20 M. Georges Bohn on the Reversal of the fossorial forms, i. e. Gorystes and certainly others besides^ the reversal of the circulation becomes, on the contrary, accentu-ated, and renders possible the adaptation of the ambulant forms to this new mode of life. In the case of Carcinus mwnas it enables the animal to live amid unwholesome surroundings by the introduction into the branchial chamber of air at a pressure even higher than that of the atmosplierc *. III. — On the Reversal of the Respiratory Current in the Decapods f-By Georges Bohn \. The reversal of the current of water in the branchial chamber, which has long been known in the case of Corystes and was recently described by me in Carcinus mcsnas, Leach, and a certain number of other Decapods §, appears to be a pheno-menon of absolutely general occurrence in this group of Crustacea. Mr. Garstang lately published an account of it in Portumnus vasutus, Latr., and I have just proved its existence in twenty-one other species selected from the various families ||. I thought it would be interesting to note the frequency and the duration of the inversions in the different cases, and I now give the principal results that I have obtained. The frequency of the inversions varies little from one type to the other : most frequently they occur at the rate of two per minute ; in one and the same species the number appears to diminish in proportion as the size increases; thus, in the case of Carcinus mannas there are on the average ten inversions * The occurroucc of tliis revorfal appeared to me to have its bearing ou the comprehension of tlie adaptations that are found in the Decapods ; M. Bouvier, -^ho lias lavished his learaed advice upon me in the course of my investigations upon this group, of wliirh he has so â– wonderful a kno-wledge, pointed out a fact of the same kind a few years ago. lie explained the adaptatiim of these crustaceans to tenestrial life as being due to an ancestral anatomical and physiological condition — to wit, the svi'plemcntary circulator;/ si/ston (the special circulation of the carapace). t A study carried out at the marine laboratory of the Must^um at Saint-Vaast-la-IIougiie. I From the ' Cnniptes Rendus,' t. cxxv. no. 15 (October 11, 1897), pp. r,3y-.j4i\ § ' Comptes Rendus,' Sept. 13, 1897, np. 441-444 ; vide supra, p. 17. II I propose to return later on to tlie mechanism of the process of reversal; at present I will simply say that the principal n/'/c seems to devolve upon the acaphognathite, and that in certain types the carapace inteiTenes ; as for the cleansing organs (epipoditcs of the crabs, posterior limbs in Galathea, Sic), I have proved that they do not take any part in it. I
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