^o A BIOLOGY OF CRUSTACEA 



Some members of the Mysidacea have lost the ability to filter 

 iced. This is so with Lophogaster typicus, which lives on muddy 

 patches of the sea floor, and feeds on large pieces of food, which 

 it holds in the same way as Hemimysis when it feeds on such 

 particles. The respiratory current is similar to that of Gnatho- 

 plumsia, but the gills are larger and smoother, probably because 

 of the creature's mud-dwelling habits. The increased size will 

 compensate to some extent for the poor aeration of the water, and 

 the smoothness helps to prevent the gills from becoming clogged 

 by the particles of mud. 



Mud dwelling has been adopted as a way of life by various other 

 Crustacea, including the Leptostraca and the Cumacea. Both these 

 groups contain filter-feeders, but the mechanisms differ from each 

 other and from that found in the mysids, nevertheless certain 

 resemblances between the three types can be traced. 



In the cumacean Diastylis the second maxilla is still the bearer 

 of the filter setae, but the current which flows through these setae 

 is produced by the action of a large epipodite on the first thoracic 

 limb. The water current, which also serves as a respiratory current, 

 is unusual in that it enters the carapace from in front and also leaves 

 from the front, though a little higher up than the site of intake (fig. 

 21). The leptostracan Nebalia bipes has adopted a different method 

 of filter-feeding, which at first sight resembles a return to the 



Fig. 21. Diastylis sp. (Cumacea) buried in sand, showing the in- 



halent and exhalent feeding and respiratory currents. (After 



Dennel, 1937). 



