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The structure of the two integumental layers comprising the carapace of female D. magna was examined at several points through the molt cycle. The epicuticle and procuticle are simple in organisation; pore canalsare absent but intracuticular fibres are present, forming complexes with invaginations of the epidermal plasma membrane similar to such complexes described in the literature for othe arthropods. The epidermis consist almost entirely of cuticle-secreting cells. Secretion of the new cuticle begins when 50-67% of the instar has elapsed by which time the epidermal cells have increased in height and their nuclei have become more rounded. However, other presumed secretory phenomena observed viz. the formation of dense core vesicles by Golgi bodies, and the occurrence of these and coated vesicles near the apical plasma membrane are not restricted to any particular period during the molt cycle. This suggests that the mechanisms of cuticle secretion do not undergo marked changes in activity as they do in decapods; presumably this relative continunity is related to the much shorter molt cycle of cladocerans.

A B S T R A C TAmphipod silk is a fibrous, self-secreted, adhesive substance employed in tube-building by amphipod species within the Corophiidea, Ampeliscoidea and Aetiopedidea. In the present study we provide a detailed characterisation of a novel, marine-based silk production system situated in pereiopods 3 and 4 in the corophioid Crassicorophium bonellii and the aorid Lembos websteri. The silk material is a mixture of protein and mucopolysaccharides. Ultrastructural and histological analyses revealed that silk in both species is produced in several rosette-type glands, presumed to be of two different types. These glands are distributed among all limb articles apart from the coxa but mainly in the basis and merus of pereiopods 3 and 4. Secretion commences in the basis and a thread-like secretion product leaves the glandular pereiopod through a cuticular pore near the dactylar tip. The silk's physical and chemical properties most likely change while moving through the dactylar duct, which subdivides into several small ductules and terminates in a spindle-shaped chamber. This chamber, which communicates with the exterior, may be considered a silk reservoir in which the silk appears fibrous. For the first time an independently evolved, marine arthropod silk processing and secretion system is described.

The ultrastructure of the nuchal organ of first instar of Daphnia magna is similar to that described in the literature for presumed ion-transporting tissues in other crustaceans. The apical and basal plasma membranes are extensively amplified through microvilli and infoldings; mitochondria are abundant and distributed unequally between the two types of cells (dark and light) composing the organ, more being present in the dark cells. The two cell types differ also in the type of microvilli projecting from their apical surface. In proecdysis these microvilli are replaced by much smaller ones which are associated with the deposition of the new cuticle.

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