GENERAL ORGANOLOGY 97 



tissues is derived either from the air or from the water, according as the 

 animal is terrestrial or aquatic. Less frequently it is the case that water- 

 dwellers breathe air, and hence are compelled, from time to time to 

 rise to the surface of the water for air; this is true for the large marine mam- 

 mals, and for many insects, spiders, and snails found in fresh water. Air- 

 and water-breathing takes place exclusively through the skin, so long as 

 this is delicate and readily permeable and no higher development of organ- 

 ization necessitates a more active interchange. If the demand for oxygen 



cxs 



j V'r V^ 'X S 



~,jp& ; ,.'/- : : 



x -rf' *'* ^"PL T < *~ , ', i 



W^j* :^'-' 



FIG. 62. Left second foot of a crayfish with ('/)/-) attached gill (after Huxley). 

 cxp, coxopodite; bp, basipodite; ip, ischiopodite; nip, meropodite; cp, carpopodite; pp t 

 propodite; dp, dactylopodite; cxs, bristles of the coxopodite; e, lamina of the gill. 



be greater, special breathing-organs are found gills (branchiai) for water- 

 breathing, lungs and trachea; for air-breathing, in addition to which the 

 skin functions as an accessory organ of more or less importance. 



Gills are usually thin-walled, frequently ciliated areas of the skin 

 which are abundantly supplied with blood-vessels, and where richly 

 branched tuftlike projections or broad leaves have grown out, thus 

 furnishing the largest possible surface for the interchange of gases; 

 these occur in such a position as to be most exposed to fresh water; in the 

 crayfish, for example, they are on the legs, where the motion drives fresh 



7 



