FISHES 273 



margin, in most bony fish, is beset with horny teeth or spines, forming 

 the so-called gill-rakers. The function of these processes is to intercept 

 any solid foreign bodies which may 

 have been taken in with the water of 

 respiration (see herring), and to pre- 

 vent their coming in contact with the 

 branchial leaflets. In the teleostean 

 and ganoid fishes (compare, however, 

 sharks, rays, and lampreys) the bran- 

 chial leaflets are outwardly protected 



against injury by the bony gill-cover, 



7 mi A j HEAD OF A FISH (HERRING), OPENED TO 



or operculum. The posterior edge of SHOW ^HE GILLS. 



the Operculum is free, leaving an aper- Kbg., gill-arches ; B., branchial laminae; 



ture, the gill opening, between it and R., gill -rakers; AU., eye-socket. 

 the body wall. 



(b) Let us now examine how respiration i.e., the exchange between 

 poisonous carbonic acid and the restorative oxygen is effected. Imme- 

 diately behind the gills, in the region of the throat, is situated the heart, 

 which consists only of an auricle (Vk) and a ventricle (Hk). The blood, 

 loaded with carbonic acid (venous blood) after passage through the whole 

 body, enters the auricle, thence passes into the ventricle, from which it 

 is pumped into a vessel which passes forwards, the branchial artery 

 (Ksch). This artery gives off on each side as many branches as there 

 are branchial arches. One of these branches (K) passes off to each 

 branchial arch, and again sends off a secondary branch (1) to each of the 

 branchial leaflets (see illustration on p. 274). The latter ramifies on 

 the inner surface of the branchial leaflets, and is finally resolved into 

 capillaries. We have now two kinds of air separated by the thin 

 membrane of the branchial leaflets viz., the carbonic acid gas in the 

 blood, and the oxygen of the atmospheric air, which is dissolved in 

 the water which bathes the gills. Hence, as in the lungs, a mutual 

 exchange of the two gases is effected (see Part I., p. 6, Section 5). The 

 carbonic acid passes through the fine membrane out into the water, 

 while the oxygen passes in a reverse direction from the water into 

 the blood. The blood thus purified (arterial blood) is collected in a 

 vessel (2) running along the outer edge of the branchial leaflet, and is 

 thence conveyed into a larger vessel (3) branchial vein which runs 

 along the branchial arch, and which also receives the oxygenated blood 

 from all the other branchial leaflets of the same arch. The branchial 

 veins of all the gills, filled with arterial blood, unite to form a large 

 vessel, the dorsal aorta (Ar.), which is continued backwards below the 

 vertebral column, and, ramifying into all the different parts of the body, 



