H NATURAL HISTORY. 



As a rule, there is a gall-bladder, but it is freqiiently absent ; it is very small in the Rays, and is 

 sometimes entirely separated from the liver, as in Lophins. The bile enters the small intestine near 

 the stomach. The internal surface of the small intestine is usually smooth, but in the Herring it 

 shows slight transverse folds, in the Sturgeon it is divided up into cells, and in the Sun-fish it is lined 

 with little tubes called villi, which absorb the nutriment from the food. The large intestine is 

 straight, and in the Ganoid fishes, Mud-fishes, Sharks, and Rays terminates in a remarkable spiral 

 coil. Though the small intestine is coiled up spirally in the Sword-fish, the coils are not in 

 contact, but in the cartilaginous fishes there is usually a spiral channel which winds around many 

 times. In the Fox Shark there are thirty-four of these turns; at the end of the intestine the 

 membrane lining the valve is deeply honey -combed. Evidence of its existence in fossil fishes is found 

 in the spirally-formed coprolites or petrified fteces which are met with in many of the geological 

 formations. 



The oi-gans for purifying the blood, by separating from it the waste products, are different 

 from the kidneys of higher animals, and correspond to organs which exist only in the embryo, and are 

 known to anatomists as Wolffiaii bodies. Their function, however, is the same in fishes as that of 

 kidneys in other animals. In most bony fishes the kidneys are long and narrow, and extend along the 

 abdomen firmly attached to the vertebrae. The tissue forming them is usually of a reddish tinge ; 

 it is soft and spongy, and supplied with arteries from the abdominal aorta, which form the minute 

 globular secreting organs termed Malpighian capsules, similar to those which abound in the outer 

 layer of the kidney in higher vertebrates. Sometimes two uretei's lead from the kidney and enter 

 a urinary bladder, but occasionally, as in the Herring, there is only one ureter, and in this and 

 several other fishes the bladder is wanting. It is largest in those fishes in which the air-bladder 

 does not exist. The kidneys are long and narrow in the Ganoid fishes, and compact and generally 

 lobulated in the cartilaginous fishes. 



The air-bladder is found in most osseous fishes ; it extends along the back of the abdomen, below 

 the kidneys, and is prolonged in some fishes below the caudal vertebrae, nearly to the end of the tail. 



Its varieties of form are very singular: it is sometimes 

 divided lengthwise into two bladders, but much more 

 frequently divided crosswise into two compartments 

 which comrmmicate with each other. In the Siluroid 

 fish Pangasius the air-bladder is said to be divided 

 into four portions longitudinally. Sometimes the 

 SWIMMING ULADDER OF CARP. air-bladder develops blind processes: in certain cases 



from the fore part, in others from the hind part, and 



occasionally from both ends. In the family Scisenidae the air-bladder often has numerous, 

 lateral branches which themselves ramify into digit-like processes. In. some species of the genus. 

 Gadus processes given off from the air-bladder line excavations in the transverse processes of the 

 abdominal vertebrae, thus, as Professor Owen has pointed out, foreshadowing the pneumatic con- 

 dition of the bones in birds. In other fishes such as Callichthys lucida the air-bladder is even more 

 singularly developed, since its many branches form a covering round the abdominal viscera. The wall 

 of the air-bladder is often shining and silvery; occasionally the interior is subdivided into small cells: 

 this condition may bs seen in the genera Erythrinus and Amia. The air-bladder here seems to be 

 taking on some of the characters of a lung, and in Lepidosiren and Ceratodus the transition is com- 

 pleted. The air-bladder is entirely wanting in the Sharks, Rays, Chimsera, Lampreys, Flat-fish, and 

 other forms, several of which, like the Angler, live habitually at the bottom of the sea. The duct 

 connecting the air-bladder with the oesophagus seems to be the rudiment of the trachea, though it does 

 not always open into the anterior end of the air-bladder. In most fresh-water fishes the air-bladder is. 

 filled chiefly with nitrogen gas, mixed with a little oxygen, while in sea fishes the gas is chiefly 

 oxygen, with a little nitrogen. Occasionally, when fishes are brought up from great depths, the air- 

 bladder expands, and forces the internal organs out of the mouth. In the Gurnards the air-bladder 

 assists in the production of sound, so that these fishes may be said to possess a voice. 



The blood in fishes is red, but small in quantity. The red blood discs usually have an 

 elliptical shape, and are largest in the Sharks. In the Lamprey the red corpuscles are nearly 



