PISCES. 211 



Immediately behind the pyloric opening of the stomach there 

 is usually a variable number of blind tubes (called the " py- 

 loric caeca ") which open into the intestine, and which are be- 

 lieved to represent the pancreas. In some fishes, however, 

 there is a well-developed pancreas, and in others even these 

 tubes are wanting. The intestinal canal is a longer or shorter, 

 more or less convoluted tube, and its absorbing surface is 

 sometimes largely increased by a spiral folding of the mucous 

 membrane, which winds like a screw in close turns from the 

 pylorus to the anus. The liver is usually of large size, and 

 saturated with oil, but in the Lancelet it is doubtfully rep- 

 resented by a hollow, sac-like organ. The kidneys in fishes 

 are of great comparative size, forming two elongated organs, 

 situated beneath the spine, and extending along the whole 

 length of the abdomen. 



Respiration in all fishes is aquatic, and is effected by means 

 of gills or branchiae, in all except the Lancelet, in which res- 

 piration is effected by branchial filaments placed round the 

 pharynx, and also by a greatly-developed pharynx perforated 

 *>y ciliated apertures (Fig. 105). The arrangement and struct- 

 .re of the gills in fishes vary a good deal in different orders, 

 and the leading modifications will be noticed hereafter. In 

 the mean while it will be sufficient to give a short description 

 of the branchial apparatus in one of the bony fishes. In such 

 a fish the gills consist of a single or double series of flat carti- 

 laginous leaflets, covered by mucous membrane, richly supplied 

 with blood, and arranged on bony or cartilaginous arches 

 which are connected with the tongue-bone (hyoid bone) below 

 and with the under surface of the head above. The branchial 

 arches and branchias are suspended in cavities placed on the 

 side of the neck, and in the ordinary bony fishes there is only 

 one such cavity on each side. The water is taken in at the 

 mouth by a process analogous to swallowing, and it gains ad- 

 mission to the branchial chamber by means of a series of clefts 

 or slits which perforate the sides of the pharynx. Having 

 passed over the gills and lost its oxygen, the effete water 

 makes its escape behind by an aperture called the " gill-slit," 

 which is placed on the side of the neck. The opening of the 

 gill-slit is closed in front by a chain of flat bones which con- 

 stitute the " gill-cover," and by a membrane which is sup- 

 ported upon a variable number of slender bony spines. This 

 is the general mechanism of respiration in one of the bony 

 fishes, but different arrangements are found in other cases, 

 which will be subsequently noticed. 



