520 CRUSTACEA MALACOSTRACA. 



position of rest, a filter through which the water is drawn on its 

 way to the gills. The water driven out by the scaphognathite 

 escapes at the antero-lateral margins of the buccal cavity. 



Garstang * has called attention to the filtering mechanisms 

 found in several groups of Decapods. In the nocturnal shore- 

 haunting crab Corystes, the setose second antennae are much 

 elongated and, when placed in apposition, form, as was recognized 

 by Gosse and by Robertson, a straight tubular channel for the 

 respiratory water when the body of the animal is buried in sand. 

 Garstang finds that during the day, when it is so buried, the 

 channel is inhalent, the water passing out at the bases of the legs, 

 but that the direction of the respiratory stream can be reversed, 

 during the activity of the crab, at night. A similar contrivance 

 is found in the anomuran Albunea, but here the channel is 

 formed by the first antennae. The oxystomatous crabs (p. 

 544) present analogous arrangements, and in many cases 

 the shape of the carapace and the limbs is so adjusted as to 

 provide narrow rifts through which the inhalent water is 

 strained. 



Land-crabs are provided with special arrangements for pre- 

 venting evaporation in their gill chambers, but in Birgus latro, 

 the branchial chambers have assumed a pulmonary character 

 (p. 540). 



Appendages. The first antenna consists of a three-segmented 

 shaft (containing, except in some Caridea, the otocyst) bearing 

 usually, as in Schizopods, two flagella of which the inner is 

 usually the thicker and more abundantly supplied with 

 sensory hairs. In the nauplius larva of Penaeus it is as usual 

 uniramous. The Palaemonidae are remarkable in that these 

 appendages bear, as in Stomatopods, three flagella two of 

 them representing the inner flagellum, and indeed uniting in a 

 common base which bears the usual relation to the outer flagellum. 

 In the Penaeidea and many Caridea a pointed spine-like or 

 lamellar process, the " stylocerite " projects forward from the inner 

 aspect of the basal segment of the first antenna. In the 

 Brachyura the first antennae are usually short and contained 

 in depressions of the frontal margin. The three segments of 



* On some modifications of structure subservient to respiration in 

 Decapod Crustacea . . ., Q.J.M.S., vol. xl. (1898), p. 211. See also the 

 literature there quoted. 



