48 SUPPLEMENT ON 



the branchiae ; but all other fish hare these branchiae free on 

 their external edge, and the water which has entered the 

 mouth is ejected by a single opening or gill on each side ; 

 the gills are of different sizes, and open more or less remotely. 

 Herrings have them large, and nearly encircling the head, in 

 eels they are small, and placed far back ; there are even some 

 species of this family, which have only a single aperture for 

 both gills. The operculum or gill plate, whose pulsations 

 serve in respiration, may vary in figure and size ; the termi- 

 nation of the membrane beneath may be united, in toto or in 

 part, with that from the other side, or with the part next the 

 shoulder. The number of rays supporting it may be more or 

 less considerable. Sometimes, as in diodon, &c. a great part 

 of this organ may be concealed by the skin, or altogether 

 wanting as in all the fish provided with several orifices. 



Some of the fins are vertical, constituting a kind of keel 

 and rudder : those on the back are named dorsals, behind the 

 vent and under the tail anals, and at the extremity of the tail 

 they form caudals. They differ in number, size, and the 

 nature of the rays which support them, being sometimes 

 spinous, and at others soft and articulated. Other fins are 

 double or form pairs, and represent the four members of other 

 classes of animals. Those corresponding to arms or wings 

 are the pectorals, and are invariably fixed behind the gills ; 

 but those which occupy the place of feet, named ventrals, 

 may be placed either forward beneath the throat, or more or 

 less backwards to the commencement of the tail ; both may 

 differ in size, in the quality of the rays, in their number and 

 structure, one or both pairs may be wanting. Eels, for ex- 

 ample, have no ventrals, munenae, have neither ventrals nor 

 pectorals ; and apterichti have no fins at all. 



Malacopterygians are such fish as have all their fins sup- 

 ported by articulated rays, and acanthopterygians those whose 

 fins are in part simple and spinous ; but some malacoptery- 



