68 INVERTEBRATE ANIMALS. 



in sand or mud : the body is long and weak and the 

 shell or crust membranaceous. Some of the exotic spe- 

 cies have the power of occasionally leaving the water and 

 remaining out of it for a considerable time. 



Astacus, the River Cray-fish, is found in fresh- 

 water streams, especially near old bridges, where cracks 

 in the masonry afford it a safe retreat. 



Nephrops, the Norway Lobster, is generally a 

 northern species, though Mr. Bell mentions having 

 received fine specimens from the Mediterranean. 

 Estimated number of species : recent, 24 ; British, 7. 



Group 170.— Genus HOMARUS (M. Edwards). Lobster. 

 The Crustacea all cast their shells annually, or more 

 frequently, until they arrive at full growth. In the 

 presence of the gigantic variety of the common Lobster, 

 Homarus vulgaris, found on the coasts of North America, 

 it seems impossible that the huge mass of muscular 

 flesh contained in the large claws should be withdrawn 

 through the narrow joints without being crushed and 

 destroyed. Reaumur states that the segments are com- 

 posed of two longitudinal pieces which open to allow of 

 the withdrawal of the soft limb, and afterwards close again 

 so accurately that it is very difficult in the cast crust to 

 discover the line of division. This explanation, derived 

 from observations on the River Cray-fish, has not met 

 with general acceptance, aud the method of the moult- 

 ing, ecdysis, of the higher Crustacea is still open to 

 discussion. 



Estimated number of species : recent, 3 (M. Edwards) : 

 British, 1. 



H Example of common Lobster, and of the gigan- 

 tic American variety. 



