. The Cambridge natural history. Zoology. 42 CRUSTACEA BRANCHIOPODA 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 s
. The Cambridge natural history. Zoology. 42 CRUSTACEA BRANCHIOPODA 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 Fig. l^.—Bythotrephes cederstrwnii, female, x 20, North Wales, from a specimen found by A. D. Darbisliire. Car, carapace. and it may be produced into a long spine, dorsal to the anus ( Bijthotrcphes, Fig. 13). The alimentary canal is extremely simple. The labrum is large, and forms a chamber above the moutli, 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 Eurycercus there is a large caecum at the junction of the rectum with the intestine. The. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colorat
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