IV. VERTEBRATA: PISCES. 



503 



for the differences in pressure due to different depths, so that the fish may 

 remain without muscular action at any desired level. (2) The bladder is a 

 hydrostatic sense organ for the recognition of water pressure and thus the depth 

 and, by reflex, for the regulation of the muscular action and muscle tonus. 

 Support for this view is found in the existence in many fishes of structures 

 adapted for conveying the variations in pressure to the ear, which is an 

 organ for equilibration. This may be accomplished, as in the Clupeidae, by 

 means of processes of the bladder which extend into the region of the ear, 

 or by the Weberia?t apparatus, a system of levers formed by appendages of 

 the vertebras and extending from bladder to ear. 



The heart, enclosed in the pericardium, lies immediately behind the 

 gill region, and is protected by the shoulder girdle. It always consists of 

 auricle and ventricle (fig. 553), separated by a pair of valves to prevent 

 back-Oow of the blood; it sends the blood to the gills by the arterial trunk 

 (ventral aorta), and receives it from the body through a thin-vi^alled sac, 



A. B. 



Fig. 553. — Forms of hearts of fishes in schematic long section (after Boas). A, 

 selachian and most ganoids; B, Amia; C. teleost. a, auricle; 6, bulbus arteriosus; c, 

 conus arteriosus; k, valves; s, sinus venosus; I, truncus aorta:; v, ventricle. 



the venous sinus, in which the hepatic veins and the Cuvierian ducts 

 (formed by union of jugular and cardinal veins) empty (figs. 66, 554). 



The most important differences lie in the development of conus and 

 bulbus arteriosus. These are muscular accessory organs, the first arising 

 from the heart, the other from the arterial trunk ; and correspondingly the 

 conus has striped, the bulbus smooth muscle fibres. The anterior end of 

 the heart contains 'semilunar ' valves, which prevent the back-flow of the 

 blood. When, by increase in the number of valves, tliis part becomes 

 elongate, a conus arteriosus (fig. 553, A) is formed. The bulbus (C) is a 

 muscular swelling in front of the valves in the arterial trunk. 



The connexion of ventral and dorsal aorta; is effected in young fishes 

 (fig. 554) by the gill arteries directly; later by means of the complicated 



