36 A BIOLOGY OF CRUSTACEA 



found in the anomuran Albunea, but the tube is formed by the 

 antennules, not by the antennae. 



Another method of obtaining a clean respiratory current is found 

 in the crab Calappa, which also burrows in sand. The chelae of 

 this species arc relatively enormous, but the biting part is com- 

 paratively small. At one time it was thought that Calappa used its 

 chelae to hide its food from other crabs and fish while it was being 

 dealt with by the mouthparts. They may in fact do this, but the 

 other function which they perform is probably more important. 

 The two chelae fit neatly together in front of the carapace, and 

 also fit closely against the front edge of the carapace. The narrow 

 gap between carapace and chelae is guarded by hairs on the edge 

 of the carapace and spines on the chelae. Water is drawn in through 

 this guarded crack, and the hairs prevent any stray particles from 

 entering. The inflowing water enters the gill chambers via openings 

 near the bases of the legs. The outflowing current is carried up- 

 wards by tubes formed from the inner branches of the first 

 maxillipeds, and is thus directed away from the current going 

 inwards between the carapace and the chelae. 



The majority of the Decapoda Reptantia are predators and 

 scavengers; they will eat practically anything of animal origin. 

 Perhaps the most remarkable record is of North American cray- 

 fish entering dens of hibernating snakes and eating the reptiles 

 which were too torpid to retaliate. 



It might be thought that once the decapods had developed 

 biting mouthparts and chelae for catching their prey they would 

 be content, but a surprising number of species, particularly 

 anomurans, have developed other methods of feeding, some of 

 which represent a return to filtration. 



The fiddler crabs of the genus Uca have one enormous chela, and 

 one small one. The males use the large one for defence and display 

 during courtship, but the small one is used to scoop up sand grains 

 and present them to the mouthparts. The first and second maxilli- 

 peds have numerous stout setae, some shaped like spoons. These 

 are used to scour the sand grains for traces of organic matter. The 

 sand is then rejected, often in the form of neat sand balls. 



Some species literally brush up their food from the surface of 

 sand or mud. Galathea dispersa has a brush on the end of its third 

 maxilliped, and the particles which collect on the brush as it 

 sweeps across the surface of the sand are removed by the second 

 maxillipeds which pass the food inwards to the mouth. 



