20 M. Georges Bolin 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 

 mcena^ 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 atmosphere *, 



]II, — On the Reversal of the Respiratory Current in the 

 Decapods]. By Georges BoHNt- 



The reversal of the current of water in the branchial chamber, 

 which has long been known in the case of Gorystes and was 

 recently described by me in Carcinus mcenas, 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 nasutus, 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 manas there are on the average ten inversions 



* The occurreuce of this reversal appeared to me to have its bearing 

 on the comprehension of the adaptations that are found in the Decapods ; 

 M. Bouvier, ■v^'ho has lavished his learned advice upon me in the course 

 of my investigations upon this group, of vs'liich he has so wonderful a 

 knowledge, pointed out a fact of the same kind a few years ago. He 

 explained the adaptation of these crustaceans to terrestrial life as being 

 due to an ancestral anatomical and physiological condition — to wit, the 

 supplementary circulatory system (the special circulation of the carapace). 



t A study carried out at the marine laboratory of the Museum at 

 Saint- Vaast-la-Hougue. 



\ From the 'Comptes Rendus,' t. exxv. no. 16 (October 11, 1897), 

 pp. 539-542. 



§ ' Comptes Rendus,' Sept, 13, 1897, pp. 441-444; vide su2)ra, p. 17. 



II I propose to return later on to the mechanism of the process of 

 reversal; at present I will simply say that the principal role seems to 

 devolve upon the scaphognathite, and that in certain types the carapace 

 intervenes ; as for the cleansing organs (epipodites of the crabs, posterior 

 limbs in Galathea, &c.), I have proved that they do not take any part 

 in it. 



