420 



VERTEBKATA. 



l.y thfir takii.i,' til.- w.it.r in at tin- mouth au-l .xixlliMj,' it from under the gill-covers. The 

 l>i».o<l, whi.-li is" const ant Iv .s.iit to tl.<- hranchiic from tlic heart, is distributed by means of the 

 art.Ti.'-s to rviTv part oftl..- \«>A\\ whence it returns tu the heart by means of the veins. 



As till- breathini; apparatus in the fish is suited to jupiatic habits, so likewise is every part of 

 its strurture. Thr bo.lv is tr.'ii. -rally .)f an elon_t,'atc, oval, compressed form, covered with scales 

 din-.-t.-.l ba.-kwar.l, an'Tfurnishcl with fins, thus being beautifully adapted for swimming. Many 

 Hshcs, moreover. iia\.- an <itr-li/(n/,/< r tilled with air. situated iiiinu'diat.-ly beneath the spine, by 



,),,. ,liiat.-iti <r eompr.-ssi.ui of whieh tlieir specitic gravity is said to be varied. The thoracic 



iiart ."f tin- bo.ly is thrown f.u-ward toward the head, so tliat the fishes may be said to have no 

 neck, an. I thus tlie hinder part of tlu- body is more free and fitted for motion. The lind>s arc 

 formed int.> fins, the f.)re-legs coustituting what arc termed the pectoral fins, and the posterior 

 extremities the ventral \ besides these fin.s, ordinary fishes are furnished with one or two dorsal 

 /ins, an anal fin, and a caudal ^fin, or tail. 



SKELETON OF THE PERCH. 



All these fins arc not always present, nor when present are they always in the same relative 

 positions ; both the absence of certain fins, and the peculiar position of these organs, affoi'd 

 characters in the classification of fishes. The fins consist of a thin elastic membrane supported 

 by rays. The rays arc of two kinds : those which consist of a single bony piece, usually hard 

 and pointed, are termed spinous rays ; and when they are formed of numerous portions of 

 bone uiuted by articulations, and frequently divided longitudinally into several filaments, they 

 are called flexible rays. The principal organ of motion is the tail ; the dorsal and ventral fins 

 apparently sei've to balance the fish, and the pectorals to arrest its progress when re<]uircd. 



The bones of fishes are of a less dense and compact nature than in the higher order of animals ; 

 in some, indeed, they are wholly cartilaginous. The skeleton may in general be divided into 

 four chief parts — the Vertebral Column, the Head, the Respiratory Apparatus, and the Limbs. The 

 Vertebral Column consists of vertebrjc which are concave at each end and pierced in the middle ; 

 and when joined together the hollow space between each two is occupied by a gelatinous sub- 

 stance, which passes from one space to the next through the hole in each bone. This hole is 

 usually very small, but in some it is so large that the bones of the vertebrae are mere rings. 

 To the vertebrae are attached the Ribs ; in fact the ribs are the main support of all the other 

 bones. The Head varies more in form than in any other class of vertebrate animals. The same 

 bones as those found in other oviparous animals are almost always traceable. The gills are fur- 

 nished with an Operculum or Gill-cover, on each side, the function of which is to close the 

 aperture in case of need, and thus protect those delicate and important organs. There are several 

 bones adapted to this particular organization. 



The teeth in fishes are entirely osseous, and are usually of a simple spine-like form. The 



