INVERTEBBATE ANIMALS. 75 



in moderately deep water. The species are not well 

 defined : one of the forms occurs in British seas. 

 Estimated number of species : recent, 32; British, 1. 



H Examples of Curtonotus, Macrophthalmus, &c. 



Skt^entlj Cabk Case. 



The Mark IT indicates specimens or other illustrations exhibited in the Cases. 



Group 181.— Family GECARCINID^ and allies. 



This group includes four families, well distinguished 

 by the habits as also by the structure of the animals. 



The Thelphusidce are fresh- water crabs, living in 

 holes on the banks of rivers. 



The GecarcinidcB, as the name implies, are Land- 

 crabs, inhabiting the warmer regions of all parts of the 

 world, except the interior of continents. Their gills are 

 adapted for respiration so long as they are kept moist, 

 a condition favoured by the nocturnal habits of the 

 animals. Once in the year, at the spawning season, 

 they are said to migrate in vast numbers together, 

 always taking the nearest way to the sea. 



The Pinnoiherida, or Pea-crabs, reside in the 

 bivalve shells of living Molluscs. 



The Mycteridcs for the most part inhabit salt- 

 marshes. 

 Estimated number of species : recent, 50 ; British, 2. 



IT Examples illustrating the four families. 



