208 LIFE, IN ITS INTERMEDIATE FORMS. 
delicate texture, to the centre of which passes the auditory 
nerve. 
The outer antennal differ greatly from the inner in their 
internal structure, though they resemble them so much 
in form. In the Crabs, the basal joints form a sort of box 
or compact mass, with an orifice on the side next the 
mouth, closed or opened at pleasure by means of a little 
door with a hinge, on the interior side of which a long 
bony lever is fastened, with the necessary muscles attached 
to it. In the Lobster and Prawn the door is wanting, 
but the orifice is protected by a thin membrane ; and in 
some of the lower forms it is placed at the end of a strong 
spine or projection. In all cases, however, the orifice “is 
so situated that it is impossible for any food to be con- 
veyed into the mouth without passing under this organ ; 
and of this the animal has the power to judge its suit- 
ability for food by raising the operculum [or door] at will, 
and exposing to it the hidden organ— the olfactory W ho 
can refrain from exclaiming, at the sight of such beautiful, 
such exquisito contrivances for the safety and wellbeing 
of creatures so mean, “ Great and marvellous are thy 
works, Lord God Almighty!” 
The “ crust,” or skin which envelopes the body, in these 
animals, differs from that of Insects, inasmuch as it gene- 
rally contains a considerable portion of carbonate of lime. 
In many of the smaller forms, as in the Shrimps and 
Prawns it takes the consistence of thin transparent horn ; 
but in the larger, as the Lobsters and Crabs, it acquires a 
great density, is perfectly opaque, and of the hardness of 
true shell, or even of stone. In the tips of the stout 
* Bate, loc. cifc. p. 44. 
