434 L. A. BORRADAILE. 



VI. THE SAND CRABS (OXYSTOMATA). 



The morphological feature by which the Oxystomata are distinguished from other crabs 

 is, at the same time, an indication of their most marked binomic peculiarity. This feature 

 is to be found in the shape of the mouth and foot-jaws. The epistome is small or wanting, 

 and the endostome is lengthened forwards, so that it is of a triangular shape. The channels 

 for the outward stream from the gill-chamber, running on this endostome, are covered in 

 by the long, leaf-like endopodites of the first pair of maxillipeds (A Fig. 110), and in this 

 way are prolonged as closed tubes to the front of the body. The whole apparatus is neither 

 more nor less than an adaptation to breathing under the special circumstances that present 

 themselves in the sand-beds which form the characteristic habitat of the group. 



The Oxystomata are singularly unanimous in their habits. Nearly all of them are crabs 

 whose strength is to sit still, trusting to concealment rather than to agility or offensive 

 tactics for protection against predaceous animals. Most of them find their concealment in 

 the sand, where they lie buried with only the eyes exposed, and can often only be detected 

 by the little whirlpool which they make in breathing. In such circumstances the advantage 

 of having the breathing channels carried right up to the surface of the sand is obvious, 

 and we have seen how this is done in the case of the outgoing stream. The incoming 

 stream is ordered differently in different families. The Calappidae' (PI. XXII. fig. 6) draw 

 in water, like many other crabs, down a passage formed, so to speak, by "holding the hands 

 against the breast," and so into the gill-chamber through an opening in front of the 

 chelipeds, guarded by the wide base of the epipodites of the third pair of maxillipeds. But 

 their hands (chelae) are peculiarly shaped for this purpose, being very broad and flat, and 

 usually having a toothed crest, which is held against the forepart of the body. Through 



1 Key to the families of the Oxystomata. B. Meropodite of 3rd maxilliped elongate and acute. 



_ „ , . ^, , , . , ,i.j u-j 1 Exopodite of same limb without flagellum. Legs 



I. Body of the shape usual in crabs. Abdomen hidden ^ . . „ . , . 



; ,, . \ ,, T 1 • -i- adapted for swimming. Orithyinae. 



under thorax. Antennae small. Legs normal m position. ^ 



, ... , , ,• ■ , i , c 4 II. Last three joints of 3rd maxilliped hidden by the mero- 



A. Afferent openings to giU-chambers lie in front of first „,., , ^ j i ^u 



., , , ,. , , ^,.,, „ , ., ,, , podite. Orbits more or less separated from the an- 



pair of legs (chehpeds). Gills 9 on each side. Male ^ , , ,r. j-^ ,^ o j n- j •*! 



^ . , „ , . , tennular sockets. [Exopodite of 3rd maxilliped with 



openings coxal. Calappidae. ^, ,, ,, j-^ .. ,■ u i .. j » 



flagellum. Meropodite ot same limb elongate and acute. 



B. Afferent openings to gill-chambers lie on either side of j^^gg j^^y (,e adapted for swimming or not.] Matutinae. 

 the mouth at the base of the third maxillipeds. Gills 



less than 9 a side. Male openings sternal. Leucosiidae. Key to the suhfamili.es of the Leucosiidne. 



II. Body more or less abnormal in shape. Abdomen not I- Meropodite of 3rd maxilliped more than half the length 

 hidden under thorax. Antennae large. Last one or of the ischiopodite. Fingers stout, gradually narrowing 

 two pairs of legs in a more dorsal position than the rest. fro™ base to tip, usually shorter than the palm. 



Leucosiinae. 



A. Carapace short. Last two pairs of legs subprehensile, 



with hook-like end-joints. Dorippidae. "• Meropodite of 3rd maxilliped never more tlian half the 



length of the ischiopodite. Fingers slender, of even 



B. Carapace long. Legs usually have the last two joints ^.^^^ ^^^^ j,^^ ^^^^ ^^ ^^^^ j,^^ jjp^ ^,^^^jly ^^^^^^ ^^^^ 



very broad. Ranimdae. p^,^_ jf..^^^^^ 



Key to the subfamilies of the Calappidae. Key to the subfamilies of the Dorippidae. 



I. Last three joints of Brd maxilliped not hidden by the I. 3rd maxillipeds leave a good part of the mouth uncovered. 



meropodite. Orbits not separated from the antennular Inward openings to the gills near the base of the cheli- 



sockets. peds. Dorippinae. 



A. Meropodite of 3rd maxilliped not elongate nor acute. II. 3rd maxillipeds almost completely cover the mouth. 



Exopodite of same limb with flagellum. Legs not Inward openings to the gills may or may not be near the 



adapted for swimming. Calappinae. base of the chelipeds. Tymolinae. 



