COMPARATIVE ANATOMY. 



105 



AVhon Stono first examined tho tooth of the shark, he WM 

 od to find a great number of tooth placed on tho inside 

 of each jaw, lying close to tho bone. From their position and 

 arrangement he thought thoy were useless. Hcrixsant after- 

 wards showed their use by proving that as tho anterior to'Ui of 

 : >w are broken off, drop out, or wear away, the posterior 

 ones oomo forward to supply their place. This act of renewal 

 seems to be persistent during life. In most fishes the tenth arc 

 constantly changing, tho older ones being succeeded by others 

 .Irvi'lopod in tho neighbourhood. Some fishes for example, 

 tho sturgeon are entirely toothless. 



kept apart by moans of an intervening clastic substance ; Ilrer, 

 large ; spleen, invariably present ; and kidneys likewise. 



The breathing apparatui consists of a number of loose fringes 

 or gills suspended in cavities, and attached to bony or cartilagi- 

 nous arches ; three or four, or more, being fixed on each side of 

 tho nock. Mattencci estimated the surface of the gills of t!io 

 common ray to measure 2,250 square inches. The cavity in 

 which the gills are suspended communicates both with the mouth 

 an<l tho outer surface of tho body. The water is taken into the 

 mouth, forced through tho inner aperture of the gill cavity, 

 whore it comes into contact with the gill fringes, bathing them 



L DIAGRAM OF THE GENERAL ARRANGEMENT OF THE VISCERA OF A FISH. II. DIAGRAM OP ARRANGEMENT OF THK VISCKKA OF A HERRING. 

 III. DIAGRAM SHOWING THE Fins OF A FISH (AFTER KNOX). IV. DIAGRAM SHOWING THE PERPENDICULAR POSITION OF THE UPPER 

 TEETH, AND THE RECCMBENT POSITION OF THE LOWER TEETH IN THE JAWS or THE SHARK. V. VERTEBBJE OF A FISH (AFTER BTXEB 

 JONES). VI. DIAGRAM SHOWING DIVIDED GILL OF A FISH. VII. CAPILLABT NET-WORK OF A PAIR OF LEAFLETS OF THK GILLS or AM 

 EEL (AFTER CARPENTER). 



Bef. to Nos. in Figs. I., II. 1, orary; 2, air-bladder; 3, oesophagus or gullet; 4, stomach ; 4,', pyloric caeca; 5, 5, intestine; 6, liver; 7, ovi- 

 ducts; 8, anus; 9, spleen; 10, heart; 11, kidney and urinary passage. III. 1, 2, the first and second dorsal fins; 3, ventral fin; 4, anal 

 fiu ; 5, pectoral fin ; 6, feelers ; 7, caudal fin. V. 1, body or central piece ; 2. spinous process ; 3, inferior spinous process ; 4, lateral or 

 transverse processes ; 5, nerve-processes and arch ; 6, hssmal processes and arch. VII. 1, 1, branches of the branchial artery convey- 

 ing venous blood ; 2, 2, branches of the branchial vein returning aerated blood. 



Tho alimentary canal consists of a simple tube, which passes 

 nearly straight through tho body. The gullet is short and 

 muscular, and the stomach largo, separated from tho intestine 

 by a small valve or curtain. Sometimes the stomach, as in tho 

 herring (Fig. II., 4'), presents a series of tubular prolongations, 

 which terminate in blind extremities. 



The inner membrane of the shark's intestine is arranged in 

 deep spiral folds, which wind from end to end. The compart- 

 ments between the spiral layers communicate through a small 

 aperture in the centre of each valvular projection. Tho object 

 of this spiral arrangement is to increase the surface over which 

 the nutrient material of tho food has to pass. The valves are 



freely, thus aerating the blood which circulates through tho 

 minute blood-vessels of the leaflets (Figs. VI., VII.). The water 

 is then expelled through tho outer aperture, which is guarded by 

 a valvular curtain. 



Most fishes possess an air-bladder, which frequently com- 

 municates with the gullet or stomach. It is furnished with a 

 muscular apparatus to regulate its capacity, so as to increase or 

 diminish the specific gravity of tho animal. Some anatomists 

 consider it to be homologous to the lungs. Its principal use, 

 however, is, as Willonghby long ago (1686) pointed out, to bring 

 their bodies to an equilibrium with the element in which they 

 swim, to enable them to impel or move themselves in any direo- 



