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, 
Fic. 13.—Bythotrephes cederstrémit, 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 forwardly-directed digestive glands 
which open into the stomach, and in Hurycercus there is a large 
caecum at the junction of the rectum with the intestine. The 
