. Bulletin. Natural history; Natuurlijke historie. 20 PEABODY MUSEUM BULLETIN 41. 1 mm. Fig. 5. Diagrammatic section through the ctenidium oiGastrochaena (Gastrochaena) hians: transverse section through the middle of the siphon. The directions of the inhalant and exhalant currents are indicated by the lower and upper arrows, respectively. Legend: O. Outer demibranch. /. Inner demibranch. S. Ctenidial septum. The inhalant siphon of S. rostrata and G. (R.) ovata differs from G. (G.) hians in having a transverse valve across its base. Similar valves have been described for 5. mytiloides by Pelsen
. Bulletin. Natural history; Natuurlijke historie. 20 PEABODY MUSEUM BULLETIN 41. 1 mm. Fig. 5. Diagrammatic section through the ctenidium oiGastrochaena (Gastrochaena) hians: transverse section through the middle of the siphon. The directions of the inhalant and exhalant currents are indicated by the lower and upper arrows, respectively. Legend: O. Outer demibranch. /. Inner demibranch. S. Ctenidial septum. The inhalant siphon of S. rostrata and G. (R.) ovata differs from G. (G.) hians in having a transverse valve across its base. Similar valves have been described for 5. mytiloides by Pelseneer (1911) and for "Rocellaria" [=Gastrochaena (G.)] cuneiformis by Purchon (1954). In S. rostrata the larger part of the valve is a crescent-shaped partition suspended ventrally from the pallial septum separating the inhalant and exhalant siphon tubes. This is accompanied by two smaller flaps projecting from ven- trolateral positions on the wall of the inhalant siphon. When contracted, these valves restrict access between the inhalant siphon and the mantle cavity. The valve in G. (R.) ovata appears more delicate and consists of a single membranous flap suspended ventrally from the siphonal septum separating the inhalant and exhalant siphon tubes. Contraction of this process blocks only one-third of the aperture at the base of the inhalant siphon. A siphonal valve does not occur in G. (G.) hians, undoubtedly because its inhalant and exhalant siphons are separated anteriorly by a ctenidial rather than a pallial septum (Fig. 5). According to Haas (1935) these siphonal processes function as check-valves to arrest the flow of water through the inhalant siphon. These valves might assist purging pseudofeces, because pseudofeces collect near the base of the inhalant. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the origin
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