Functional morphology of the heart/kidney complex, digestive system and mantle of dentalium rectius (mollusca, scaphopoda)




Reynolds, Patrick Dennis

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The functional morphology of the heart/kidney complex, digestive system and mantle was investigated in Dentalium rectius (Mollusca, Scaphopoda). While encompassing in-depth examination of the diverse roles of each organ system, these studies also contribute towards an overview of metal processing by the organism. The heart/kidney complex departs substantially from typical molluscan form; morphological features demonstrate that the heart is reduced to a perianal sinus adjacent to the pericardium. Excretory function appears to be maintained, however, pericardial podocytes and a right renopericardial connection indicate that a blood ultrafiltrate passes to the kidney. Urine is modified by two nephrocyte types. While one may effect reabsorption, both secrete calcium, zinc and phosphate-containing granules into the urine. Intracellular granules of the digestive system also contain calcium phosphate; iron is the only other metal accumulated, principally by oesophageal and stomach epithelia. Iron uptake occurs via digestive cells and by both undifferentiated and specialized mantle epithelia. Iron-containing granules, released into the haemocoel by the mantle epithelium, are phagocytosed and transported by amoebocytes. Iron is not excreted by the kidney, but by oesophageal secretion into the gut lumen and by radular mineralization, differing significantly from iron processing reported in other molluscs. In addition to iron uptake, mantle functions include the creation of respiratory currents, gas exchange and sensory reception; the respective epithelial specializations described here constitute functional equivalents to ctenidia and osphradia, organs which are absent in this molluscan class. The ciliated bands of the mid-mantle region include supporting cells which possess high-domed, microvillous apices that facilitate diffusion between the mantle cavity and underlying haemocoel. The posterior region of the mantle is richly endowed with innervated cells, considered putative sensory receptors. Cilia number, length and ultrastructure define three receptor types. They are heterogeneously distributed among specific regions of the pavilion, and probably function in respiratory current testing. The maintenance of respiratory current passage to the scaphopod mantle cavity requires a secondary increase in posterior aperture size, which is otherwise progressively diminished by normal shell growth. Such an increase occurs in D. rectius, and is effected by periodic shell decollation through dissolution by the posterior mantle.



Scaphopoda, Morphology, Dentalium