390 



CKUSTACEA. 



(998.) The tail is the great agent of locomotion in all the Macroura 

 or large-tailed Decapods ; and for this purpose it is terminated by a fin, 

 formed of broad calcareous lamella, so arranged that, while they will 

 close together during the extension of the tail, and thus present the 

 least possible surface to the water, they are brought out to their full 

 expansion by the down-stroke of the abdomen ; and such is the impulse 

 thus given, that, as we are credibly informed, a Lobster will dart itself 

 backwards to a distance of eighteen or twenty feet by one sweep of this 

 remarkable locomotive instrument. 



(999.) If we now pass on to the consideration of the Anomourous 

 Decapods, we find that the external organs above enumerated, although 

 existing in precisely similar situations, are so far modified in their con- 

 struction and relative proportions as to become suited to a mode of life 

 widely different from that led by the members of the last division. The 

 Anomoura, as their name imports, have tails of very unusual conforma- 



Fig. 199. 



Hermit Crab. 



tion. Instead of being encased in a hard coat of mail, as in the Macroura, 

 the hinder part of the body is soft and coriaceous, possessing only a few 



