CONTRIBUTIONS TO MARINE BIONOMICS. 225 



in the middle, and the prelabial plate behind. It is flanked by the two 

 basal joints of the second antennre in front/ and by a forward process of 

 the pterygostomial region of the carapace behind. Its floor is imperfect, 

 and is formed by the anterior part of the third maxillipeds behind, and 

 by a quadrangular sieve in front, furnished by the hairs springing from 

 the two basal joints of the second antennae, the anterior pterygostomial 

 processes, and a special anterior process of the fourth joint of the 

 external maxillipeds. The hairs from all these parts are directed 

 inwards towards the centre of the quadrangular space outlined by the 

 boundaries of the prostomial chamber, and constitute a complete sieve- 

 like floor to the chamber in question. On each side this prostomial 

 chamber leads by a wide aperture into the branchial cavity. 



The participation of the epistome together with the prelabial space in 

 the formation of a prostomial chamber is one of the features which 

 strongly distinguishes the Corystoid crabs from typical Cyclometopa, 

 Catometopa, and Oxyrhyncha. The arrangements of these parts approxi- 

 mates in some respects to that found in the Oxystomata, where the 

 buccal frame or the peristome is prolonged anteriorly as a definite 

 prostomial chamber to the very tip of the snout. This chamber in the 

 Oxystomata, however, is completely closed in by the third maxillipeds, 

 and is very narrow anteriorly ; in the Corystoidea, on the other hand, it 

 is broad in front, and is imperfectly closed by the third maxillipeds. 



Ill, Previous Observations on Habits. 



In Bell's " British Stalk-Eyed Crustacea " (1853) a brief reference is 

 made to the sand-burrowing habits of Corystcs cassivelcamus. Couch 

 had already described the crab as "burrowing in the sand, leaving 

 the extremities of its antennte alone projecting above the surface." 

 The actual process of burrowing appears not to have been observed 

 at the time when Bell wrote, for he quotes Couch's suggestion that the 

 elongated anteuncc possibly " assist in the process of excavation." This 

 theory of the function of the antennae was subsequently rejected by 

 Gosse (1865), as a result of his own observations on the habits of the 

 crab, and again by Hunt (1885), who correctly states that the crab 

 descends into the sand backwards with the greatest agility, " thus leaving 

 the antennae no opportunity of assisting in the operation." 



The first writers to offer anything approaching a real explanation of 

 the use of the antenucC were the veteran naturalist of Cumbrae, 

 Mr. David Robertson, and Mr. P. H. Gosse. It is difficult to say, and 

 would indeed be ungenerous to enquire, which of these two naturalists 

 has the priority in the matter. Gosse, in 1855, described the outer 

 antennte of Corystes as " together forming a tube " {Manual of Maritic 



K 2 



