CONTRIBUTIONS TO MARINE BIONOMICS. 405 



position — not the upright attitude exhibited by Corystes cassivdaunus 

 (this Journal, 189G, p. 223). The anterior part of the body is, however, 

 generally a little higher than the posterior. 



6. Eespiratory Currents. 



Under these circumstances, Portumnus nasidus exhibits a reversed 

 water-current through its branchial chamber, though this is much more 

 diflicult to demonstrate in the present species than in the case of 

 Corystes. The method I adopted was as follows : — 



The depth of gravel in the aquarium was so regulated that the crab 

 could not burrow far beneath the surface. The fragments of sand 

 and shell which lay upon the front of its carapace and upon its inter- 

 orbital lobe were then gently removed, one by one, with a pair of fiue 

 forceps, until the aperture of the buccal frame was exposed. These 

 proceedings were, however, incessantly watched by the crab, which, not 

 unnaturally, did not hesitate to disturb ray preparations whenever it 

 conceived that there was due cause for alarm. I therefore took the 

 precaution to leave some fragments of shell over its eyes, and thus did 

 not seriously disturb its impression that it was safely ensconced. I 

 eventually succeeded in getting the crab so suitably situated that, on 

 the addition of a little black-coloured water by means of a pipette to 

 the region in front of the crab's maxillipeds, I had the satisfaction of 

 seeing the water sucked inwards on both sides, to reappear again in a 

 pair of streams at the base of the chelipeds. The two exhalant streams 

 rose above the surface of the sand in a pair of clouds, one on each side 

 of the body. Suddenly, and without warning, the normal current was 

 set up, and then the lateral clouds of inky water were rapidly sucked in 

 again on each side, to re-emerge again a second or two afterwards in a 

 continuous stream in front of the mouth. Without this kindly co- 

 operation on the part of the crab it would have been ditticult, if not 

 impossible, to get so successful a demonstration of the reversal of the 

 currents. One of the most interesting phenomena presented by this 

 crab is indeed the frequency with which, when under observation, it 

 will alternate the direction of the respiratory currents."*^ It may even 

 suspend the respiratory currents altogether for long intervals ; e.g., iov 

 as long as fifty-live seconds. At such times there is absolutely no 

 movement in the surroundintj water. 



'to 



7. Utility of Specific Characters. 



The interorbital prolongation of the frontal area, which gives both 

 its name and most peculiar feature to the species Portumnus nasutus, is 



* Probably to eject distasteful particles. This is undoubtedly the explanation of similar 

 phenomena in the case of Corystes. (See this Journal, vol. iv. 1896, p. 230.) 



