MANUAL OF NATURAL HISTORY. 79 



pheric air, the oxygenation of their blood is depend- 

 ant on the air dissolved in the water, which requires 

 a special adaptation of their breathing apparatus. 

 Accordingly we find that their permanent condition 

 resembles that which is seen to exist in the young 

 of the immediately preceding class of Amphibians, 

 that is to say, they respire throughout their life by 

 means of aquatic lungs or gills. By their agency 

 the venous blood is exposed in a state of minute sub- 

 division to the influence of streams of water, which 

 is effected by the blood-vessels leading to them di- 

 viding again and again until they become nearly 

 microscopic, in which state they are supported by a 

 thin, delicate membrane, the whole being arranged 

 as a series of plates or processes placed on what are 

 named the branchial arches. In the greater number 

 of fishes the water meant to supply these vessels 

 enters by the mouth, and, being directed backwards, 

 passes over them, and is finally expelled by the gill- 

 opening under the gill-cover or operculum. Fishes 

 are cold-blooded animals ; that is, their temperature 

 hardly exceeds that of the medium in which they 

 live; yet to this there are some exceptions, as in the 

 case of the " Bonito/' mentioned by Dr. John Da- 

 vey, whose temperature, he ascertained, surpassed 

 that of the surrounding water by nearly ten degrees 

 of Fahrenheit. Their chief propelling power is their 

 tail, which acts by alternate strokes on either side, 

 similar to the action of an oar in sculling. It is 

 observed that their swiftness varies with the form 

 of the tail ; that those whose velocity is very great, 



