This account is the result of investigations on living and preserved specimens of Pomacea canaliculata as well as preserved specimens of Pila globosa, Turbinicola saxea and Lanistes ovum bangweolicus.
Pilids are amphibious prosobranchs with both a lung and a ctenidium in the mantle cavity. The mantle cavity is broad and shallow, and in addition to the normal pallial complex in these there is a highly extensible pallial inhalant siphon on the left of the head, allowing ventilation of the lung under water, and a fold bordering a longitudinal groove to the right of the head, similar to that in viviparids which creates a separate exhalant channel leading to a samll exhalant siphon. the ctenidium is displaced by the lung to the right and its small leaflets are curved so that they overhang the pallial groove. The hypobranchial gland to the right of the ctenidium extends over the wall of the rectum and genital duct along the right side of the groove.
Blood is carried to the lung by the extension of the afferent ctenidial vein and drained from it by the efferent ctenidial vein. In Pomacea an additional afferent vein to the floor of the lung is apparently developed by the horizontal division of the efferent ctenidial vein. To maintain an ample supply of blood at high pressure to the respiratory organs the visceral vein leads directly on to the mantle skirt and has lost its connection with the afferent renal vein. This results in a highly vascular mantle skirt so that a large volume of blood would be forced into the heart on retraction of the snail into its shell; to accommodate this a highly extensible ampulla is developed on the anterior aorta within the pericardial cavity.
The kidney, like that of other freshwater prosobranchs, is complex and contains an accessory chamber, overlying the posterior part of the mantle cavity, and which appears to be mesodermal in origin. It is much folded and highly vascular, and is supplied by the afferent renal vein and the renal nerve. The epithelium of the chamber excretes some purines other than uric acid as very fine spherules, whereas in the posterior part of the kidney, large concretions containing uric acid are built up. Blood from the anterior chamber drains into the afferent ctenidial vein; the efferent vessel of the posterior chamber is the homologue of the efferent nephridial vein, although the nephridial gland itself is vestigial. The efferent renal vein to the mantle skirt, which is present in other prosobranchs, has lost its connection with the visceral part of the kidney in pilids.
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