NO. 2123. CRUSTACEA COLLECTED IN COLOMBIA— PEARSE. 535 



the Reptilia all breed on land; even the marine turtles and sea snakes 

 which spend their hves in the open ocean come back to the shore to 

 lay their eggs. Among invertebrates representatives of but two 

 phyla have taken generally to terrestrial habitats — the arthropods 

 and the mollusks, both provided with a strong exoskeleton. In the 

 Arthropoda the Crustacea hold a position like the Amphibia among 

 the vertebrates, for they are in the midst of a transition from aquatic 

 to terrestrial habitats. In fact this class shows every gradation in 

 mode of life — marine, fresh-water, terrestrial, arboreal. Everything 

 indicates that crustaceans had their origin in the ocean and subse- 

 quently invaded other habitats. What are the factors, then, which 

 have been influential in enabling these animals to take up new 

 modes of existence or have prevented them from doing so? How 

 have they left the ocean and become adjusted to new habitats ? 



Crustaceans have two peculiarities which are of considerable eco- 

 logical importance. They carry their eggs about with them so that 

 the distribution of the various species is not closely limited by breed- 

 ing conditions, as is the case with many animals which deposit eggs; 

 as, for example, insects whose larvae require particular food plants, 

 fishes which can nest only on certain kinds of bottom, etc. Further- 

 more, the class as a whole is strongly committed structurally to 

 respiration by means of gills and this fact has apparently offered the 

 most serious obstacle to the invasion of the land. 



The lessened salinity of fresh water has apparently not been 

 important in limiting crustaceans in their migration in streams, 

 swamps, or lakes. Several orders are now confined almost exclusively 

 to fresh water. Yet some species which live on land or in fresh 

 water always go back to the ocean to breed (Coenohita, Birgus). 

 On the other hand, there are many which pass their whole life cycle 

 away from the sea (many Entomostraca, Isopoda, Potamonidae, 

 Potamobiidae) . 



The food habits of Crustacea permit them to live almost every- 

 where. Most species are omnivorous, and they frequently do much 

 good as scavengers. If feeding habits alone were considered, it 

 would seem remarkable that more crustaceans have not left the 

 ocean. 



Admirable protection is afforded by the exoskeleton which not 

 only serves as an armor but prevents desiccation. Most species lurk 

 in lioles or crevices during the day and are active at night. Those 

 which feed in daylight have very keen vision and are quick to react 

 to any moving object, retreating to some appropriate hiding place 

 when threatened with danger. 



In changing from salt to fresh water or from water to land respi- 

 ration must undergo marked changes, and this process is therefore 

 of great importance in considering the ecology of Crustacea. The 

 simpler crustaceans breathe thi^ough the general body sm'face but 



