23-1: 



VEETEBIIATE ANIMALS. 



vj,- 



ou each side. The water is taken in at the mouth by a process 

 analogoiis to swallowing, and it gains admission to the branchial 

 chambers by means of a .series of clefts or slits which perforate the 



sides of the pharynx. Having" 

 passed over the gills and lost 

 its oxygen, the effete water 

 makes its escape behind by 

 an aperture called the "gill- 

 slit," which is placed on the 

 side of the neck. The open- 

 ing of the gill-slit is closed in 

 front by a chain of flat bones 

 which constitute the " gill- 

 cover,'' and by a membrane 

 which is supported upon a 

 variable number of slender 

 bony spines. This is the gen- 

 eral mechanism of respiration 

 in one of the Bony Fishes, but 

 different arrangements are 

 found in other cases, which 

 will be subsequently noticed. 

 The heart in fishes may 

 be regarded as essentially a 

 branchial or respiratory heart, 

 being concerned chiefly with 

 driving the venous and im- 

 pure blood to the gills. It 

 consists in almost all cases of 

 two cavities, an auricle and 

 a ventricle (fig. 169). The 

 auricle (au) receives the ven- 

 ous blood which has passed 

 through all the vnxious parts 

 of the body, and ])ropels it 

 into the ventricle (v). From 

 the ventricle proceeds a single 

 great vessel (the "branchial 

 ai'tery "), the base of which is 

 usually developed into a mus- 

 cular cavity, the " bulbus arteriosus " (ah), which acts as a kind of 

 additional ventricle. By the ventricle and hn/hns nr/eriosiis the 

 venous blood is driven to the gills, where it is subjected to the 

 action of the water. The aerated blood is not returned to the 



Fig. 109. — Diagram of tlie circulatory .sjsti'ui 

 in a Fibli, tlie vessels containing venous 

 blood being longitudinally shaded, and 

 those containing arterial blood being cross- 

 shaded, vc Vena cava; vpYcna. i)Ortai : cu 

 Auricle ; v Ventricle , a'' Bulbus arteriosus ; 

 b Branchial artery ; ba One of the divisions 

 of the branchial artery going to the gills, 

 from which jiroceeds one of the correspond- 

 ing branchial veins, by the union of which 

 the subvertebral aorta {.^(() is formed ; i In- 

 testine ; k Kidney. 



