THE SMELT 307 



flotation, and in it the lungs of land vertebrates seem to be 

 foreshadowed. 



Respiration. — The first stage in respiration is carried out 

 in the gills, where the oxygen in the water is picked up by the 

 blood which circulates in them. The second stage of the res- 

 piratory process occurs in the tissues to which the blood gives 

 up its oxygen. 



Organs of circulation. — Beneath the pharynx of the trout 

 there is a heart, consisting of two chambers, an auricle and a 

 ventricle. The auricle receives the venous Ijlood from the 

 trunk directly and that from the intestine after it has passed 

 through the liver, where it rehnquishes most of its food. The 

 ventricle is more muscular and passes into the aorta, as the 

 main vessel leading out from it is called. Together they force 

 the blood through the gills, where it is purified and from which 

 it passes to the head, trunk, trunk-walls, and viscera. The 

 blood, as in other vertebrates, contains red l:)lood corpuscles. 



The organs of excretion are a pair of kidneys {kd), lying all 

 along the dorsal wall of the abdomen above the air-bladder. 

 In early development each kidney consists of a series of tubules, 

 one for each segment of the body, as in annelids. These 

 tubules collect fluids from the body-cavity as well as from the 

 blood-vessels and convey it to the outside. 



Organs of reproduction. — The number of eggs laid by 

 most fishes is very great, and so fishes have correspondingly 

 large ovaries. The germ-glands both of the female and male 

 lie along the sides of the bodj^-cavity, and both open to the 

 exterior at a point immechately behind the vent {u.g.s). The 

 eggs and sperm are both thrown out into the water. There 

 fertilization of the eggs (i.e. union \vith a sperm) takes place 

 and development begins. 



