8 Introduction 



The blood of all animals is saline. Many invertebrate animals 

 that live in the sea have bloods which contain the same salts in the 

 same proportions as those that are present in the surrounding me- 

 dium. But the bloods of marine elasmobranch and bony fishes 

 differ in salinity from sea water, the salt content being considerably 

 lower. The bloods of fresh-water and land animals also contain less 

 salt than those of marine invertebrates (Pearse, 1932b) . Sea water 

 is a medium which contains all the elements which are necessary for 

 building and maintaining protoplasm. The general similarity of 

 the bloods of animals to sea water has been interpreted as indicating 

 that all animals originated in the sea. To explain qualitative and 

 quantitative differences which exist between the salts in the bloods 

 of both marine and non-marine animals and sea water, some have 

 assumed that blood salinities in certain cases became established and 

 stabilized long ago when the salts in the ocean differed in amount 

 from those which are characteristic today. Such questions will be 

 discussed more fully in Chapter 4. While there are many discrep- 

 ancies to be explained, it is a common belief that "blood is modified 

 sea water" (Pantin, 1931). 



The Ufe histories and breeding habits of certain types of animals 

 indicate that they originated in the ocean and subsequently migrated 

 to land. For example, crabs and hermit crabs which live in the 

 ocean generally pass through characteristic swimming larval stages; 

 certain land crustaceans of the same types spend most of their 

 lives on land but return to the ocean once each year and leave their 

 young for a short sojourn in their ancestral home. The larvae which 

 thus require a marine existence are quite like those of crabs which 

 never leave the sea. On the Irish coast three species of the same 

 genus of shore snails are arranged in zones. The snails of the spe- 

 cies that is found at low tide lay eggs which hatch into swimming 

 veliger larvae, and these spend some time in the sea; those that live 

 in the middle zone of the tidal area spend less time as swimmers; 



