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 diff'erent 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 



