STUDY OF FISHES 



143 



water passes out by the spiracle. Senior pupils cannot fail to 

 be interested in the vestigial gill, which lies on the wall of the 

 spiracle. It is a remnant of a gill that was large enough to be 

 of use in the ancestors of the present-day gristly fishes. The 

 idea of a vestigial organ may be illustrated by referring to the 

 useless letters in many words, e.g. the o in leopard, or to the often 

 useless buttons, etc., on the cuffs of a man's coat and on other 

 places. (5) Hanging down from the front of the lower jaw in 

 the cod there is a sensitive barbule, and small pore-like openings 

 on the jaws and other parts of the head are also sense-organs, 

 in part tactile. As an illustration of the frequently minute 

 differences between related species, it may be shown that the cod 

 Gadus morrhua has a long barbule ; the 

 haddock Gadus ceglefinus has a short one ; 

 the whiting Gadus merlangus has none. 



(g) The Gill Region. In a dogfish there 

 are five gill-clefts opening from the pharynx 

 on each side of the neck. They are separated 

 from one another by complete partitions, 

 and these partitions bear numerous folds 

 or plates. All the lamellae or plates on one 

 side of a partition constitute half a " gill." 

 Where the gill-cleft opens into the pharynx 

 there is a strong arch of gristle (the gill- 

 arch), the outer convex edge of which 

 bears a fringe of gristly rods strengthening 

 the partition. 



The state of affairs in bony fishes is 

 very different, (i) In a bony fish, such as 

 a haddock, the gill-clefts open into a gill- 

 chamber formed by a movable fold or flap 

 called the gill-cover. The gill-chamber 

 opens to the exterior by a slit between the 

 free margin of the gill-cover and the body- 

 wall. (2) The partitions between the several 

 gill-clefts have practically disappeared. 



(3) The gill consists of a double series of triangular filaments 

 projecting from the convex outer margin of the gill-arch. 



FIG. 51. A section across 

 the gill of a bony fish, 

 showing two triangular 

 gill-filaments (g-f-} sup- 

 ported by a gill -arch (g.a.}. 

 Each gill bears a double 

 series of these filaments. 

 The " impure " blood 

 from the heart ascends 

 one side of each filament, 

 and after "purification" 

 descends along another 

 line 



