. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. API ACOPHORA: PROGENETIC COELOMATES 61. Figure 3. Midsagittal section of the pelagosphera larva, Phascolosoma agassi:ii (from Rice, 1973, pi. 5). 1 buccal organ, 2 lip gland, 3 pore of lip gland, 4 mouth, 5 lip, 6 stomach, 7 coelom, 8 esophagus. of older echiuran larvae is the prototroch anterior to the mouth. In sipunculan pelagosphera. the metatroch below the mouth, not the prototroch, is the major swimming organ. Indeed, the region in pelagosphera that forms the head with its locomotory lip, lip gland, and buccal organ, i
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. API ACOPHORA: PROGENETIC COELOMATES 61. Figure 3. Midsagittal section of the pelagosphera larva, Phascolosoma agassi:ii (from Rice, 1973, pi. 5). 1 buccal organ, 2 lip gland, 3 pore of lip gland, 4 mouth, 5 lip, 6 stomach, 7 coelom, 8 esophagus. of older echiuran larvae is the prototroch anterior to the mouth. In sipunculan pelagosphera. the metatroch below the mouth, not the prototroch, is the major swimming organ. Indeed, the region in pelagosphera that forms the head with its locomotory lip, lip gland, and buccal organ, is represented in echiuran larvae by only a few rows of cells between the prototroch and metatroch, and no larval organs are present. If sipunculans are sister taxon of the Mollusca, they must have arisen, like mollusks, early in the evolution of metazoans. One piece of evidence for an early sipunculan history is the mid-Cambrian genus Ottoia from the Bur- gess Shale. Considered priapulids by Conway Morris (Whittington, 1985) and close to priapulids by Banta and Rice (1976), the genus indicates great diversity of spe- cialized sacciform, coelomate or pseudocoelomate, worm- like animals already in the early Paleozoic. Sipunculans therefore could have a very long, but unobservable and unverified, geologic history. A second piece of evidence is that sipunculans contain hemerythrins, found also only in priapulids, lingulid brachiopods, and some annelids (Curry and Runnegar, 1990). Because lingulids and prob- ably priapulids and annelids are known from the early Cambrian, the presence of hemerythrins indicates a very long history for all forms having these oxygen transport molecules. Si:e of the pericardium in "primitive" mollusks The pericardium is larger relative to the heart in Apla- cophora, Monoplacophora, and Polyplacophora than it is in Gastropoda, Pelecypoda, and Cephalopoda (Schel- tema, 1973, 1988; Scheltema and Kuzirian, 1991) (Figs. 5, 6A). Ontogenetically, the
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
