398 CONTRIBUTIONS TO MARINE BIONOMICS. 
afferent (inhalant) 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 orifice 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 Hbalia and other Leucosiide. 
M. Alphonse Milne-Edwards (1860, p. 207) states that in the 
Portunidee “les mains ne sont jamais conformées de facon a pouvoir 
s'appliquer exactement contre la région buccale, ainsi que cela se voit 
chez quelques autres Brachyures nageurs tels que les Calappes et les 
Matutes.” This contrast is quite in accordance with my view, that the 
afferent channel of the Portunide 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 Portunide 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 aquarium, 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 afferent aperture at the base of 
the cheliped, and that it entered the branchial chamber by this aperture. 
