ALLIES OF THE CEAYFISH. 257 



bi'anchite are terminated by short hooked spines ; and the 

 coxopoditic setae, as well as those which beset the stems 

 of the podobranchise, have hooked apices. 



The definitions of the genera would in Uke manner be 

 given by adding the distinctive characters of each genua 

 to the definitions of the family ; and those of the species 

 by adding its character to those of the genus. But at 

 present it is unnecessary to pursue this topic further. 



There are no other inhabitants of the fresh waters, or 

 of the land, which could be mistaken for crayfishes ; but 

 certain marine animals, familiar to every one, are so 

 strikingly similar to them, that one of these was formerly 

 included in the same genus, Astacus ; while another is 

 very often known as the " Sea-crayfish." These are the 

 " Common Lobster," the " Norway Lobster," and the 

 " Eock Lobster " or " Spiny Lobster." 



The common lohster {Homarus vulgaris, fig. 67) 

 presents the following distinctive characters. The last 

 thoracic somite is fijmly adherent to the rest ; the exo- 

 podite of the anteima is so small as to appear like a mere 

 movable scale ; aU the abdominal appendages are well 

 developed in both sexes ; and, in the males, the two an- 

 terior pairs are somewhat like those of the male Astacus, 

 but less modified. 



The principal difierence from the Astacina is exhibited 

 by the gills, of which there are twenty on each side; 

 namely, six podobranchise, ten arthrobranchiae, and four 



