44 2 MOTEP. 



the gill-veins, directjy into the ventricle of the heart. The other 

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



Nute 71, page 171. Besides the twc 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 occur. In 

 all moUusca, 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 pores and the margin of the mantle ; others 

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

 absent from moUusca. 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, jMge 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 7-4 of oxygen (see Bert, Leqona sur la 

 Physiologie compared de la ResjnTation, 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 well 

 worthy of attention. 



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

 as to the interesting phenomenon of inteslinal respiration in Cohitis 

 fossilis. 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 Cijprimis (see Note 75 

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

 in the same way as ihe Gobitis, and even have in the intestine certain 

 processes or folds of the mucous membrane which seem especially 

 adapted to 1 hat end ; these are species of the genera C'alichthys 

 (Siluridfu), Ik'ras, and Jli/postomus. We might almost venture to ask 

 whether the Cgprliiidce of European waters, when they take in air 

 ihrough 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 Leuciscus from coming to the surface of an aquarium by 

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



