230 CONTRIBUTIONS TO ^lARINE BIONOMICd. 



the crab's body, aud is scattered outwards and laterally by the vigorous 

 lashings of the exopoditic palps. 



Gosse's observations on the respiratory currents of Corystes cassive- 

 launus are thus seen to be incomplete rather than inaccurate. A 

 current may be directed outwards through the antennal tube, and the 

 effete water from the branchial chamber may be carried away by that 

 channel ; but such a direction of the current in Corystes cassivelaunns is 

 not constant, as Gosse believed, or even usual. Moreover, when the 

 crab is imbedded in sand, the current is always reversed, except for 

 a few seconds now and then, when the crab desires to eject distasteful 

 particles which have entered the prostomial chamber with the respira- 

 tory current. Under such circumstances the reversed inhalent current 

 through the antennal tube is temporarily replaced by a forcible exhalent 

 current. But as soon as the desired ejection has been effected, the 

 reversed current is again set up. This voluntary inhibition of the 

 reversed current can be easily demonstrated by the addition of carmine 

 to the water setting through the antenna). Oddly enough, a weak 

 solution of Chinese ink is less distasteful to Corystes than a mixture 

 of powdered carmine and sea-water. 



(c) Cause of the Currents. The direction of the respiratory currents 

 is exclusively due to the movements of the scaphognathite, the valve- 

 like and highly muscular appendage of the second maxilla, which 

 is known to produce the regular respiratory currents of other Decapoda. 

 H. Milne-Edwards first demonstrated the important role played by the 

 scaphognathite in Decapod Crustacea; and he maintained that the 

 direction of the respiratory current was absolutely constant, i.e., from 

 behind forwards in all Decapods (1839, p. 136). De Haan (1850, 

 p. 117) has indeed suggested that the current to the branchiae passes 

 from before backwards ; but his remarks on this subject are obviously 

 the result of mere inference, and are not determined by actual experi- 

 ment. He states, for example, that in Portunus the inhalent current 

 sets inwards not only through the aperture between the base of the 

 cheliped and the edge of the branchiostegite, but also through the 

 anterior aperture at the side of the mouth. Experiments on Portumis 

 have shewn me that this is quite devoid of foundation ; the water 

 certainly enters — in part — through the former of these apertures, but 

 the aperture at the side of the mouth is invariably exhalent in function. 



In the case of Corystes I observed the action of the scaphognathite by 

 removing the three maxillipeds and the edge of the pterygostomial 

 fold of a living specimen. The scaphognathite was completely ex- 

 posed by this preparation, and its movements were readily followed. 



When the normal current — from behind forwards — was at work, the 

 propulsion of the water could be seen to be effected by a sharp, prompt 



