82 LIFE IN THE SEA [PART I 



eggs which are carried by the currents over a very wide area of 

 sea. In this way the species becomes distributed over as wide an 

 extent of sea as is consistent with the climatic conditions under 

 which it is fitted to live. Now it is the existence of the method 

 of reproduction by the production of free-swimming larvae which 

 is the cause of the transitory appearance of a whole host of 

 different classes of marine life among the plankton. 



Marine animals exhibit various methods of development. The 

 two highest gi'oups of vertebrates in the sea — the mammalia (the 

 seals, whales, &c.) and the reptiles (the turtles and water-snakes) — 

 reproduce in the same way as their relations on the land, that 

 is the reptiles lay large eggs which develop in the same way as 

 those of birds, while the mammalia are viviparous. Among the 

 fishes there are some which reproduce in a manner which is very 

 similar to that found among the mammalia ; others lay large eggs 

 which develop in a manner not unlike that seen in the eggs of the 

 birds and reptiles ; and others again lay eggs which develop in a 

 very short time into a larva. 



In the development of a warm-blooded animal the embryo 

 grows within the uterus of the mother and is nourished by a 

 placenta, that is a structure in which the nutritive matter of the 

 maternal blood diffuses through into the blood of the foetus. This 

 kind of development occurs exceptionally among the fishes. There 

 are a few British forms, and many tropical ones, in which a vivi- 

 parous mode of reproduction obtains and the little fish is born 

 from the body of the mother just as a mammal is. Not only so 

 but in some fishes there is even a structure which resembles the 

 j^lacenta of the mammalia. Thus in some tropical rays there is a 

 functional placenta. The yolk sac comes into relation with the 

 internal wall of the uterus, and little projections grow out from the 

 latter and come to fit into depressions in the wall of the yolk sac, 

 and the nutritive matter required by the growing embryo diffuses 

 through the walls of these blood-vessels. Then in the British 

 spurdog (Acantkias) we have an example of a viviparous fish. If 

 a gravid female caught during the summer be opened four or more 

 young dogfishes will be found in the uteri and if the size and 

 weight of these be compared with those of the undeveloped eggs 

 it will be seen that a notable increase in both weight and size has 



