202 BONY FISHES vu. 8- 



contains a series of three chambers, sinus, auricle, and ventricle, but the 

 muscular conus arteriosus is absent, there being only a thin-walled 

 bulbus arteriosus at the base of the ventral aorta. The walls of the 

 bulbus are elastic but not muscular, and study of its action by means 

 of X-rays shows that it is dilated by the ventricular beat and then 

 contracts, thus maintaining the pressure against the capillaries of the 

 gills. The ventral aorta is short, but the arrangement of the afferent 

 and efferent branchial vessels is essentially as in elasmobranchs. 



The blood-pressure in the ventral aorta is less than 40 mm Hg in 

 most fishes at rest, and in the dorsal aorta about half this. The venous 

 pressures are around zero, the pericardium being fibrous but not 

 rigid as it is in elasmobranchs (p. 160). There is no communication 

 between the pericardial and peritoneal chambers. There is a vagal 

 cardiac depressor nerve, but no sympathetic nerve to the heart. 



There is a well-developed lymphatic system beneath the skin and 

 in the muscles and viscera. Lymphoid tissue is abundant in various 

 organs but there are no lymph-nodes along the vessels. There is a 

 large spleen concerned with haemopoiesis, which also proceeds in the 

 kidneys. The red cells are smaller in bony fishes (8-10 ix) than in 

 elasmobranchs (up to 20 /z). A continuous series of white cells is 

 present and acidic and basic granules may occur in the same cell. 



9. Urinogenital system and osmoregulation 



The kidneys are mesonephric in the adult and consist of an elon- 

 gated brown mass above the air-bladder. The ducts of the two kidneys 

 join posteriorly and are swollen to form a bladder which, being meso- 

 dermal, must be distinguished from the endodermal cloacal bladder 

 of tetrapods. The urinary duct opens separately behind the anus, 

 there being no common cloaca. 



Nitrogenous elimination is a function mainly of the gills, which 

 excrete as ammonia and urea more than six times as much nitrogen as 

 the kidneys. The latter excrete creatine, uric acid, and the weak base 

 trimethylamine oxide, which is present in large amounts in the blood 

 of marine teleosts. 



One of the most striking features of the life of bony fishes is that 

 they occur both in fresh water and the sea, and many, such as the 

 trout itself, can move from one to the other. It is supposed by some 

 that the earliest gnathostomes were freshwater animals (p. 187), and 

 the bony fishes might be said to show evidence of this in that the 

 concentration of salt in their blood is always less than in the sea, in the 

 neighbourhood of 1-4 per cent. NaCl against 3-5 per cent, outside. 



