CONTRIBUTIONS TO MARINE BIONOMICS, 405 
position—not the upright attitude exhibited by Corystes cassivelaunus 
(this Journal, 1896, p. 223). The anterior part of the body is, however, 
generally a little higher than the posterior. 
6. RESPIRATORY CURRENTS. 
Under these circumstances, Portwmnus nasutus exhibits a reversed 
water-current through its branchial chamber, though this is much more ~ 
difficult 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 fine 
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 my 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 difficult, 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; eg., for 
as long as fifty-five seconds. At such times there is absolutely no 
movement in the surrounding water. 
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.) 
