ICHTHYOLOGY. 547 



whose branches hang into a pond, pass over to the opposite 

 branches and drop into an adjoining stream, and thus migrate to 

 far distant waters. The trees at such times appeared to be quite 

 alive with the eels. (Gosse.) Eels descend rivers to spawn in the 

 brackish waters of estuaries. Salmon ascend rocky rivers from 

 the sea, leaping cascades and overcoming various obstacles to de- 

 posit their eggs in fresh water. 



The form of the fishes is decidedly the one best adapted to facili- 

 tate progression through such a medium as water, being commonly 

 that of a spindle, swelling in the middle and tapering to each 

 extremity. To this, however, there are exceptions. The 

 Skates, JRaiidce, and Flat-fishes, Planida, are flattened horizon- 

 tally; the Chaetodons, or Hair-Tooths, Chaetodontidce, and the 

 famed Dories, Zeina, (Gr. from Zeus, Jupiter,) a branch of the 

 Scombridce, (Lat. scomber, a mackerel,) are flattened vertically ; 

 some are of a globe-like form, as the Diodon and the Sun-fish, 

 Orthagoriscus, (Gr. a sucking-pig :) some of serpent-like form, 

 as the Eels and Lampreys, Petromyzon, (Gr. stone-sucker ;) and 

 some, as the Ribbon-fishes, Cepolida, resemble in length and 

 thinness the fabric after which they are named. 



The organs of motion in this class are fins. These have 

 the form of a delicate membrane, investing a series of bony 

 or cartilaginous rays, and which is more or less transpar- 

 ent. These rays are slender bones, consisting, in some cases, of 

 a single piece, stiff and spinous ; in other instances, they are 

 made up of several pieces jointed together, and hence flexible ; 

 the latter are frequently divided each into two or more branches 

 at the tip. The bony character of the fin rays affords a basis for 

 two of the orders, viz. : ACANTHOPTERYGII, Spiny-finned Fishes, 

 and MALACOPTERYGII, Soft-finned Fishes. 



The fins of Fishes are of five kinds, which have received 

 their names from their position upon the body, viz. : (1) the 

 dorsal or back fins, (Plate XII. fig. 3a,) usually single, but some- 

 times divided into two or three fins, at varying distances from 

 each other ; (2) the caudal or tail fins, (c.) which in the true 

 fishes are vertical, but in the fish-like mammalia are horizontal ; 

 (3) the anal or vent fins, (d ;) (4) the pectoral or breast fins, (f;) 

 (5) the ventral or belly fins, (e.) 



The pectorals and ventrals are arranged in pairs, and corres- 

 pond to the fore and hind limbs in other vertebrate animals ; the 

 pectorals, for instance, representing the wings in birds ; the ven- 

 trals the feet. The dorsal or medial fin aids in keeping the body 

 in a perpendicular position in the water; scarcely any fishes are 

 without this fin, many have two dorsal fins, and a few, as the 



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