Morphology, morphometry and ultrastructure of the maxillary gland of the terrestrial isopod Porcellio scaber (Crustacea, Oniscidea)

Abstract

The ultrastructure of the maxillary gland of the terrestrial isopod Porcellio scaber is described. The gland is composed of an end sac, an excretory duct and a terminal duct which opens by a valve at the base of maxilla 2. An epithelium of podocytes in the end sac enables passive ultrafiltration by haemolymph pressure. The excretory duct shows ultrastructural adaptations to secretion and resorption. SEM micrographs reveal the location and the morphology of the valve at the excretory pore. A model reconstructed from serial sections allowed the calculation of morphometric data of the gland. The ultrafiltration area of the end sac and the area of resorption and secretion of the excretory duct amount to 0.091 and 0.157 cm2 per 1 g of fresh body weight, respectively. The total volume of the gland is calculated to be 0.04 mm3 in a specimen of 13.7 mm length. In comparison with the marine species Mesidotea entomon, the relative areas for ultrafiltration and resorption of the gland of P. scaber are more than twice as large as the corresponding areas of the marine species. This relative enlargement of the gland in P. scaber and the form of the valve at the excretory pore are seen as adaptations to terrestrial life.