ResjJi'ration o/Cai*cinus maenas, Leach. 19 



experimentally, and have proved that this is the case by 

 mixing carmine with the water in order to show the respi- 

 ratory currents. 



Now here are facts that remind us of the phenomena which 

 take place in the case of Corystes, the crab which buries 

 itself in the sand during the day, and the mechanism of 

 whose respiration was recently the subject of a pleasing 

 memoir by Mr. Garstang, of Oxford ; the reversal, says the 

 autlior, takes place during the day — that is to say, when the 

 crab is buried. Mr. Garstang did not seek the reason of this 

 reversal, and left the fact isolated, without explanation. The 

 physiological explanation that I have just given with reference 

 to Carcinus mcenas evidently applies to Gorystes. I would 

 add that the act of reversal is not jjeculiar to these two species 

 of crabs ; everywhere where I have sought for it among 

 crabs I have found it — in Portunus, where, as in Carcinus^ 

 there is a medium amount of differentiation, and in Hya or 

 Maia^ which are so highly specialized and have a mode of 

 life so different from the foregoing; in the case of these latter 

 examples the periods of reversal never last longer than a few 

 seconds *. Since in this instance the reversal cannot be 

 explained by the mode of life, I sought for the origin of it 

 among the ancestors of the crabs or their near relations. I 

 have observed it in Palcemon, as also in the 2Iegalopa-\avv9Q 

 and in the crayfish. When a prawn is placed in water 

 charged with carmine, from time to time jets of water are 

 seen to issue from the inferior margin of the carapace, from 

 the spot at which inspiration usually takes place ; at 

 these moments, owing to the reversal of the movement of 

 the scaphognathite, there occurs, within the branchial 

 chamber, a regular rush of water from the front to the rear, 

 which ejects all the foreign bodies with which it is encum- 

 bered. The same thing happens in the case of the crayfish. 

 The reversal of the circulation of the water is therefore 

 phylogenetically a very ancient fact, and it should be met with 

 in the Peneida, just as in the prawns and lobsters, their descen- 

 dants; in the natant, as in the ambulant, forms the reversal 

 has no other object than to produce currents of water in a 

 direction opposite to that of the normal stream, in order to 

 cleanse the branchial chamber. In the case of crabs like 

 MaiQf where the cleansing process is performed by means of 

 other mechanical arrangements, the reversal is met with again 

 as a survival (" comme un souvenir ancestral "), but in the 



* But in none of these crabs does the reversal of the motion of the 

 scaphognathite produce spontaneously the entrance of air into the 

 briiui.hial chamber. 



2* 



