INTRODUCTION xxi 



the Perch and Ruffe. In the Bull-head scales are 

 absent, in the Eel they are quite abnormal, and in the 

 Stickleback replaced by bony scutes. The muciferous 

 channels on the head are continued on the body as 

 the lateral line, which appears as a series of tube- 

 bearing scales, usually running to the base of the 

 caudal fin ; sometimes the lateral line is incomplete 

 (Smelt, Minnow), sometimes it is absent (Shad). Each 

 tube of a lateral line scale opens to the exterior 

 behind, and in front communicates with a continuous 

 longitudinal canal ; this contains a series of sense 

 organs, which are probably concerned with the per- 

 ception of movements in the water. 



The fins are formed of rays connected by membrane, 

 and may be divided into median fins, comprising the 

 dorsal, on the back, the caudal, at the posterior end 

 of the fish, and the anal, on the lower edge of the tail 

 between the vent and the caudal fin, and paired fins, 

 pectorals and pelvics, corresponding respectively to 

 the anterior and posterior limbs of other vertebrates. 



The fish swims by lateral flexions of the tail, aided 

 by movements of the caudal fin, and, as a rule, the 

 other fins chiefly function in keeping the fish upright, 

 or assist in turning movements, or in slowing down ; 

 however, some fishes normally progress by paddling 

 with the pectoral fins, and in the Flat-fishes and Eels 

 undulating movements of the dorsal and anal are of 

 importance. 



In the more generalized Teleosteans (Isospondyli, 

 Haplomi) Ostariophysi) the fins are composed of 

 flexible articulated rays, the anterior of which may 

 be simple, whilst the majority are branched ; the 

 dorsal and anal are distinct from the caudal, the 

 pectorals are placed low, just behind the head, and 



