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 ■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 



