File:American malacological bulletin (1988) (17534109264).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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144 AMER. MALAC. BULL. 7(2) (1990) cardium as described by Plate (1891, 1892), Boissevain (1904) and Distaso (1905) for other species of Dentalium. Without the benefit of ultrastructural study, it is likely that these early investigators considered the contractile dorsal pericardial wall as a ventricular epicardium. The lack of a myocardium or any ultrastructural differentiation of this portion of the pericardial epithelium discounts this interpretation, and no evidence sup- porting homology with the molluscan ventricle exists. While the scaphopod stomach is described as possessing a muscular tunic (Salvini-Plawen, 1988), the musculature in D. rectius is discontinuous and closely applied to the stomach wall and as such is not associated with the pericardium and does not enclose a portion of the haemocoel. The homology of the pericardial coelom described by Lacaze-Duthiers (1857), Plate (1891, 1892), Boissevain (1904) and Distaso (1905) with that of other molluscs is based sole- ly on its general anatomy, i.e. a closed sac composed of squamous epithelium within the haemocoel. Ultrastructural features of the pericardial epithelium in Dentalium rectius sup- port this homology; the presence of long cytoplasmic branches, desmosomes and few other organelles suggest a simple delimiting epithelium, similar to that found in other molluscan peri- and epicardia. Furthermore, an excretory role inferred from the presence of podocytes and a renopericar- dial connection also supports this homology. On this basis the term pericardium should be retained in describing this structure in scaphopods. The arrangement of musculature in the Dentalium rec- tius pericardium suggests transverse contractions of the dorsal pericardial wall, which are supported by the observations of live animals. The contractile pericardia in the Polyplacophora have been suggested by Okland (1981) to function in the cir- culation of pericardial fluid as part of the excretory system, with little effect on the contraction mechanisms of the heart. In Tonicella, the epithelial and muscle cells are separated by collagen and basal lamina, which is, however, continuous with the basal lamina lining the epithelial cells (Okland, 1981). In comparison, no extracellular material exists between cell types in the pericardium of D. rectius, and the basal lamina is limited to an unbranching layer between the pericardial elements and haemocoel. Contractions of the dorsal pericardial wall in D. rectius undoubtedly contribute to the circulation of blood through the relatively large abdominal sinus. These contrac- tions do not lead directly to ultrafiltration of blood through the dorsal pericardial wall due to the absence of podocytes in this area of the pericardium. However, it is possible that local in- creases in blood pressure could be transferred anteriorly to the perianal sinus and ultimately facilitate ultrafiltration via the podocytes lining the perianal sinus. The irregular in- vaginations of the dorsal pericardial wall of D. rectius seen in section and considered by Plate (1891, 1892), Boissevain (1904) and Distaso (1905) in other Dentalium species to be the heart, are due to the state of contraction of the dorsal pericardial wall at fixation and do not represent a permanently enclosed contractile vessel. The ventral pericardial wall was always observed in close adherence to the body wall in both fixed and live material. The presence of podocytes is a development of the pericardial epithelium that is commonly observed and inter- preted in other molluscan classes as the site of ultrafiltration of blood and production of primary urine. While ultrastructural features such as apposition of basal lamina, fenestration width and the presence of slit diaphragms in the podocytes of Dentalium rectius are consistent with such a function, there is no evidence of an extensive array of pedicels and ultrafiltra- tion slits as seen in representatives of some other molluscan classes (Andrews, 1979; Meyhdferef a/., 1985). Thus, the area available for ultrafiltration appears to be quite limited in D. rectius. The renopericardial connection with the right kidney is small (20 in diameter), and was only noted in inciden- tal thin (1 /itn) sections. A left renopericardial canal in Dentalium was described by Distaso (1905), although it ap- pears from reading his account that the dorso-ventral orien- tation of the animal is opposite to that generally accepted by other investigators of the Scaphopoda. Therefore, consider- ing the mantle cavity as ventral and the larger aperture as
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Fig. 25. Podocytes of the pericardium. Note raised pedicels (arrows) (h, haemocoel; pc, pericardial cavity). Scale bar = 0.3 ^m. Fig. 26. Podocytes of the pericardium. Note fenestrations in epithelium ap- posed by basal lamina (arrowheads) and bridged by diaphragms (ar- rows) (h, haemocoel; pc, pericardial cavity). Scale bar = 0.3 nm.

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