ALLIES OF THE CRAYFISH, 257 
branchie are terminated by, short hooked spines; and the 
coxopoditic sete, as well as those which beset the stems 
of the podobranchie, have hooked apices. 
The definitions of the genera would in like manner be 
given by adding the distinctive characters of each genus 
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 
© Rock Lobster ” or ‘‘ Spiny Lobster.” 
The common lobster (Homarus vulgaris, fig. 67) 
presents the following distinctive characters. The last 
thoracic somite is firmly adherent to the rest; the exo- 
podite of the antenna is so small as to appear like a mere 
movable scale; all 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 difference from the Astacina is exhibited 
by the gills, of which there are twenty on each side; 
namely, six podobranchiz, ten arthrobranchie, and four 
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