FISHES. 233 



(called the " pyloric creca ") which open into the intestine, and which 

 are believed 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 spii-al folding of the mucous membrane, which winds 

 like a screw in close turns from near the pylorus to the anus. The 

 liver is usually of large size, and saturated with oil, but in the 

 Lancelet it is doubtfvilly represented 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 efiected by means of 



-b 



Fig. 168. — Gills and heart of the Perch exposed hy tlie removal of the gill-cover on the 

 left side, a First of the four bony arches which caiTy the gills (b &) ; &' The lower 

 edges of the gills ou the right side ; h Heart. (After Van der Hoeven.) 



gills or hranchice, in all except the Lancelet, in which respiration is 

 effected by branchial filaments placed round the pharynx, and also 

 by a greatly developed pharynx perforated by ciliated apertures 

 (fig. 171). The arrangement and structure of the gills in fishes vary 

 a good deal in different orders, and the leading modifications will be 

 noticed hereafter. In the meanwhile 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 cartilaginous leaflets, covered by mucous membrane, richly sup- 

 plied 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 (fig. 168). The branchial arches 

 and branchiae are suspended in cavities placed on the side of the 

 neck, and in the ordinary Bony Fishes there is only one such cavity 



