140 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
same may be said of the crayfish’s neighbours, the perch 
and the water-snail; and they do all these things neither 
better nor worse, in relation to the conditions of their 
existence, than the crayfish does. Yet the most cursory 
inspection is sufficient to show that the “styles of archi- 
tecture ” of the three are even more widely different than 
are those of the Gothic, Italian, and Queen Anne houses. 
That which Architecture, as an art conversant with 
pure form, is to buildings, Morphology, as a science 
conversant with pure form, is to animals and plants. 
And we may now proceed to occupy ourselves exclusively 
with the morphological aspect of the crayfish. 
As I have already mentioned, when dealing with the 
physiology of the crayfish, the entire body of the animal, 
when reduced to its simplest morphological expression, 
may be represented as a cylinder, closed at each end, ex- 
cept so far as it is perforated by the alimentary aper- 
tures (fig. 6); or we may say that it is a tube, inclosing 
another tube, the edges of the two being continuous at 
their extremities. The outer tube has a chitinous outer 
coat or cuticle, which is continued on to the inner face 
of the inner tube. Neglecting this for the present, the 
outermost part of the wall of the outer tube, which 
answers to the epidermis of the higher animals, and the 
innermost part of the wall of the inner tube, which is 
an epithelium, are formed by a layer of nucleated cells. 
A continuous layer of cells, therefore, is everywhere to 
