406 CONTRIBUTIONS TO MARINE BIONOMICS. 
a feature usefully correlated with a habit of burrowing in coarse shelly 
gravel. It acts as an efficient buckler for the protection of the anterior 
sense-organs ; but its unusual size and its downward bend seem to be 
more directly correlated with the reversal of the branchial currents, 
which I have shown to take place when the crab is imbedded. The 
advantage of reversal in the present case is a point to which I shall 
recur when dealing with the phenomenon in a more general manner ; but, 
eranted the reversal, the utility of the possession of a stout triangular 
shelf over the inhalant orifices is obvious after a study of the animals 
habits and of the nature of the objects amid which the crab excavates 
its dwelling-place. In Corystes, which lives in fine sand, the inhalant 
antennal tube has been shown (1896) to subserve the double purpose of 
a supply pipe and a sieve. In P. nasutus a sieve is unnecessary so long 
as the crab inhabits coarse shell-gravel, the fragments of which are too 
large to enter the respiratory channels; and this appears to be the 
specific habit of the crab. But if the anterior inhalant apertures (during 
reversal) were altogether unprotected, the pointed fragments of shell 
might easily penetrate the inhalant orifices (during reversal), and so 
occlude their lumen. Such occlusion would prevent the crab from 
burrowing in the kind of material most suitable to its respiratory 
organisation, and thus expose the animal to increased risks of destruction 
by its ever-watchful enemies among fishes. The overhanging buckler 
provided by the prominent frontal lobe acts, however, as a very 
efficient means of supporting the shell-fragments well above the 
inhalant orifices—a function the existence of which I do not throw 
out as an academical suggestion, but the value of which I had frequent 
opportunities of observing and appreciating in my aquaria. 
The interorbital lobe of P. nasutus is remarkably similar to the frontal 
protuberance of Carcinus maenas in the Megalops stage, which becomes 
reduced in later stages of development. Since I have found no 
indications of a reversal of the respiratory currents in the latter species, 
I am inclined to believe that the retention of this larval feature in 
P. nasutus is to be correlated with the reversal of the currents which 
occurs, as I have shown above, in this type; while its eventual loss in 
Carcinus maenas is to be indirectly attributed to the lack of any further 
use for it after the larval stages. The larval forms of P. nasutus are at 
present, however, unknown, and it is impossible to support this view 
with the necessary embryological facts. 
The other specific characters of P. nasutus (viz., breadth of carapace, 
retention of two supra-orbital fissures, mobility of basal joint of second 
antenna) are not new features acquired within the history of the present 
species, but are merely heirlooms from Portunid ancestors of less 
specialised habits, It is not their presence in P. nasutus which is to 
