The fine structure of the carapace integument of Daphnia magna Straus (Crustacea Branchiopoda).

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.