INTRODUCTION. 



xiu 



or intermaxillary. (4) The palatine bones. (5) The vomer. 

 mandible or lower jaw. (8) The branchiostegal rays. 



(6) The hyoid bones. (7) The 



Head of Pike. 



a, operculum. 6, suboperculum. r, preoperciilum. i?, interoperculum. c, vomer. /, iirffimaxiUary. g, palatine, h, maxillary, i, hyoid. 



k, branchiostegal rays hyoitl. /, mandible. 



Fish as a rule have their external integuments covered with scales, though there are fish 

 quite destitute of scales ; amongst the scaleless fresh-water fish may be mentioned the Miller's 

 Thumb and the Lampreys. Important characters may sometimes be drawn from the form of 

 the scales. Agassiz enumerates four kinds of scales, which he termed cycloid, ctenoid, placoid, 

 and ganoid. 



(i.) Cycloid scales (from kvkKo^, "a circle,") are thin scales more or less circular, with a 

 smooth margin ; they occur in most of our fishes. 



(2.) Ctenoid scales (from k.tu'^, Kreim, "a comb,") have their hinder margins cut into 

 comb-like spines, as in the Perch. 



(3.) Placoid scales (from -TrXa^, ttXo/co?, "anything flat and broad,") consist of detached 

 bony plates scattered through the skin; these scales are not unfrequently armed with pro- 

 jecting spines, as in the common Thornback Ray. 



(4.) Ganoid scales (from 7ai'os^ "brightness," "polish,") are generally much thicker than 

 other scales; they are often oblong or rhomboidal, or lozenge-shaped in form, and seldom 

 overlap one another, as in Lcpidosteiis, or Bony Pike of North America. 



In nearly all fish a peculiar line, called "the lateral line," is to be seen; this line 

 consists of a number of perforations in the scales, each scale having a pore with a minute 

 tube leading into a longitudinal canal, which has the power of secreting a mucus to lubricate 

 the surface of the whole body; a very desirable object, whereby the fish is enabled to dart 

 through the watery medium in which it passes its life. 



The digestive system in fishes consists of an cesophagus, a stomach, and an intestine. 

 The mouth is usually furnished with teeth, which present greater diversity in their mode, as 

 well as in their place of attachment, than is observable in any other class of animals. In 

 some fishes almost every bone of the mouth is provided with teeth, and even the tongue is 

 armed with these weapons; notably I may instance the teeth in the genus Esox (Pike), and 

 in that of Salnio. Everyone knows what a formidable dental armature the Pike possesses; 

 there are large and strong teeth of unequal size on the mandible, the maxillary is destitute 

 of teeth, but the premaxillary, the vomer, the palatine, and the hyoid bones are thickly 

 studded with cardiform* teeth, of which those of the palatines are the largest and disposed 

 most irregularly. So again, what an effective apparatus for seizing and retaining hold of a 



* Cardiform is from carduiis, "a thistle;" or more directly from the brush set with wire-teeth for "carding" wool, 

 cotton, etc. 



