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 antenne are frequently seen to be rubbed 
obliquely against one another for two or three strokes, whereby the hairs 
on the antenne are cleansed from adhering particles. This very charac- 
teristic action of the antennz 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 cassivelawnus 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. Iam inclined to think that if the water and sand 
provided be of a perfectly suitable character, Corystes will remain 
imbedded throughout the day. (cf. Robertson, l.c. supra). 
I have noticed, however, that individuals which were inactive and 
concealed beneath the sand during the day, shewed 2 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 cassivelawnus 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. 
(b) Respiratory Currents. We have seen that Gosse observed a 
current of water setting upwards from the buccal region of the crab 
