. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. NEOMENIOID DISCRIMINATION AND PHYLOGENY 139 the denticle-bearing membrane; (3) a pair of large, lateral denticles flanking two smaller ones above the buttress; and (4) a medial and distal hemispherical extension of the bar. Original, triangular tooth retained in anteroventral radular pocket (Fig. 17H-K). Copulatory spicules long, with four distinct sections: (1) a terminal, thickened, conical, curved tip (Fig. 18A-C. F, H): (2) a long, distal, thin-walled, twisted hemispherical channel (Fig. 18F, H) followed by (3) a short,
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. NEOMENIOID DISCRIMINATION AND PHYLOGENY 139 the denticle-bearing membrane; (3) a pair of large, lateral denticles flanking two smaller ones above the buttress; and (4) a medial and distal hemispherical extension of the bar. Original, triangular tooth retained in anteroventral radular pocket (Fig. 17H-K). Copulatory spicules long, with four distinct sections: (1) a terminal, thickened, conical, curved tip (Fig. 18A-C. F, H): (2) a long, distal, thin-walled, twisted hemispherical channel (Fig. 18F, H) followed by (3) a short, narrow, solid, twisted and furrowed section (Fig. 18E, G, H); and (4) a broad, hollow basal portion (Fig. 18D, E, G). Paired bundles of curved, hollow, thick-walled, accessory copulatory spic- ules bearing distal protuberances (Fig. 18J, K) present amongst thickly set epidermal spicules adjacent to mantle- cavity opening. Remarks. No entire, fully developed copulatory spicule was recovered from 10 dissected Plawenia individuals. In two small individuals ( mm in P. sphaera, mm in P. argentinensis) the spicules were composed only of distal sections 1 and 2 from just above the solid, yet-to-be-formed midsection (Fig. 18F). In five of six larger individuals, the distal sections of the spicules were missing and the spicules broken, either through the solid midsection (Fig. 18D, G), or somewhat above it (Fig. 18E). In one individual of inter- mediate size, the basal section was beginning to form while the tip was retained (Fig. 18H). Such a constancy between body size and absence of the distal section perhaps indicates that the distal end is deciduous rather than missing as an artifact of dissection. The biogeographic questions raised by the distributions of the three Plawenia species herein described are provoc- ative. P. schizoradulata lives at 4758 m on the Pacific side of the great eastward-bending parabolic arc that stretches from the southern tip of South America through
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
