18 M. Georges Bolm on the 



behind forwards, to issue by orifices situated on the anterior 

 margin of the buccal framework, and that the current is 

 occasioned by the scaphognathite of the jaw *. In fact, this 

 is what is observed under ordinary conditions in crabs. In 

 Carcinus mcenas, in consequence of a modification of the 

 movements of the scaphognathite, the direction of the current 

 may undergo a change f. 



The observation may easily be repeated. All that is 

 necessary is to place a crab in a crystallizing- pan containing 

 just sufficient water to cover it ; as a rule the crab quickly 

 raises itself on the posterior part of its body in such a way as 

 to cause its front and the so-called expiratory orifices to 

 emerge. Then the water is sometimes seen to continue to 

 issue by these orifices; sometimes, on the contrary — and this 

 occasionally during long periods of time — bubbles of air pass 

 out in a continuous fashion from the so-called inspiratory 

 orifices which have become expiratory : the air naturally 

 enters by the orifices raised above the water. During this 

 latter period the epipodites may remain motionleas, and I 

 have succeeded in observing the movement of the scapho- 

 gnathite undergo a modification. Thus, then, through the 

 instrumentality of the scaphognathite, a direct circulation of 

 water is succeeded by an inverse circulation of air. Without 

 doubt a portion of the oxygen in the air is absorbed j ust as it 

 is, while another portion is dissolved in the water still con- 

 tained in the branchial cavity. 



The experiment succeeds all the better when the crab is 

 immersed in water which is less fit for respiration. The 

 reversal of the circulation of the water has a notable physio- 

 logical advantage for the animal : instead of fatiguing itself 

 to no purpose in causing the circulation in the branchial 

 chamber of water which is badly aerated, or in any way 

 insufficient for respiration, it utilizes the energy of the motor 

 muscles of the scaphognathite in making air penetrate into 

 the branchial chamber, to aerate the water which bathes its 

 gills. 



In the natural state the advantages of the reversal often 

 make themselves felt. When, for example, the crab lives 

 upon a muddy bottom, it can absorb air instead of muddy 

 water, which would clog its gills with slime, or, at any rate, 

 can take in water from the surface, which is always purer 

 than that in the depths. I have reproduced these conditions 



* Haan alone maintained the contrary — that is to say, that in the 

 branchial chamber the current passed from the front to the rear; and a 

 year ago Mr. Garstang, of Oxford, frankly said that he was mistaken. 



t Which explains the observation of Haan. 



