TRANSACTIONS OF SECTION D. 829 
power of free movement is necessary to enable the crab to detect the direction of 
odoriferous bodies in its neighbourhood. At the same time the situation of the 
antenne in front of the body renders these organs particularly liable to injury 
unless specially protected. 
In regard to the denticulation of the lateral margins of the carapace experi- 
ments show that in sand-burrowing species a most important function of the 
denticulated margins is in connection with the process of respiration. It may be 
termed the ‘sieve-function.’ 
It is not generally known that a crab’s chelipeds are in many cases not merely 
organs of prehension, but important agents in the respiratory process. The 
principal afferent apertures to the branchial chambers are situated at the base of 
the chelipeds. When the chelipeds are folded’ against the sides of the carapace 
(for which purpose they are in many forms specially curved and moulded) a pair 
of lateral slit-like channels is produced which lead directly downwards to the 
afferent apertures at the basement of the chelipeds. The lateral denticulated 
margins of the crab’s carapace overhang the slit-like orifices of these accessory 
water-channels. When the crab is partially imbedded in sand it is possible, by 
the addition of colouring matter to the water, to demonstrate that a constant 
stream of water flows from above downwards through these accessory channels 
between chelipeds and carapace. The stream enters through the gaps between 
the teeth or spines on the lateral margins of the carapace. The teeth act asa 
coarse sieve or grating over the slit-like orifice, and prevent foreign bodies, such 
as particles of sand and shell, from falling into the channel and blocking its 
lumen. The water, after traversing these channels, enters the branchial chambers 
by the afferent apertures at the base of the chelipeds, and emerges in front by the 
lateral apertures at the sides of the mouth. 
As examples of sand-burrowing crabs to which the above remarks apply, 
Bathynectes longipes and Atelecyclus heterodon may be mentioned. In each case 
the lateral denticulated margins of the carapace subserve this sieve-function. The 
number of teeth is five in Bathynectes and nine in Atelecyclus, but in each case 
the extent of the denticulated area is commensurate with the extent of the lateral 
inhalant gap between chelipeds and carapace. 
This view is confirmed by the fact that in Ebalia and other Leucosiide, in 
which the afferent water-channel is entirely independent of the chelipeds, the 
lateral margins of the carapace are smooth and free from denticulations. 
In Calappa granulata of the Mediterranean the chelipeds can be pressed 
against the smooth sides of the carapace with extreme nicety. The author has 
not yet had an opportunity of studying this crab alive; but, if the chelipeds are 
held tightly to the body when the animal is buried in the sand, it must be 
impossible for water to enter between them and the carapace, except at one point 
on each side, between the anterior margin of the carapace and the curious cock’s- 
comb-like crests with which the chelipeds in this genus are provided. The 
antero-lateral margin of the carapace is smooth throughout, but the crests of the 
chelipeds are conspicuously denticulated. The structure of the surrounding parts 
renders it extremely probable that the inhalant current of water passes to the 
afferent aperture through the notches between the spines on the crest-like 
expansions. 
In Matuta victor, an East Indian sand-burrowing crab, the inhalant current 
actually seems to enter through the crab’s orbits, flowing thence downwards 
through a special pair of orbital gutters. Here also we find the marginal teeth of 
the carapace obsolete and scarcely recognisable. 
A complete reversal of the ordinary branchial currents may take place in 
certain sand-burrowing crabs, as the author has experimentally determined in the 
ease of Corystes cassivelaunus, Atelecyclus heterodon, and Platyonichus nasutus. 
A similar reversal probably occurs also in Albunea symnista, Platyonichus latipes, 
and several other forms. 
In Corystes and Atelecyclus filtration is effected during reversal by an inhalant 
sieve-tube formed by the second antenne, with the participation of the third 
maxillipeds. In A/bunea a similar tube is formed by the apposition of the first 
