The genus has the following characters: head and first thoracic segment are fused, and so are the fourth with the fifth; lateral corners of last thoracic segment produced into an acute spine on each side; abdomen 4-jointed in female; first antenna 23-jointed, joints 8 and 9, joints 24 and 25 are fused; second antenna with endopodite about as long as exopodite; mandible has exopodite longer than endopodite; maxillae and maxillipede are of usual type; first leg with 3-jointed exopodite and 1-jointed endopodite, outer marginal spine only on second and third joints of exopodite; second leg with 3-jointed exopodite and endopodite; third and fourth leg with 3-jointed exopodite and endopodite; fifth leg absent in female. Male resembles female in general appearance, differing from it in the absence of rostral spine, in the number of abdominal segment, and in the structure of fifth pair of legs. G. 0. SARS divided the genus in two; Aetideus and Euaetideus on the ground that the shape of rostral spine is different; to the latter genus belong A. giesbrechti, A. bradyi, and A. acutus. Aetideus armatus (BOECK) (Fig. 23, a-i)
Length. Female, 3.10~3.26 mm; male, 3.50~3.64 mm. Remarks. The present female specimen belongs to the small-sized example which has been suggested by SEWELL (1947).
The movement of the cold Oyashio water and the occurrence of the cold water mass in Kumanonada were discussed through the distribution of four cold-water copepods, Calanus plumchrus, Calanus cristatus, Eucalanus bungii bungii, and Metridia lucens, in the waters off east-central Honshu. The selected copepods which are known to be most dominantly distributed in the shallow layer of the Oyashio area, north of 40•‹N in general, were found in the present area in the depths greater than 400 m where the temperature was lower than 10•‹C. Their distribution seemed to agree with the layers where the Pacific Intermediate Water exists. Great differences were observed between C. cristatus obtained in the present area and that from the Oyashio area on comparing their composition of developmental stages, population, and body weight. These facts indicated that the reproduction of the cold-water copepods does not occur in the present area but they are only supplied from the Oyashio area. It is evident from their distribution pattern that the Oyashio water sinks and spreads widely to southward at the levels between 400 m and 1,000 m, and furthermore it extends westerly as far west as about 138•‹E after passing over the Izu ridge. Probably, the water finally reaches Kumanonada. There might exist two branches of the submerged Oyashio water. One flows along the coast of Honshu, north of the Kuroshio, and the other runs in offshore area, south of the Kuroshio. The appearance of the copepods suggested that the former retains the characteristics of the Oyashio water rather than the latter.
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