300 LIFE BY THE SEASHORE. 



correspond to the fore and hind limbs of a terrestrial ver- 

 tebrate, one would naturally expect that the pectoral pair, 

 which are equivalent to the fore limbs, should lie in front 

 of the pelvic pair. This is the case in many fish, but in 

 the cod family they have been shunted forward till they 

 actually lie in front of the pectorals (see Fig. 85). This 

 shifting seems to be associated with the general moving 

 forwards of the organs of which we have already spoken. 

 As regards the unpaired fins we have in the saithe three 

 dorsals on the back, and two anals on the ventral surface 

 behind the anus, in addition to the tail fin of which we 

 have already spoken. 



In regard to the other characters, the gills are of special 

 importance. In a living fish there will be noticed a flat 

 plate, or operculum, behind the mouth on either side, which 

 is constantly opening and shutting. It is easy to observe 

 that water is constantly entering by the open mouth, and 

 leaving by the opening at the side of the throat which is 

 disclosed when the operculum is raised. A more careful 

 examination will show that internally the sides of the 

 mouth are perforated (usually) by five clefts, bounded by 

 bony arches bearing red gill-filaments. Externally these 

 openings are not obvious, as they are covered by the 

 operculum, beneath whose posterior margin the water taken 

 in by the mouth escapes. As the water passes out it purifies 

 the blood contained in the gills, so that the mouth-cavity, 

 or pharynx, of the fish, like the pharynx of a Tunicate, has 

 a respiratory function, as well as its nutritive one. Other 

 important peculiarities are the teeth, not confined to the 

 margin of the jaws, but also found on the walls of the 

 mouth-cavity, and the "lateral line" a series of superficial 

 sense-organs which run down the sides of the body, forming 

 a conspicuous black line in the haddock, a pale one in the 

 saithe. The scales should of course also be noticed, and 

 the flat, lidless eyes, so arranged as not to interfere with the 

 general curve of the body, and so offer no resistance to the 

 passage through the water. Into the anatomical details of 

 structure we cannot go, but the external form and the move- 

 ments are worth careful study, and your appreciation of the 

 graceful movements will probably increase as you learn more 

 of the mechanical adaptations which render them possible. 



