MODIFICATIONS OF STRUCTURE IN DECAPOD CRUSTACEA. 217 



iufra-orbital region of the carapace, where the teeth on the 

 margin of the propodial crests of the chelipeds have taken 

 over the sieve-function which in more primitive types is dis- 

 charged by the marginal spines of the carapace. 



A similar argument may be employed to explain the absence 

 of marginal teeth on the carapace in Ebalia and the Leucosiidse 

 in general. In these forms, also, there is a very highly 

 specialised exostegal afferent canal, the aperture of which is 

 restricted to a very narrow area beneath the orbit of the crab. 

 Since the canal is completed in this group by the exopodite of 

 the external maxilliped alone, the respiratory process is in- 

 dependent of chelipeds and carapace margins alike, and there 

 is consequently no necessity for sieve-forming teeth on either 

 of these parts of the body. Whetlier, however, this inde- 

 pendence was maintained throughout the whole ancestral 

 history of the Leucosiidse is another matter ; the considerations 

 advanced in the present paper seem to me to render it probable 

 that the peculiar respiratory adaptations of these forms have 

 also been derived from the more generalised type of adaptation 

 found in the Cyclometopa. In that case the Leucosiidae have 

 lost the spines on the carapace margins pari passu with the 

 restriction of the area occupied by the exostegal afferent current 

 of water ;^ and the chelipeds have re-acquired their inde- 

 pendence simultaneously with the expansion of the third 

 maxillipeds to form an opercular floor to the exostegal gutter. 

 In Calappa, both the chelipeds and the third maxillipeds are 

 concerned in forming the walls of the gutter. It is quite con- 

 ceivable how the maxillipeds could gradually usurp the whole 

 opercular function to the exclusion of the chelipeds, especi- 

 ally as the specialisation of the Leucosiidse has clearly been 

 accompanied by a gradual diminution of size, rendering possible 



' It is interesting to note that the larger types of Leucosiid appear to 

 have acquired anew set ofdenticulations at the anterior (infra-orbital) 

 extremity of the narrow afferent gutter. I have observed the presence of 

 such denticulations in species of Ilia, Ipliis, and Philyra. Their function 

 is probably the same as that of marginal spines in the Portuuids, and of the 

 teeth on the crests of the chelipeds in Calappa. 



