84 LIFE IN THE SEA [PART I 



and find its nutriment in matters outside the fund of food provided 

 for in the egg. All this time it is an inhabitant of the plankton. 



Here then in the marine mammalia, the viviparous fishes, the 

 marine reptiles, and in those fishes which lay large-yolked eggs 

 we see what is knoAvn as the direct mode of development. 

 Whether the period of incubation or gestation is long or short, 

 that is relatively so, the little animal which is born out into the 

 world is a fully-formed member of the species to w^hich it belongs 

 and it can be recognised as belonging to this species, for most of 

 the characters of the adult are to be seen in the newly-born 

 animal. This is because provision is made for the nutriment of 

 the embryo and the process of development is carried to nearly its 

 conclusion when birth takes place. 



But this is not the case among the vast majority of the inver- 

 tebrata of the sea. The direct mode of development is exhibited 

 only very exceptionally by these animals. We find here that small 

 eggs are produced, that these contain little food yolk, that de- 

 velopment is a comparatively rapid process and that at the end of 

 the incubation period a creature is hatched which is as a rule quite 

 unlike its parents. It is a larva and unless the complete life- 

 history of the species to which it belongs is known it cannot be 

 recognised. On the land we have the same thing in the manner 

 of reproduction of the insects. Here a " maggot " or larva is 

 hatched out from the egg, and when the life-history is unknown, 

 the mere form of this is no indication of the identity of the species 

 which has produced it. Among marine invertebrata, as among 

 the terrestrial insects, the mode of reproduction is an indirect one, 

 that is the development of the adult conformation is not a simple 

 continuous process but is interrupted by the formation of a larval 

 stage, or of a series of such. 



The characters of the plaice are well known to every one. At 

 the beginning of the year this fish spawns about three hundred 

 thousand eggs, which then drift about in the sea while undergoing 

 development. At the end of a fortnight or so these eggs hatch 

 and the little newdy-born plaice are liberated into the sea. The 

 parent fish is a fiat creature coloured brown and red on one side 

 but colourless on the other. It swims on one side of its body ; its 

 eyes are apparently both on the same side of its head ; and its 



