Abstract
Two megalopa larvae of Percnon planissimum (Herbst) were obtained on the coasts of Hayama, Sagami Bay (Sep., 1964) and Hachijo Island (Aug., 1966), and reared in the aquarium until they reach the first crab stage. Megalopa stage: (The carapace 5mm in length, 3.8mm in width.) The carapace has no dorsal spine. The dorsal surface of carapace is thickly covered with a fine tomentum. The rostrum is horizontally projecting forwards. The postero-lateral portion of the carapace is somewhat swollen. The flagellum of second antenna has ten segments and its distal segment is furnished with a short hair and two long ones. In the third maxilliped the endopodite has the distal five segments setose and its exopodite one-segmeted and about half as long as the ishium. The dactylus of the fourth ambulatory legs is provided with two long feelers at the distal end. The second to fifth abdominal segments are respectively provided with a pair of biramous pleopods. There are 33〜44 swimming hairs on the lateral margin of the exopodite of pleopods. The small hooks are growing along the inner margin of the endopodite of pleopods. The sixth abdominal segment is provided with a pair of uropods, which are rather smaller in size, bearing 25〜26 swimming hairs on the lateral margin. The number of segments of exopodite and that of swimming hairs are compared with those of the megalopa of Plagusia dentipes and Plagusia depressa tuberculata in Table I. First crab-stage: (The carapace 5.5mm in length, 5mm in width.) The aspect of the body approaches that of the adult crab. The carapace is slightly larger than that of the megalopa. In the first crab-stage, the abdomen has seven distinct segments, while in the next stage (carapace 8mm in length, and 8mm in width), the third to fifth abdominal segments are almost coalescent with each other as in the adult crab.
