398 CONTlilBUTlONS TO MARINE BIONOxMlCS. 



afTeroiit (iulialaut) aperture of the branchial chamber, which is situated 

 at the base of the cheliped, and opens above through the notches 

 between the teeth of the antero-lateral margins of the carapace. Since 

 the back of the crab is covered with sand, it will readily be understood 

 from this description that the antero-lateral teeth act as a coarse sieve 

 or grating placed over the orilice of this accessory channel, and that 

 they prevent the accidental intrusion of sand-particles into the lumen 

 of the channel, a function which it was easy to determine that they 

 efficiently discharged. 



The pair of accessory channels produced by the approximation of 

 chelipeds to carapace I propose to term the " exostegal channels," owing 

 to their situation on the external face of the branchiostegite. I show 

 elsewhere (1897) that these channels probably represent in a generalised 

 condition certain remarkable accessory afferent branchial canals of the 

 Oxystome Brachyura, which attain their most specialised form and 

 relations in Ehalia and other Leucosiida^. 



M. Alphonse Milne-Edwards (1861), p. 207) states that in the 

 Portunidic "les mains ne sont jamais conformues de facon a pouvoir 

 s'applit|uer exactement contre la region buccale, ainsi que cela se voit 

 chez quelques autres Jjrachyures uageurs tels que les Calappes et les 

 JMatutes." This contrast is quite in accordance with my view, that the 

 afferent channel of the Portunida3 represents a primitive and relatively 

 unspecialised type, from which the highly elaborate canals of the 

 Oxystomata have been derived. 



That these accessory channels in the Portunidai are functionally 

 connected with the respiratory process, was demonstrated by me in 

 the case of Bathynectes longipes in the following manner : — 



When the crab was partially imbedded in sand with its face close to 

 the front of a square glass acj^uarium, in the attitude already described 

 it could be seen that beneath the body of the crab was a shallow ventral 

 water-chamber, free from sand. The crab was resting with its body in 

 an approximately horizontal plane. Sand-particles were supported over 

 the orifice of the exostegal channel by the sieve-like row of teeth along 

 the antero-lateral margins. Some water, coloured black with Indian 

 ink, was then added by means of a pipette to the water lying above the 

 slit between cheliped and carapace. The coloured water was at once 

 sucked downwards between the grains of sand into the exostegal 

 channel in waves which apparently corresponded to blows of the 

 scaphognathite, and after a few seconds emerged in a black stream 

 out of the afferent orifice of the branchial chamber situated in front 

 of the mouth. It was quite clear that the water passed downwards 

 through the exostegal channel to the afi'erent aperture at the base of 

 the cheliped, and that it entered the branchial chamber by this aperture. 



