42 CRUSTACEA BRANCHIOPODA CHAP. 



with a gnathobase on the inner side in the Polyphemidae, 

 but not in Leptodora. The number varies from four to six 

 pairs. 



The abdomen bears no appendages. The telson is compressed 

 in the Calyptomera, and is produced into two flattened plates, 

 one on each side of the anal opening. The backwardly-directed 

 margins of these plates are commonly serrated, and the lower 

 corner of each is produced into a curved spine, which carries 

 secondary teeth. The number and arrangement of these teeth, 

 though often extremely variable in the same species, are used 

 extensively as specific characters. Above the anus the telson 

 commonly bears two long plumose hairs, which are directed 

 backwards. 



In the Gymnomera the telson is not bilaterally compressed, 



FIG. 13. Bythotrephes cederstromii, female, x 20, North Wales, from a specimen found 

 by A. D. Darbishire. Car, carapace. 



and it may be produced into a long spine, dorsal to the anus (e.g. 

 Bythotrephes, Fig. 13). 



The alimentary canal is extremely simple. The labrum is 

 large, and forms a chamber above the mouth, into which 

 food is driven by the limbs, as in the Phyllopoda, food being 

 taken while the animal swims or lies on its back. The 

 oesophagus runs vertically to join a small stomach, which bends 

 sharply backwards and passes gradually into an intestine. In 

 the last segment of the abdomen the intestine joins a short, 

 thin-walled rectum, provided with radial muscles, by means of 

 which it can be dilated. The dilatation of the rectum leads to 

 an inhalation of water through the anus, which may possibly 

 serve as a means of respiration. In the Daphniidae and 

 Bosminidae there are two forw^ardly- directed digestive glands 

 which open into the stomach, and in Eurycercus there is a large 

 caecum at the junction of the rectum with the intestine. The 



