CRUSTACEA OF THE SEASHORE 99 



the larger Decapods, which can be watched in their 

 natural haunts, or more closely in aquaria, many 

 interesting adaptations to burrowing in sand have 

 been discovered. Many Crabs belonging to the tribe 

 Brachyrhyncha often take refuge in sand or gravel, 

 burying themselves till only the eyes remain exposed. 

 The Swimming Crabs (Portunidae Plate XIII.) of our 

 own coasts have been found to use the paddle -shaped 

 last pair of legs for digging as well as for swimming. 

 In the sand, the Crab keeps its large claws, or 

 chelipeds, folded close up to the front edge of the 

 carapace, which is cut into sharp, saw-like teeth. 

 Between these teeth the water passes, to reach the 

 entrance to the gill chamber which lies at the base of 

 each cheliped, and in this way an efficient strainer is 

 provided, which in coarse sand at least prevents the 

 clogging of the respiratory passages. The out-going 

 current of water from the gills passes through chan- 

 nels that open on either side of the mouth-frame. 



A more complex adaptation of structure to the 

 habit of sand-burrowing is found in the Masked Crab 

 (Corystes cassivelaunus Plate XIV.). This Crab is 

 common on the British coast, living in moderately 

 deep water wherever the bottom is sandy, and it has 

 received its English name from the fact that the 

 furrows on the back of the carapace give it a grotesque 

 resemblance to a human face. It is noteworthy, 

 among other things, for the marked difference between 

 the sexes, the male having very long, slender chelipeds, 



