CH. IV] LIFE IX THE SEA 85 



mouth is twisted to one side. But the newly-born plaice is round 

 in cross section, it SAvims at first on its back and latterly with its 

 back upwards in the proper manner, it is clear and glassy in 

 appearance except that the tail is dotted over with black and 

 some yellow pigment and it is quite symmetrical, one eye being 

 situated on each side of the head. The plaice when it is recently 

 hatched from the egg cannot be recognised as such, and we only 

 know that it is a plaice because we know its life-history and we 

 can easily watch the process of development. It is really a larval 

 stage in the development of the fish, and the larva differs strik- 

 ingly from the adult, because the egg from which it was hatched 

 contains so little food yolk that the process of development has 

 to be interrupted in order that the larva may grow and obtain 

 food so that it may be able to complete unfolding of the adult 

 structure. 



Now it is such a process that is characteristic of the inverte- 

 brate population of the sea. Among the Crustacea there is no 

 animal which gives birth to one like itself. The eggs of these 

 shellfish develop into larvae which are quite unlike their parents, 

 at least when they are just bom from the egg. Among the 

 Crustacea there are two principal larval forms, the ncmplius and 

 the zoea. The nauplius is the creature which is hatched from the 

 eggs of all these shellfish with the exception of the Decapoda (the 

 crabs, lobsters, prawns and their allies, where the larval form is 

 the zoea). Even in these latter animals there is a disguised nau- 

 plius stage in the embryology of the zoea. When the egg of a 

 crustacean hatches a nauplius issues and for a time this creature 

 leads an independent life in the plankton. It is quite unlike the 

 young of any of the higher animals, which is for a time cared for 

 and fed by its parents, but it behaves from the time of its birth 

 exactly like a fully developed animal except that it is not sexually 

 mature. It swims about by itself and seeks for and finds its own 

 food, behaving to the extent of its powers as a minute predatory 

 animal. 



The development of the Crustacea is really more complicated 

 than I have indicated, in that more than one larval stage is usually 

 intercalated between the egg and the adult form. Thus the zoea 

 larva hatched out from the egg of the decapod passes through 



