268 FISHES. 



its aperture ; in the Herrings it is enormous, and is continued round 

 the whole side of the head : in the Eels it is rather small, and placed 

 backwards ; some species of this family, the Synbranchiie, have no 

 more than one opening for the two gills. 



The operculum, which, by its motion, is an agent in respiration, 

 varies both in its size and shape : the membrane which completes it 

 beneath, is joined either wholly or partially to its fellow of the oppo- 

 site side, or to that portion of it which is adjacent to the shoulder. The 

 number of rays which support it is more or less considerable ; some 

 times as in the Tetrodons, the Diodons. and the Ostracions, a large 

 portion of this apparatus is concealed beneath the skin, and it can 

 only be seen by dissection ; it is altogether wanting in those small 

 species which have numerous orifices. 



A part of the fins are vertical, and become to the fish what the 

 keel or rudder is to the ship ; they are fixed either to the back, when 

 they are called Dorsal, or beneath the tail Avhen they are called Anal, 

 or at the end of the tail when they are called Caudal ; they vary 

 in the number, in the height, and in the structure of the rays which 

 support them, and which, in some cases, are spinous, and in others, 

 branched and composed of many small joints. Other fins are ar- 

 ranged in pairs, and represent the four extremities of the higher 

 classes. Those fins which correspond with the arms and wings, are 

 called pectoral fins, being always attached behind the gills; the fins 

 which answer to the feet are called ventral fins, and may be placed 

 more or less forward or behind, from the lower part of the throat to 

 the origin of the tail. Both the fins are variable in their size re- 

 spectively, in the number of their rays, in the circumstance of their 

 being a simple or a jointed structure. Sometimes one of the pairs, 

 sometimes the two pairs are altogether wanted ; the Eels, for exam- 

 ple, have no ventral fins, and the Murpena not only have no ventral, 

 but they have no pectoral fins : the Apterichtes have no fin at all. 



We call those osseous fishes, the rays of whose fins are articulated 

 or jointed, Malacopterygians; and the name of Acanthoptery- 

 gians is given to those fishes which have the fins partly simple, and 

 in the form of spines ; but in some Malacopterygians, as the Carps 

 and Siluridee, the soldering of the articulations gives to certain of the 

 rays, the appearance of spines. The anus is sometimes very far back 

 behind the ventral fins ; it approaches them in some fishes or gets 

 before them, and when the fins do not exist at all, it is found to have 

 its opening just under the throat, as may be seen in the Sternarchus. 

 In certain species, as the Gobies and Blennies, the anus has, behind it, 

 a tongue-like body which would appear to have something to do with 

 generation, but it cannot be a penis, inasmuch as it is common to 

 the two sexes ; it is not to be found, however, in the greater 

 number of other fishes. 



The whole of the distinctions to which we have now been ad- 

 verting, belong to the intimate structure, to the frame- work of the 

 fishes ; there are other distinctions of a more superficial kind. 



The jaws are armed with teeth of all sorts, and these are sometimes 

 placed in all parts of the mouth and even in the gullet. 



The lips are furnished with appendices or cirri, which differ either 



