78 STRUCTURE OF FISHES. 



represent the extremities of other vertebrate animals, the ante- 

 rior extremities which correspond to the arm in man, and the 

 wing in birds, are fixed on each side of the trunk, immediately 

 behind the -head, and are called pectoral fins, (Fig. 41, a.) The 

 abdominal extremities (6.) less distant from each other, generally 



occupy the inferior 

 Fig, 41.* face of the body> 



and may be placed 

 more forward or 

 backward, from be- 

 neath the throat to 

 the origin of the 

 tail : they are called 

 ventral fins. The 

 single or unpaired 

 fins occupy, as we have just said, the middle line of the body, 

 and are distinguished into first dorsal, (c.) second dorsal, (d.) 

 anal, (e.) and caudal fins, (/.) according to their situation on the 

 back, under the tail, or at its extremity. They are all nearly of the 

 same structure, and almost always consist of a fold of the skin, 

 sustained by bony or cartilaginous rays, very much in the same 

 manner that the wings of bats and dragons are sustained by the 

 fingers or toes, or by the ribs of those animals. 



3. We also observe on the external surface of the body, large 

 slits placed, on each side, immediately behind the head, which 

 serve as an outlet to the water which has laved the branchiae : 

 they are openings of the gills. Generally, there is but one on each 

 side, and their anterior edge is moveable, and resembles a shutter. 

 Along the whole length of the body, on each side, there is a series 

 of pores which form what ichthyologists call the lateral line. 



4, The skin is sometimes nearly naked, but is almost always 

 covered with scales. Sometimes these scales are in the form of 

 rough grains ; sometimes they are very stout tubercles, or plates 

 of considerable thickness ; but, in general, they are very thin 

 lamellae, covering each other like shingles or tiles, and let into 

 folds of the skin. They may be compared to our nails ; though 

 they contain more calcareous salts. The colours with which these 

 animals are adorned, are astonishing in their variety and bril- 

 liancy. Sometimes they can only be compared to the most glitter- 



* Explanation of Fig. 41 A common Perch, a. the pectoral fin of one 

 Bide, b. ventral fin, c. first dorsal fin, d. second dorsal fin, e. anal fin, 

 /. caudal fins. 



3. What are the gill openings ? What is their use ? What is meant by 

 the lateral line ? 



4. What is the character of the skin ? What is the nature of scales? How 

 do they obtain their colour ? 



