442 NOTES. 



the ^11- veins, direct jy into the ventricle o£ the heart. The other 

 species of this genus are not endowed with these mantle-gills. 



JVWe 71, page 171. Besides the two modes of respiration proved 

 by experiment to exist in water-animals (by the outer skin or by the in- 

 testine), yet a third mode of respiration seems sometimes to occmr. In 

 all mollusca, beyond a doubt, a certain amount of water is taken up into 

 the body and actually into the blood ; this certainly serves to dilate 

 the tissues — e.g. in the foot— but it is probably useful also for respira- 

 tion. But this is at present merely an assumption, founded on no exact 

 experiments. The ways in which water, whether fresh, or salt, is said 

 to penetrate to the blood are twofold. Many authors assert that it 

 takes place through the porea and the margin of the mantle; others 

 say that it must first pass through the. renal organs, which are never 

 absent from mollusca. After carefully weighing all the treatises on 

 the subject, even the most recent labours of Griesbach, I must declare 

 that neither the one nor the other is absolutely proved ; the second 

 hypothesis, however, seems to me, judging too from my own investiga- 

 tions, to be the more probable. Both perhaps may be correct, but here, 

 as in all physiological questions, experiment can alone supply' the 

 answer. 



Note 72, page 172. Comparing an eel with a gudgeon of equal 

 weight, the cylindrical form of the eel giving it a much greater extent 

 of surface, the gudgeon consumes in the same time — three hours — on an 

 average 13-8, and the eel only 7-i of oxygen (see Bert, Leqom mr la 

 Physiohgie comparee de la Sespiratian, 1870). His critical observations 

 on the popular, but erroneous, hypothesis that fishes which are tenacious 

 of life, as the eel, can live for a long time on land because their gills 

 are kept free by means of the water contained in the gill-sac, are weU 

 worthy of attention. 



Note 73, page 172. Many important observations have been made 

 as to the interesting phenomenon of intestinal respiration in CoMtis 

 fosdUs. This fish swallows the air, taking it in through its mouth, and 

 it is deprived of a portion of its oxygen in the intestine. Many other 

 fishes, however, do the same, as species of Cyprinus (see Note 75 

 below). Jobert has recently shown that various Brazilian fishes breathe 

 in the same way as the Cobitis, and even have in the intestine certain 

 processes or folds of the mucous membrane which seem especially 

 adapted to that end ; these are species of the genera Caliohthyi 

 (Siluridas), Doras, and Hypostomus. We might almost venture tq ask 

 .whether the Gypriiddm of European waters, when they take in air 

 through the mouth, do not send only a portion of it through the gills 

 and truly, swallow the remainder, so as to keep the mucous membrane 

 of the intestine directly supplied with oxygen. If we prevent the 

 species of Leuciscm from coming to the surface of an aquarium by 

 [lacing a wire net just below the surface of the water, so that they 



