LARVAE AND METAMORPHOSES 27 



at the anterior end above the mouth of the larva. The body 

 develops a hump on its back, and this is soon protected by a primitive 

 shell, and, on the lower side, behind the mouth, a flattened mass 

 forms the beginning of the muscular foot, the slimy organ on which 

 a slug or a snail crawls. The veliger gradually assumes the shape of 

 the kind of mollusc in which it is to grow. 



It would have required a great deal of elaborate description and 

 the explanation of many details of structure familiar only to ad- 

 vanced zoologists, to give a just idea of the remarkable resemblances 

 between the larvae of Echinoderms, the trochophores of Worms and 

 Molluscs, and the similar larvae of some other marine invertebrates. 

 It is tempting to suppose that these different creatures follow 

 the path of a common ancestor while they are living the free- 

 swimming life of that ancestor, and then sharply diverge to reach 

 their different goals. But we have to remember that a meta- 

 morphosis cannot be a primitive mode of development, and that 

 where it exists a long history has been blotted out. And we have 

 also to remember that the resemblances of the larvae are in plain 

 relation to similar habits, and may have no ancestral meaning. 



The great class of Crustacea includes crabs, lobsters, crayfish, 

 prawns, shrimps, sandhoppers, woodlice, barnacles and water -fleas 

 and many less well known creatures. Like insects and spiders, 

 they have jointed limbs, arranged in pairs, and the body is covered 

 by a hard external case to the inside of which the muscles are 

 attached, and which is usually known as the shell. Most of them 

 live in or near water, and the terrestrial forms show plain traces of 

 aquatic ancestry. The young of many of them, especially those 

 that live in fresh water or on land, pass through their period of 

 youth in fashions that are quite clearly direct adaptations to the 

 special circumstances of their lives. The marine crustaceans 

 usually lay small eggs which hatch out into larvae extremely unlike 

 their parents, although the external shell and jointed limbs show 

 plainly that they are crustaceans and betray no resemblance with 

 any other group of the animal kingdom. The larvae swim about, 

 feed, and after a few days or weeks the hard shell becomes too tight 

 for the plump body, and splits open, setting free the animal, clad 

 in a soft skin and at once swelling to a size rather larger than that of 

 the case from which it emerged. Very quickly the skin hardens 

 to form a new shell, and this second larva is not exactly like the 

 first larva, but rather more complicated, and more near the adult 

 form. The same sequence is followed again, and may be repeated 



