FISHES. 345 



in promoting the current of water through the gills, or per- 

 haps forms a current over the gills, when the mouth is oc- 

 cupied in seizing prey. 



The gill-flap furnishes to the systematic ichthyologist 

 some of his most useful characters. He seldom pays atten- 

 tion to its form, but its rays are eagerly counted, as he 

 finds that they are not subject to vary. Species of the same 

 genus have, in general, the same number of rays, and 

 many of the Linnaean genera depend on this circumstance 

 for their character. But in counting their number, care 

 must be taken to examine the structure of the gill-lid at 

 the same time, as the student sometimes enumerates, 

 among the rays of the gill-flap, the posterior divisions of 

 that organ, when present, and hence finds his observations 

 at variance with the descriptions of authors. 



The gill-opening in the osseous fishes, and among the 

 branchiostegi, is a simple aperture behind the gills on each. 

 It is sometimes round, or semi-lunar, and in relative posi- 

 tion it differs according to the species or genera. In the 

 cartilaginous fishes, the opening on each side is subdivided 

 into as many apertures as there are gills, the gills in this 

 tribe being fixed to the membranes which act as partitions 

 in the opening. In such fishes, these openings are on the 

 summit, at the sides, or underneath, according to the ge- 

 nera. 



In the fishes with free gills, these organs are in general 

 eight in number, four on each side. Each gill consists of 

 three parts, a cartilaginous or bony support, and its convex 

 and concave sides. The support of each gill consists of a 

 crooked bone or cartilage, in general furnished with a joint. 

 At its base, it is united with the bones of the tongue, and 

 above with those of the head. At both extremities it is 

 moveable, and throughout is flexible like a rib. Its posi- 

 tion is nearly vertical. From its exterior or convex side. 



