. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 146 R. A. TANKERSLEY AND R. V. DIMOCK. Figure 1. Scanning electron micrograph of a frontal cross section of the marsupial gill of Atwdonta cataracta showing the position of the glochidia larvae (G) in the brood chambers and the location of the sec- ondary water tubes (SWT) and interlamellar septa (1LS). Additional ab- breviations: F, gill filament. role of demibranchs as sites of larval storage during re- production have been reported previously (Peck, 1877; Lefevre and Curtis, 1910; Ortmann, 1911; Richard et al, 1991).
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 146 R. A. TANKERSLEY AND R. V. DIMOCK. Figure 1. Scanning electron micrograph of a frontal cross section of the marsupial gill of Atwdonta cataracta showing the position of the glochidia larvae (G) in the brood chambers and the location of the sec- ondary water tubes (SWT) and interlamellar septa (1LS). Additional ab- breviations: F, gill filament. role of demibranchs as sites of larval storage during re- production have been reported previously (Peck, 1877; Lefevre and Curtis, 1910; Ortmann, 1911; Richard et al, 1991). The non-marsupial gills (outer and inner demi- branchs of males and inner demibranchs of females) of A. cataracta possess continuous interlamellar septa that run dorso-ventrally at right angles to the gill surface and form evenly spaced, uninterrupted water tubes (Fig. 2) (Ridewood, 1903; Heard and Guckert, 1971). The primary water tubes of the marsupial lamellae are more numerous than those in non-marsupial gills and during gravid pe- riods are divided into three separate compartments: a central brood chamber serving as an ovisac, and two tem- porary secondary water tubes located on both the lateral and medial ends of the brood chamber parallel to the surface of the demibranchs (Fig. 2). These secondary water tubes are formed from extensions of the interlamellar septa prior to larval incubation, and may be associated with the long brooding period of this species; in particular, they are thought to be responsible for maintaining water transport across the gill surface for respiration, filtration, and aeration of developing larvae (Ortmann, 1911; Heard, 1975; Richard et 1991). Investigations of the flow dynamics associated with cil- iary suspension feeding in bivalves (see Silvester and Sleigh, 1984;J0rgensentVa/.. 1988; Silvester, 1988) have prompted several studies of the functional anatomy and ultrastructure of the bivalve gill (Moore, 1971; Owen, 1974; Way et 1989
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