MARINE CRUSTACEANS. 435 



the notches between the teeth water can enter, while at least the coarser sand-grains are 

 kept out. Matuta (PL XXII. fig. 4) replaces these teeth, functionally, by a sieve of hairs. 

 In the Leucosiidae, on the other hand, the openings for the ingoing stream are situated 

 at the base of the maxillipeds of the third pair, and the water is led thither by a groove 

 on the body, outside the efferent channel and parallel with it, covered by the third pair 

 of maxillipeds. The Dorippidae (PI. XXII. fig. 1) are probably divided in this respect, some 

 breathing like the Leucosiidae, while others do not. In the Raninidae (PI. XXII. fig. 5) 

 a third method seems to be adopted. There are no obvious afferent openings in the forepart 

 of the thorax, but spaces can clearly be seen between the hind edge of the carapace and 

 the bases of the last pair of legs, and it is through these that the water probably enters. 



Besides the conformation of the breathing organs just described we can trace other 

 adaptations of the structure of Oxystomes to a sandy habitat. The comparatively simple 

 egg- or pear-shaped body of some Leucosiidae is eminently suitable to a life spent buried 

 in the sand, and the hard carapace found in most of these crabs is no doubt a last resource 

 of the defenceless. Calappa, when seized, often draws up its legs under the broad shields 

 formed by the overlapping edges of the carapace, thus seeming to sham death (PI. XXII. 

 fig. 6, right side). No doubt it is more difficult to dismember in this attitude, but it is 

 quite possible that the action has no such special significance in this case, being instinctively 

 adopted whenever the animal is moved against its will'. The colouring of many Calappidae 

 is another feature in which they are adapted to their environment, being sand-like, and 

 clearly protective. 



Notwithstanding these generalisations, it is by no means likely that all Oxystomes hide 

 in sand. Donppe (PI. XXII. fig. 1) conceals itself by holding a sponge or some other object 

 over its back with its two hinder pairs of legs, which are dorsally placed and subchelate 

 with hooked end-joints. Many of the Leucosiid genera show so strong a likeness to the 

 Spider crabs (Oxyrhyncha) in their form of body and limbs (Fig. 117, and PI. XXII. fig. 2) 

 that they probably have a similar habit of weed-haunting, a view confirmed by the fact 

 that several were taken in weed by the Expedition^. Another habitat is taken up by 

 Oreophorus and Tlos (Fig. 115), which closely resemble water- worn coral pebbles, and live on 

 shingle made up of such pebbles. 



As a whole the Oxystomata are of distinctly sluggish habits, and this peculiarity reaches 

 its height in the Leucosiidae. Calappa is perhaps somewhat more active. MaUita is not only 

 a good swimmer, by means of paddle-like feet (PI. XXII. fig. 4), but also an excellent digger, 

 burying itself with wonderful rapidity in the sand. Borippe is said to be able to run rapidly 

 by means of its two pairs of very long walking-legs (PL XXII. fig. 1). And, lastly, the 



Explanation of Plate XXII. crabs, it should always be remembered that actual obaerva- 



Fio. 1. Dorippe dorsipes. Fig. 2. Arcania quinquespi- tion in shallow water is far more convincing proof than 



nosa. Fig. 3. Leucosia marmorea. Fig. 4. Matuta banksi. dredging on any particular bottom. For instance, a large 



Fig. 5. Ranina scrrata. Fig. 6. Calappa hepatica, legs on crab dredged on a bottom of coral blocks may fairly be 



right side withdrawn under the shield. 6 a, outside of concluded not to be a sand-haunting one, but a small species 



hand, showing crest. The figures are not drawn to one scale. may well make use of the hollows in the blocks, often as 



' Calappa is generally to be found in spots where deep large as a man's fist, which are found to be filled with sand, 



sand is freely exposed to currents or wave motion, and in and may have been dislodged in the dredge ; and, again, the 



this compact form would probably undergo rolling about on presence of weed or coral in a dredging always opens the 



a soft bottom without much injury. possibility that some of the crabs found free may have 



^ In considering the habitat of Oxystomes, as of other belonged to them. 



G. ' 56 



