228 CONTRIBUTIONS TO MARINE BIONOMICS. 



renders easier and quicker the descent of the crab into its sandy 

 burrow. Briefly stated, in fact, the four hindmost pairs of legs are 

 all engaged in pulling the crab downwards, while the first or chelate 

 pair is engaged in pushing away the more superficial sand in the neigh- 

 bourhood of the crab's maxillipeds. The two actions combine to drive 

 the crab downwards and obliquely backwards. The main object of this 

 latter motion appears to be the prevention of any forcible intrusion 

 of sand into the buccal apparatus. 



When the carapace of the crab has completely disappeared beneath 

 the surface of the sand, the antenna) are frequently seen to be rubbed 

 obliquely against one another for two or three strokes, whereby the hairs 

 on the antennse are cleansed from adhering particles. This very charac- 

 teristic action of the antenna? was noticed long ago by Couch, and 

 correctly recognised by him as a process of cleansing {vide Bell, p. 161). 

 After this cleansing process, however, the crab proceeds still further in 

 its act of burrowing, and descends deeper and deeper until nothing 

 is visible above the sand but the most distal portion of the antennal 

 tube. 



Resting passively in its bed of sand, Corystes cassivelaunus spends the 

 daytime thus concealed from all observation. In aquaria an individual 

 will occasionally emerge and remain on the surface of the sand for some 

 time, but this can usually be attributed to the restlessness resulting from 

 strange conditions. I am inclined to think that if the water and sand 

 provided be of a perfectly suitable character, Corystes will remain 

 imbedded throughout the day. (cf. Eobertson, 1. c. supra). 



I have noticed, however, that individuals which were inactive and 

 concealed beneath the sand during the day, shewed a marked tendency 

 to activity at night. I have observed on several occasions that my 

 aquarium, containing some half-dozen of these crabs, was the scene 

 of distinct excitement and activity late at night; the crabs had 

 emerged from the sand, and were restlessly hobbling about on the 

 surface, as though in search of food. Although I cannot make a final 

 statement upon the point, all my experiences incline me to the view 

 that Corystes cassivelaunus is a nocturnal animal ; it conceals itself 

 in the sand by day as a protection from sight-feeding fishes, but 

 emerges at night for food and recreation. If these habits were ab- 

 solutely constant, we should expect to find the eyes of Corystes 

 undergoing retrogressive changes, as, for example, in the case of 

 Pinnotheres. Such is not the case, however, for the eyes are capable 

 of forming distinct images, as well as, no doubt, of distinguishing light 

 from darkness. 



(h) Respiratory Currents. We have seen that Gosse observed a 

 current of water setting upwards from the buccal region of the crab 



