222 WALTER GARSTANG. 



The chelipeds are smooth and concave on their inner face, 

 and are capable of being closely apposed to the inferior wall 

 of the carapace, so that they cover the afferent aperture and 

 the whole carpet of hairs in this region, as is well shown in 

 the figure here provided. Their upper margins do not coin- 

 cide with the edge of the carapace, as they do in Calappa, 

 but during flexion appear to come as far forwards as the 

 posterior aperture of the orbital gutter. 



Water clearly seems to enter the orbits, travelling back- 

 wards through the orbital gutter into the carpet of hairs 

 (which, when the chelipeds are flexed, must furnish a most 

 efficient sieve for the finer particles of mud and sand) , through 

 which it no doubt eventually makes its way to the afferent 

 aperture at the base of the chelipeds. 



This aperture is also furnished with a special hair-sieve of 

 its own, since the edge of the branchiostegite which forms its 

 anterior wall is fringed with a special line of stiff hairs, and 

 there is a corresponding series of hairs on the opposing sur- 

 face of the basal joint of the cheliped. When the cheliped is 

 apposed to the carapace, the two sets of hairs interdigitate and 

 constitute a sieve completely covering the aperture. A similar 

 arrangement exists in Calappa also, but in the latter form 

 the basal portion of the epipodite of the third maxilliped 

 furnishes a much more obvious operculum to the aperture 

 than is the case in Matuta. 



The remarkable course of the exostegal canal in Matuta, 

 with the restriction of its principal aperture to the cavity of 

 the orbit, appears explicable to me only on the view which I 

 have set forth in the case of Calappa and the Leucosiidse, 

 viz. that the common ancestor of all three types possessed a 

 continuous waterway along the whole extent of the antero- 

 lateral margins of the carapace to the base of the chelipeds ; 

 that the antero-lateral margins were denticulated ; and that a 

 process of restriction of the inhalant orifice began, by which 

 the closure of the whole inhalant gap between chelipeds and 

 carapace became gradually effected, except in the infra-orbital 

 region, This process of restriction was effected as a con- 



