CONTRIBUTIONS TO MARINE BIONOMICS. 231 



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. loO) 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 cassivdauntis 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 antenme 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. 



