. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. A. H. SCHELTEMA Figure 2. Peiagosphera larvae of Sipuncula. (A) Frontal view of head. Siinmailu* sp. (from Rice, 1981, fig. 4). (B) Entire larva. Aspidosiphon sp. (from Rice, 1981, fig. 6). Numbers as in Figure 3: I huccal gland, 3 pore of lip gland, 4 mouth, 5 lip. which opens either directly, or by way of a ciliated duct or ducts, into the lip pore. In comparison, the anterior pedal gland in larval chitons and in Aplacophora is duct- less ( Figs. 2A, 4B, 6C). Aplacophora, but not chitons, have a central ciliated pit. S
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. A. H. SCHELTEMA Figure 2. Peiagosphera larvae of Sipuncula. (A) Frontal view of head. Siinmailu* sp. (from Rice, 1981, fig. 4). (B) Entire larva. Aspidosiphon sp. (from Rice, 1981, fig. 6). Numbers as in Figure 3: I huccal gland, 3 pore of lip gland, 4 mouth, 5 lip. which opens either directly, or by way of a ciliated duct or ducts, into the lip pore. In comparison, the anterior pedal gland in larval chitons and in Aplacophora is duct- less ( Figs. 2A, 4B, 6C). Aplacophora, but not chitons, have a central ciliated pit. Similarities in form and function in these three struc- tures—lip and foot, lip glands and pedal glands, and buc- cal organ and radula with its sac—are striking. Their morphologies are particularly clear in sagittal sections of a pelagosphera and a chiton larva (Figs. 3, 4). There are also similarities in their development, as they all arise from posttrochal ectoderm, with these differences: in si- punculans, the origin of all three structures is stomodeal, whereas in mollusks, the ventral somatic plate, usually from cell 2d, gives rise to the foot and its glands, and only the radula sac is stomodeal (Raven, 1966). In Sipuncula as well, cell 2d gives rise to the somatic plate, which forms the ectoderm of the trunk (Rice, 1976). In mollusks, how- ever, the proximity and functional interdependence of the somatic and stomodeal structures are indicated by the pedal contribution to feeding in veliger larvae. An anterior, medial ciliary tract is formed on the foot by which particles unsuitable for ingestion are rejected (Moor, 1983). Only the head region of the pelagosphera, which is rad- ically altered during metamorphosis to a juvenile sipun- culan. can be compared to the Mollusca. The posterior part of the body with its large coelomic sac, nephridia, mid-dorsal anus, and ventral nerve cord, are already de- finitive adult structures. Evidence from the presence of the molluscan c
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
