In Simocephalus, as in the other Daphniidae, there are five pairs of thoracic limbs, of which the third and fourth are alike; in the female each limb of the first pair consists of a jointed axis, with strong biting hairs on the inner border, and a rudimentary epipodite (Fig. [12], A), the second limb being more like the third, but with a more prominent gnathobase and a narrower exopodite (B), while the limbs of the fifth pair have the gnathobase and the exopodite filamentous (D).
In the Sididae there are six pairs of thoracic limbs, which are nearly alike in the female; in the Bosminidae there are six pairs, the first two modified for prehension, the last much reduced.
Fig. [12].—Thoracic limbs of female Simocephalus vetulus. A, The first; B, the second; C, the third; D, the fifth. Br, Bract; Ep, epipodite; Ex, exopodite; Gn, gnathobase.
In the male, the first thoracic limb is usually provided with a long sensory process and a prehensible hook (Figs. [11], 19).
In the Gymnomera the limbs are cylindrical, jointed rods, 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.
Fig. [13].—Bythotrephes cederströmii, female, × 20, North Wales, from a specimen found by A. D. Darbishire. Car, carapace.
In the Gymnomera the telson is not bilaterally compressed, and it may be produced into a long spine, dorsal to the anus (e.g. Bythotrephes, Fig. [13]).