34 A BIOLOGY OF CRUSTACEA 



of their thoracic limbs, which appear to serve as respiratory 

 surfaces. 



A return to filter-feeding has been made by the amphipod 

 Hailstorms arenarius. This species lives in sand containing a fair 

 amount of organic detritus. The filter is made up of the usual setae, 

 and is situated on the second maxilla, but it is not the same as the 

 original mysid filter. The water current is produced by a rotatory 

 movement of the second maxilla, and the food sieved out of the 

 current is removed by a comb-like series of setae on the first 

 thoracic limb, which is so modified that it is considered as one of 

 the mouthparts and is called the maxilliped. An important differ- 

 ence from the mysid is that the palp of the maxilliped bears the 

 comb which removes food from the filter; in the mysid the work 

 is done by a group of setae at the base of the limb. The palp of the 

 maxilliped passes the food forwards, around the second maxillae to 

 the maxillules, which in turn pass it to the mandibles and so into 

 the mouth. 



It is interesting to note that in the two examples of redevelopment 

 of a filter mechanism that we have studied the organ removing 

 trapped food from the filter is a palp : In Neb alia and Nebaliopsis 

 it is the mandibular palp, and in Hanstorius the maxillipedal palp. 

 Palps are highly mobile structures, and are usually well provided 

 with setae, so that the development of a brush on some part of them 

 does not present any great difficulty. The mobility of the palps may 

 well confer advantages which are denied to a brush situated in 

 the primitive position at the base of a limb. 



In the decapods the three front thoracic legs have become 

 modified to form maxillipeds; they help in the general dismember- 

 ing and passing of food into the mouth. Swimming, in the adult 

 stage, is performed by the movements of the pleopods in shrimps 

 and prawns, or by movement of the last thoracic legs in the swim- 

 ming crabs (fig. 24). Walking is the main function of the thoracic 

 legs, though the first pair behind the maxillipeds are modified to 

 form the chelae, which are used to catch and hold food. The gills 

 are housed in chambers on either side of the body, between the 

 body and the carapace, and a current is caused to flow over them 

 by the beating of a backwardly projecting process from the 

 exopodite of the second maxilla. This process, or scaphognathite 

 as it is called, causes water to flow into the gill chamber through 

 small openings between the edge of the carapace and the bases of 

 the legs. The current flows between the gills and then out through 



