NO. 2123. CRUSTACEA COLLECTED IN COLOMBIA— PEABSE. 537 



The behavior of crustaceans is such that life in any sort of a habitat 

 might be possible. As a rule the reactions of different species are 

 rather stereotyped and are suited to a particular habitat. Those 

 species which are closely related taxonomically may have widely 

 different types of behavior. Fiddler crabs are diurnal, while Ocypode 

 hunts at night, yet both live in burrows along the seashore and have 

 close systematic relationship. Isopods Hve in burrows m wood, 

 cling to aquatic vegetation, run over the ground, live as parasites, 

 or have other habitats, each uivolving a different set of reactions. 

 Balanus and Polyonyx both get their food by net fishing, yet one is a 

 Cirriped and the other a Porcellanid. The behavior of crustaceans 

 shows great specificity in relation to particular habitats which 

 indicates that the class as a whole is plastic and has been able to 

 become adjusted to a great variety of conditions. 



Summarizing the last few paragraphs, it may be said that respira- 

 tion appears to be the chief factor which has kept crustaceans in 

 aquatic habitats. Reproduction, salmity of water, protection, food, 

 and behavior may also be of more or less importance but their 

 influence is, as a rule, distinctly secondary. 



The next point to be considered is the routes which crustaceans 

 have followed in their migration from the ocean to other regions. 

 I think we may assume three such highways: (1) Through the rivers 

 to the land, (2) from ocean directly to land, (3) from marshes to 

 swamp habitats in fresh water. 



At the mouths of rivers there is a mingling of fresh and salt water 

 and a variety of crustaceans live there, some wandering from sea to 

 river and back. The prawns, so characteristic of the rivers in the 

 Tropics, doubtless came originally from the ocean, for they are 

 closely related to marine species; in fact, some species now live in 

 both places. The crayfishes and river crabs probably followed the 

 same route. Cardisoma still remains near the mouths of rivers, but 

 other genera of river crabs (Trichodactylus, etc.) never enter the 

 ocean and are found far inland. The prawns and shrimps never 

 wander from rivers over the land, but crayfishes commonly do so 

 and some live in holes far from water. Some crabs pass most of their 

 time on land even when they are carrymg eggs (PseudothelpJiusa) . 



Along the ocean there are many crabs, isopods, and amphipods 

 which feed between the tide marks (CMridotea) or above {Orcliestia, 

 Uca, Ocypode, etc.), or even climb shrubs or trees in search of food 

 (Sesarma), yet aU of them remain near the seashore. Doubtles£ 

 the terrestrial hermit crabs (Birgus and Coenohita) started in this 

 way, but now do not return to the ocean except to breed. Caiman 

 ('11) points out that the more primitive land isopods are found 

 nearer the sea and most of the terrestrial representatives of that 

 group appear to have taken their way from sea dnectly to land. The 

 terrestrial amphipods (Orcliestia, Tdlitrus) took the same route. 



