ICHTHYOLOGY. 



679 



ST.'.-- : 



et tuho. 





Kune* of the F Proportion of Oxygen. 



Sparua sargus, female 0.09 



Ditto, mole O.iJO 



Holocentrus marinui 0.12 



Labrus turdus 0.1(5 



Spurns melanunis 0.20 



LJibrus turdus var. 0.24 



Scuena nigrs, female 0.27 



male 0.25 



Labras turdus, female 0.24 



' male 0.28 



Spams dentex, female 0.40 



Sphynena spet( Esox sphyra-na, Lin.) 0.44 



Sparus argenteus 0.50 



Holocentrus gigas 0.69 



Gadus merluccius 0.79 



Trigla lyra 0.87 



The depth at which the fishes had been caught in- 

 creases from the beginning to the end of the table, and 

 the proportion of oxygen observes the game rule. This 

 l.i-t circumstance induced Biot and his friend De Laroche 

 to endeavour to ascertain the proportion of oxygen con- 

 tained in sea water at different depths. They were un- 

 able to perceive any difference. M. Configliachi has 

 more lately repeated these experiments, and found that 

 the proportion of oxygen in sea water bore no relation 

 to the depth from which the water had been obtained. 

 These experiments lead to the conclusion, that the 

 air contained in this sac is a secretion of the organ ; 

 that in fishes which live near the surface azote is se- 

 creted ; but in fishes which live at great depths, the 

 quantity of oxygen is proportionally increased. The 

 pmrposes accomplished by this arrangement have never 

 been explained in a satisfactory manner. The red or- 

 gan which we have already taken notice of as existing 

 in tome fishes, is now generally considered as the part 

 which separates this air from the blood. But as this 

 organ is not always present when there is an air-bag, 

 we are still left in doubt on the subject. 



To the systematic ichthyologist, the characters fur- 

 nished by the air-bag are of considerable importance, 

 although seldom sufficiently attended to. They are 

 easily traced, and they are not subject to variation. 



To the economist, the air-bag or sound is considered 

 as an article of value. This organ in the cod or ling, 

 when salted, forms a nourishing and palatable article 

 of food, held in high estimation in the northern islands 

 of this country. But it is chiefly in the manufacture 

 of the substance called I*I>OLASS- that the sobnds of 

 Ashes are extensively employed. The sounds of va- 

 rious kinds of sturgeon are chiefly made use of for this 

 purpose. The external membrane is removed, and the 

 remaining part is cut lengthwise, ami formed into rolls, 

 and then dried hi the open air. The sounds of cod 

 and ling are frequently employed as a substitute for 

 those of the sturgeon. They require some dexterity 

 to separate them from the back-bone. But when the 

 membranes are well scraped on both sides, steeped for a 

 few mhrates in lime water to absorb the oil, and then 

 washed in dean water and dried, they form an isinglass 



iiiderable value. 



Isinglass consists almost entirely of gelatine, and is 

 ned either M food, or for the purpose of fining liquors, 

 50O grains of H yielded to Hatchet by incineration 1.5 

 grains of phosphat of soda, mixed with a little phosphat 

 rf lime. An inferior kind is manufactured from the 

 bone*, fins, and useless parts of ffohes. These mate- 

 rials are boiled hi water, the fluid skimmed and filtered, 

 i concentrated, until it readily gelatinizes 



Structur? 

 and 



Function* 



on cooling. This kind is much used in various manu- 

 factures, and might be obtained in considerable quan- 

 tity at all the fishing stations in this country where of p is]les 



the materials abound, but which are at present left as > - ' ^ 



a nuisance on the adjoining beach. 



6. ML-COUS DUCTS. The surface of the skin of MUCOUS 

 fishes is almost always covered with a slimy fluid, to ducts. 

 protect them from the penetrating influence of the 

 surrounding element. This mucus is poured out from 

 small pores, situated under the scales in every part of 

 the body of some fishes, while in others these excretory 

 ducts are arranged in a determinate order. These 

 ducts were first observed and described by Steno, in 

 his works, " De Miucnlit el Glandtilis, p. 42. and 

 Elementorum Myologice Specimen, 166Q, 8vo. p. 72. 

 The subject was afterwards investigated by Perrault, 

 Lorenzini, and Revinus, and more recently by Monro. 

 To this last author we are indebted for many excellent 

 observations and sketches. " In the skate," (he says. 

 Struct, and Phys. p. SI.) " numerous orifices, placed 

 pretty regularly over the surface, have been observed by 

 Steno to discharge this slimy matter. With respect to 

 these last, I have remarked some memorable circumstan- 

 ces. First, I have discovered one very elegant serpen- 

 tine canal between the skin and muscles, at the sides 

 of the five apertures into the gills. Farther forwards it 

 surrounds the nostrils ; then it passes from the under 

 to the upper -part of the upper jaw, where it runs 

 backwards as far as the eyes. From the principal part 

 of this duct, in the under side or belly of the fish, there 

 are not above six or eight outlets ; but from the upper 

 part near the eyes there are upwards of thirty small 

 ducts sent off, which open upon the surface of the 

 skin. The liquor discharged from these has nearly 

 the same degree of viscidity as the synovia in man. 

 But besides the very picturesque duct I have been de- 

 scribing, I have remarked on each side of the fish, a 

 little farther forwards than the five breathing holes, a 

 central part from which a prodigious number of ducts 

 issues, to terminate on almost the whole surface of the 

 skin, excepting only the snout or upper jaw. At 

 these centres all the ducts are shut ; and in their 

 course they liave no communication with each other. 

 In these two central parts, or on the beginning of the 

 mucous ducts, a pair of nerves nearly as large as the 

 optic, terminate ; and, which is a curious circumstance 

 with respect to them, they are white and opake in their 

 course, between the brain and their ducts ; but when 

 they divide, they become suddenly so pellucid, that it 

 is impossible to trace them farther, or to distinguish 

 them from the coats of the ducts." 



In the osseous fishes, the openings of the mucous Lateral line, 

 ducts are chiefly observable in the fore part of the 

 head, and in the lateral line. This line extends from 

 the head to the tail, along each side of the fish, and 

 exhibits several striking peculiarities. It is not ob- 

 servable in the larnpry ; in general it is single, but in 

 the sandeel it has the appearance of being double. It 

 is usually of a different colour from that of the sides, 

 and varies according to the species in position and di- 

 rection. After death it sometimes disappears, and 

 hence some difficulties have arisen with regard to the 

 discriminating marks which it furnishes. 



Tlie mucus which is poured out upon the skin by 

 these ducts, in some cases appears to be the liquid 

 known by chemists under that name, while in other 

 instances it appears to be of the nature of albumen. 

 Chemists, however, have not turned their attention to 

 the subject. 



