ON THE NATURAL ORDERS. 101 



that it is somewhat difficult to find any more certain 

 indication than that presented by their fins : the ventral 

 fin, however, is always present ; and the branchial aper- 

 ture, with one or two exceptions, is unconfined. We 

 thus get three characters ; one of which separates this 

 order from the last, another detaches it from the next, 

 and. the osseous skeleton from all other divisions. 

 Their organisation, as fishes, appears less perfect than 

 the more typical group ; for it is among these we find 

 all the ground fish, — those which are restricted to fresh 

 waters, and such as lie in wait for their prey. In this 

 order, also, we have a small group of viviparous fish, 

 analogous to the blennies in the last. The salmons, 

 pike, herrings, cods, carps, and flat fish, have been 

 justly included in this order, v/hich, in regard to the 

 subsistence it furnishes to man, becomes the most im- 

 portant of all others. 



(88.) In the next order, the typical structure begins 

 to disappear, and is finally lost. The skeleton, in some, 

 is still osseous ; but in m^any others is sub- cartilaginous ; 

 and even finally becomes membranaceous: the fins, which 

 represent the feet, entirely disappear : the branchial 

 aperture assumes the form of a slit, and is termed a spi- 

 racle : the shape is long, and like that of a serpent : the 

 dorsal, anal, and caudal fins, when all are present, are 

 generally united : the body is slimy and naked ; or the 

 scales are very minute, and imbedded in the cuticle. 

 The reader cannot fail to recognise, in this description, 

 the essential characters of the eels, lampreys, and other 

 similarly formed families, which have as much the 

 outward aspect of serpents as of fish. To this order 

 we retain the original name of Apodes bestowed upon 

 it by Linnaeus. 



(89.) Having entirely quitted the osseous structure 

 of bone in the last tribe, we next come to such families 

 as have their skeleton fibro- cartilaginous: these, also, 

 breathe by a spiracle ; the operculum being either obso- 

 lete, or entirely concealed beneath the common skin. 

 They differ, however, materially from the last, by pos- 



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