90 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



apodemes are calcified and make up a kind of open framework, the 

 endophragmal system, which imperfectly roofs over a cavity known 

 as the sternal canal, which is traversed by the ventral nerve-cord. 



It will be seen from the foregoing that the body of the cray- 

 fish may be regarded as made up of twenty segments or metameres 

 all serially homologous that is, reducible to the same type. The 

 differences between the several regions are chiefly differences of 

 proportion, while certain parts are suppressed in some segments, 

 and the anterior segments have fused together. These various 

 differences have been brought about by division of physiological 

 labour. The head bears the chief sense-organs and contains the 

 ganglia which supply them. Its three posterior pairs of append- 

 ages are jaws, the last of which help to renew the water in the 

 gill-chambers. The anterior thoracic appendages are jaws, then 

 follow forceps which seize food, serve as weapons, and enable the 

 animal to climb, lastly come walking-legs. The thorax also 

 undertakes the work of respiration, and affords firm points of 

 origin to the powerful muscles which move the tail. This is the 

 swimming organ, and it also has secondary functions in relation 

 to reproduction. 



2. Skin. The true skin is formed by an epidermis with a thin 

 underlying dermis. The former secretes the exoskeleton, a many- 

 layered pigmented cuticle, differing in degree and not in kind 

 from that of a worm, and largely impregnated with salts of lime 

 (seven-eighths carbonate and one-eighth phosphate). The numer- 

 ous setae are cuticular structures. 



The epidermis is a single layer of columnar cells, the outlines 

 of which are indistinct. Slender prolongations of the epidermis 

 traverse the cuticle and end at the bases of the setae. Numerous 

 tubular cement-glands, lined by glandular epidermal cells, open on 

 the ventral surface of the abdomen in the female. The thin 

 dermis is composed of connective tissue, in which delicate blood- 

 vessels and nerves run. The exoskeleton is made up of numerous 

 closely-united layers, which are traversed by an immense number 

 of delicate vertical " pore-canals." Most of the setae are two- 

 jointed pinnate bristles slightly sunk in small pits of the outer 

 surface. Each of these setae is hollow and contains a granular 

 core, which is only separated by a thin transverse cuticular layer 

 from the epidermic process which occupies the underlying pore- 

 canal. 



