TEMPERATURE. CIRCULATION. xllX 



TEMPERATURE OF FISHES. 



The temperature of the blood of fishes is much the same as that of ' the 

 fluid in which they reside, but in some forms wherein there is great muscular 

 activity, as in the tunnies, the respiratory process is so energetic that it 

 raises it to a much greater heat. Davy when making investigations upon 

 the heat of a tunny fish, Thynnus (vol. i, page 100), observed that the 

 temperature would appear to be about 12 deg. above the medium in which 

 they swim, and at least 9 deg. above that of the surface of the water. 



ORGANS OF CIRCULATION. 



Fishes are provided with an arterial and venous circulation similar to what 

 obtains in the higher classes of vertebrates, and possess one for general 

 nutrition, one for respiration, and also a portal system. But of these only 

 the respiratory circulation possesses any muscular contractile system at its 

 commencement, while it corresponds to the right or venous side of the 

 heart of birds and mammals. The heart, which is absent in the Amphioxus, 

 in other fishes is small compared with the size of their bodies and lodged 

 in a cardiac chamber or pericardiac cavity which is closed in osseous fishes, 

 but communicates with the peritoneal cavity in the sturgeon and among the 

 plagiostomes, while in the myxine this cavity is simply a continuation of the 

 peritoneum. Although the heart is usually placed a short distance behind 

 the lower jaw and between the branchial and abdominal cavities, variations in 

 its position are found to exist. In the true Apodes it is placed far back 

 and behind the scapular arch. It is mostly free in the cavity it occupies, 

 but sometimes, as in the sturgeon or eel, it has ligamentous attach- 

 ments to the walls of the pericardium. It consists of an auricle or atrium, 

 having thin walls, and into which a large venous sinus empties itself, having 

 brought the blood from the veins of the body : a thick walled and 

 muscular ventricle, and an arterial bulb. The venous sinus is situated out- 

 side the pericardiac cavity in teleosteans, but within it in plagiostomes. 

 The arterial bulb or enlargement at the base of the arterial system in teleos- 

 teans, termed the bulbus arteriosus, is a pear-shaped dilatation of the artery, 

 elastic, but destitute of any contractility, while internally it may contain 

 many trabeculas and irregular pouches but no valves, but has at its base one, 

 two, or three valves, or even four in the sunfish.' Among the Chondrop- 

 terygii this swelling differs in size, while internally its valvular system has 

 been found useful in classification. Externally this bulb is but little 

 developed among the Chimeras, is mostly conical among the sturgeons and 



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