FI8HE8. 233 



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

 are believed to represent the panci-eas. In some tishes, howevei-, 

 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 near the pylorus to the anus. The 

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

 Lancelet it is doulitfully 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. 



Eespiration in all fishes is aquatic, and is effected by means of 



Fig. lOS. — Gills ami he.art of the Perch exposed hy the removal of the gill cover on tlie 

 left side, a First of the four lioiiy arctics vi'liieli c^rry tlic pills {b h); 1/ The lower 

 edges of the gills on the right side ; h Heart. (After Van dei- Jl..c\'eii.) 



gills or hranchia;, in all excei)t the Lancelet, in wliicli le.spiration is 

 effected by branchial filaments placed round the ])harynx, and also 

 by a greatly developed pharynx jierforated by ciliated apertures 

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

 a good deal in different orders, and tlie 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 sucli 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 (/i_?/o((/ bone) below and with the 

 under surface of the head above (fig. 168). The branchial arches 

 and branchiaj are suspended in cavities placed r>n the side of the 

 neck, and in the ordinary Bony Fi.shes there is only one such cavity 



