Among the curious examples of the morphological adaptation of an 

 organ is that of the evident modification by which the univalve Grastero- 

 pod Navicella has been developed from the Neritina. There is prac- 

 tically no difference between them except in the form of the foot and its 

 cover or operculum. The Neritina when alarmed instantly draws into 

 its shell, pulling its foot after it. And the operculum, like a hoof, closes 

 up the mouth of the shell snug and tight. But with such a habit he 

 can live only in comparatively still water, or he would be carried off or 

 dashed to pieces when he loosened his foot-hold. The Navicella, on 

 the contrary, when alarmed does not let go, but hugs down upon the 

 object he happens to be on all the harder, covering his foot under him 

 with his shell over his back. He has come to do this by a habit of liv- 

 ing in a current where it was unsafe to let go, and those of his kindred 

 who did were probably selected out of existence. But by thus keeping 

 this foot always extended and the operculum always open, the foot has 

 grown to such a form that it can no longer pull the operculum shut, and 

 if it could, the cover has grown so distorted by disuse that it would no 

 longer fit or nearly cover the opening, so it is, in fact, reduced to a use- 

 less rudiment. 



Dessication. 



Among the curious habits to which animals may become differentiated, 

 is that of dessication. This dessication takes place in the egg and 

 among invertebrates only. The eggs of the following animals may be 

 dried and kept, some of them for years, and then moistened and 

 hatched a great number of infusoria, sponges and worms ; and of 

 Crustaceans, the Apus, Branchipus, Brine Shrimps (Artemia), Water- 

 flea ( Cypris ), Branched-horn Water-flea ( Daphnia ), Cypridina, Limna- 

 dia, Estheria and many Copepods (Cyclops ). Some of these eggs are 

 very small, so that when dry they can be carried about by the wind. 

 They have attained the abilit}' to stand this drouth by living in places 

 subject to being dried up in the summer. The mud containing them 

 can be kept for years, and the eggs then hatched by moisture at a 

 proper temperature. But the most curious feature connected with it is 

 that the habit of dessication is so fixed as a part of their development, 

 that some of them, for example those of the Apus, never will hatch out 

 till they have been dried, and lain for a certain time in the dry mud. 



Reptiles do not incubate their eggs. In the snakes the young are 



