49 
1917-18.] Respiratory Mechanism of the Shore-Crab. 
prostomium in the exhalent stream. As soon as the exhalent current 
ceased no more ink was inhaled. 
At intervals the ink, instead of being inhaled, was expelled or exhaled 
from the vicinity of the cleft. This exhalent current was of short duration, 
however, the direction being again changed and the usual inhalent current 
once more established. On using a more concentrated suspension of ink 
the frequency of this momentary reversal of currents increased. 
Exp. III. — A small opening, about a quarter of an inch in diameter, 
was made in the branchiostegite opposite the third thoracic leg, in order 
to determine the direction of the currents in the branchial chamber. An 
inhalent current was observed. 
Experiments with Crabs immersed as before, but with the 
WHOLE OF THE RESPIRATORY TRACT EXPOSED. 
The branchiostegite and the maxillipedes were removed on both sides 
in order to expose the scaphognathites and branchial chambers. The 
characteristic undulating movements of the former, described first by 
H. Milne Edwards (7) and later by Garstang (3), were observed in a 
postero-anterior direction. These movements were not rhythmical, being 
sometimes fast and at other times very slow. 
Occasionally they were “ reversed,” i.e. antero-posterior, when they 
became extremely rapid. 
The action of the scaphognathites has already been noted to be irregular, 
while not infrequently it was entirely suspended. Often both sides did not 
work together ; one side would stop or go more slowly than the other. 
Exp. IV. — For this, Experiment II. was repeated with the branchial 
chambers thus exposed. During the postero-anterior action of the scapho- 
gnathites the same result was obtained, i.e. an inhalent current was 
observed at the enlarged sub-branchial clefts. Following the current 
through the branchial chamber, the ink could be seen being drawn forward 
by the scaphognathite, between it and the roof of the pre-branchial chamber^ 
and being finally exhaled through the prostomium. 
When the action was “ reversed,” injection at the prostomium showed 
the ink to be sucked inwards ( inhaled ) in the same manner as it was 
exhaled, i.e. between the scaphognathite and the pre-branchial roof. It 
then passed backwards through the branchial chamber and outwards 
through the enlarged sub-branchial cleft. 
No current could be demonstrated when the scaphognathite was. 
stationary. 
VOL. XXXVIII. 
4 
