CLASSIFICATION AND DISTRIBUTION OF FISHES. 27 



divided into those having the extremities of the gill-filaments 

 unattached (as in Osseous Fishes) ; and those having them 

 fixed. The former group constitutes but a single order; but 

 the latter is again subdivided into those which have the jaws 

 moveable and adapted for mastication ; and those which have 

 them soldered into a ring for suction. The latter are termed 

 Cyclostomi, or circular-mouthed Fishes. 



553. The following Table will perhaps enable this arrange- 

 ment to be more easily understood. "We shall presently find, 

 however, that there are one or two small groups not included in 

 it, which are separated from the rest by characters so . remark- 

 able, as perhaps to entitle them to be ranked as distinct orders. 



C Acanthopterygii^ spiny-rayed Acanthopterygii. 



o_*J f*S; ^ 



\_Matacopterygii, soft-rayed-' ventral fins beneath 



shoulder . . . . J 

 V. without ventral fins Apoda. 



{gills free Sturiones. 

 n fi A Jj aws moveable . Plagiostomi. 

 fixed 1 jaws soldered . . Cyclostomi. 



554. The class of Fishes is probably the most numerous of 

 the whole Vertebrated series, both in regard to the number of 

 families, genera, and species which it contains, and in regard to 

 the number of individuals of the same species. The structure of 

 a very large proportion of it has been but very imperfectly 

 investigated; and of the habits of these animals still less is 

 known. Instead, therefore, of giving a technical description of 

 every family, such as will be found in Systematic Treatises on 

 Zoology, we shall confine ourselves to a notice of those groups 

 which are of most general interest, either on account of pecu- 

 liarities in their structure, or their importance to Man. It has 

 been calculated that, exclusive of lakes and rivers, the seas 

 occupy full seven-tenths of the earth's surface. These seas yield 

 food even to the depth of several hundred feet ; and, as there is 

 no obstacle in the water to bar the movements of Fish, we see 

 that their pasture-grounds are almost unlimited in extent. 

 Different tribes are formed to inhabit different situations ; thus 

 we have some Fish adapted to live in fresh-water only, others 



