406 CONTRIBUTIONS TO MARINE BIONOMICS. 



a feature usefully correlated with a habit of burrowing in coarse shelly 

 gravel. It acts as an ellicieut 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 tu 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, 

 granted the reversal, the utility of the possession of a stout triangular 

 shelf over the inhalant orifices is obvious after a study of the animal s 

 habits and of the nature of the objects amid which the crab excavates 

 its dwelling-place. In Corystcs, which lives in fine sand, the inhalant 

 antennal tube has been shown (189G) to subserve the double purpose of 

 a supply pipe and a sieve. In P. nasuius 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-watchfnl enemies among fishes. The overhanging buckler 

 provided by the prominent frontal lobe acts, however, as a very 

 eificient 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 F. nasuius 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. nasuUis (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 i'rom Portunid ancestors of less 

 specialised habits. It is not their presence in P. nasutus which is to 



