100 CRUSTACEA. 



not concealed, but simj)ly bent under the breast, and termi- 

 nated by a fan-like aj)j)aratus or swimmeret, having also, on 

 its under side, five pairs of small appendages or false legs, 

 with two filaments at the extremity of each. The abdomen 

 is always seven-jointed, the antenna? generally long and ex- 

 serted, the outer foot-jaws long, and not entirely covering 

 the inner parts of the mouth. The shell, or carapax, is 

 longer and narrower than in the Brachyura, or cral)s, and 

 generally terminated in front by a spine. These Crus- 

 tacea are marine, and do not (piit the water like the land 

 crabs. 



This sub-order is divisible into five families, namely : — 



1. Hippidas. 



2. Paguridcc, or hermit crabs. 



3. Scyllarida, including the scaly lobsters, Palinu/us. 



4. AstacidcE, or lobsters and crayfish, including the 



GalathecB and Porcellance. 



5. PalamonidcB, or shrimps. 



The hermit crabs are of very common occurrence upon 

 the shore, and are well known from their singular and soli- 

 tary mode of life, which is passed entirely in old and for- 

 saken univalve shells, exhibiting a most beautiful instance of 

 that connexion which is always found to exist between the 

 structure and functions of animals ; hence it is that we see 

 the reason why the abdomen of these crabs is of a soft and 

 (kishy nature, since, if it were hard and shelly, like that of 

 the lobster, it would be imj)Ossible for it to acconmiodate 

 itself to its tortuous al)odc, whereby it is enabled to es(;ape 

 from those dangers to which it woidd l)e exposed from the 

 boisterous element in which it resides, if it were not thus 

 amj)ly secured from harm. The species of which this genus 

 is composed are very numerous, and are found in all parts of 

 the iilobe. 



