RESPIRATORY SYSTEM 589 



remains moist and unprotected by a thick epidermis it is inevitable 

 that diffusion along a concentration gradient will take place. 

 About a third of the ox^^gen uptake, and three-cpiarters of the 

 carbon dioxide loss, goes on through the skin in a resting edible 

 frog, and some salamanders have neither gills nor lungs, and so 

 presumabty breathe entirely through the skin. In most verte- 

 brates, however, the skin is too thick to be more than an accessory 

 respiratory organ, and in the primarily acpiatic forms, gills, 

 developed on the walls of the gill slits which are characteristic 

 of chordates, are the chief site of gas exchange. We have already 

 seen that Branchiostoma has a large number of these, and that 

 in the fishes there is on the w^hole a reduction to about five pairs, 

 with an anterior smaller pair, the spiracles. Like the gills of 

 lamellibranchs (p. 274) these gill slits were probably originally 

 nutritive, not respiratory, in function. All the early vertebrates, 

 as well as the protochordates, appear to have been microphagous, 

 and to have got their food by maintaining a current of plankton- 

 containing water over a mucous surface. For this purpose 

 some division of the stream of water into small channels is clearly 

 useful, so that the advantage of the gill slits is obvious. They 

 could, incidentally, if their walls were thin enough, absorb oxygen, 

 and it might be an advantage for them to increase their surfaces 

 to do so. With the abandonment of microphagy their respiratory 

 function would be left, and would increase in importance with 

 increasing size of the animal. 



The gill slits are formed by pockets which grow out from the 

 pharynx and meet corresponding depressions growing in from 

 the skin ; they are thus lined partly by endoderm and partly 

 by ectoderm (p. 638). In fish the gills are developed from the 

 outer ectodermal part, so that they are, as most gills are, ex- 

 pansions of the skin. The posterior surface of the last sHt generally 

 has no gill, and in many teleosts the anterior surface of the first 

 slit is often bare, but otherwise each slit generally has a gill on 

 each surface ; such a gill is called a hemibranch. to show that it 

 is half the complete complement of a slit. In sharks the gills 

 are merely crinkled expansions of the wall of the slit, and so are 

 called lamellar, but in teleosts they are produced into long 

 streamers projecting into the outer world — the filamentar type 

 (Fig. 458). The sUts are here close together, and their external 

 openings, and the gills, are covered with a backwardl\- projecting 

 operculum, supported by special bones (p. 577). 



