LARVA AND EMBRYO. 47 



The larva is an immature organism functionally adapted 

 for its external environment at every stage. Very often the 

 larva passes through a succession of environments before 

 becoming adult, and the series is known as the ontogenetic 

 migration of the species. For example, a cod is a ground- 

 feeder and lives at moderate depths near the sea-bottom, but 

 the egg and larva are pelagic, living in the surface-water of 

 the open sea. The larva migrates inshore to the shallows 

 before moving out to join its fellows, thus performing an 

 ontogenetic migration. At each stage its structure is adapted 

 for its particular environment ; whilst pelagic it is transpar- 

 ent, when inshore its coloration helps to hide it, and so on. 



In the second, or Embryo7tic . iy^e^ the developing ovum 

 is supplied with, nourishment^ in one form or another, from 

 the parent and is protected from the outside world by a 

 shell, or by the body of the parent, until all its earliest stages 

 are passed, when it leaves its protecting envelope more 

 or less like its parent. 



In the ideal embryonic type cell-formation is completed 

 before differentiation commences, a condition nearly attained 

 in the embryo of some vertebrates. As a general rule, the 

 lower and more primitive members of a marine phylum 

 develop by the larval method and the higher members of 

 marine phyla, together with nearly all terrestrial forms, have 

 an embryonic development. 



The past descent of a group of animals is known as phytogeny^ and 

 in nearly every known instance this past descent reveals a long change 

 of environment of the successive generations, ox phylogenetic migration. 

 Thus it is usually held that our land amphibians, like the frog, are 

 descended from aquatic ancestors which must have gradually, as time 

 went on, migrated from the sea to fresh water and from fresh water 

 to marsh and eventually to dry land. Doubtless these ancestors were 

 fish-like in their characters at the epoch when they lived in the sea 

 and the rivers, but they gradually acquired amphibian characters as the 

 dry land was reached. 



If we picture to ourselves the succession of individuals in this 

 instance we see that each must have, passed through the same stage of 

 structure as its predecessor and then passed a little further on. Thus 

 the individual A was a fish and lived an aquatic existence. The in- 

 dividual B, its progeny, lives in the same surroundings, and by the 

 primary law of heredity he develops like his parent, but as he has taken 

 to slightly more air-breathing habits his structure adapts itself slightly 

 to this change of environment and traces of amphibian characters begin. 

 His progeny C will tend to resemble his parent and will pass through 

 the fish-structure of A to the partially amphibian structure of B. Hence 



