CONTRIBUTIONS TO MARINE BIONOMICS. Zo 
blow dealt by the posterior lobe of the scaphognathite, which was 
succeeded by an undulatory movement from behind forwards of the 
remaining part of the scaphognathite. As the crab lay on its back 
the anterior lobe could finally be seen to descend slowly and gently 
upon the anterior edge of the roof of the chamber, gliding along, and, 
as it were, stroking its polished surface. 
When the current is reversed, however, the action of the anterior 
lobe is quite different; it strikes the water in front with a prompt, 
decisive blow, and this is succeeded by an undulatory movement of the 
rest of the scaphognathite from before backwards. The water lying 
between the valve and the roof of the chamber is thus driven back- 
wards into the branchial cavity. The action of the scaphognathite 
is fairly rapid, but after a little observation, checked by the employment 
of coloured water to test the currents, it becomes quite easy to detect 
with certainty the direction of the current by inference from the move- 
ments of the scaphognathite alone. 
The action of the exopoditic palps of the maxillipeds in causing 
currents has already been described. Such currents are purely 
accessory, and Gosse (1865, p. 130) and De Haan (1850, p. 117) 
have undoubtedly erred in assigning to the maxillipeds an important 
share in the production of respiratory currents. 
V. EVOLUTIONAL SIGNIFICANCE. 
The habits of Corystes cassivelaunus described above seem to me 
to demonstrate the adaptive nature of the entire organization of this 
Crustacean, and slight consideration is all that is required to enable 
a naturalist to recognise the utility of these adaptive features, 
The burrowing habit is useful as a mode of concealment from enemies. 
The elongation and smoothness of the carapace, and the elongated claws 
of the four hindmost pair of thoracic legs, are all features usefully 
correlated with the specialization of the crab for a sand-burrowing 
existence. 
The elongation of the antennie and the arrangement of the hairs upon 
them, the double bend of their basal joints, the structure of the parts 
bounding the prostomial chamber, and the arrangement of hairs upon 
them, are characters which, in conjunction with the reversal of the 
respiratory current, adapt the respiratory mechanism of the crab in 
a remarkably complete manner to an arenicolous mode of life. The 
antennal tube enables the crab to draw its supplies of water directly 
from the superincumbent reservoir of water, while the arrangement 
of hairs is such as to constitute a sieve, keeping the sand away from the 
respiratory organs. 
