De Jong‐Moreau, L. and Casanova, J.‐P. 2001. The foreguts of the primitive families of the Mysida (Crustacea, Peracarida): a transitional link between those of the Lophogastrida (Crustacea, Mysidacea) and the most evolved Mysida. —Acta Zoologica (Stockholm) 82: 137–147 The morphology of the foregut, which is a good indicator of phylogenetic relationships, has been studied within the two suborders of the Mysidacea. Special attention has been focused on rare species belonging to the Lophogastrida and the three primitive families of the Mysida, i.e. Stygiomysidae, Lepidomysidae and Petalophthalmidae. It appears that the foregut of Gnathophausia gracilis (Lophogastrida) is the most primitive in the Mysidacea, and that in the Petalophthalmidae the foregut exhibits transitional stages between that of the Lophogastrida and of the evolved Mysidae. Moreover, in the four families of Mysida, ancestral characteristics of the foregut remain, thus strengthening the hypothesis of the unity of this order.

Scanning microscope investigations were carried out on mandibles, labrum and paragnaths of several species of Mysidacea and Euphausiacea. Gut content analyses were in agreement with morphological observations. It appears that the morphology of the peri-oral structures and especially of the mandibles reflect the feeding habits, and that the well known relationships between the size of the molar process and the incisor one, i.e. a large molar process is associated with herbivorous feeding, is not always verified. Bacescomysis abyssalis and Bentheuphausia amblyops are believed to be mostly saprophagous, Boreomysis inermis and Meganyctiphanes norvegica are mostly phytophagous, Hemimysis speluncola is omnivorous, while Siriella armata and Thysanopoda orientalis are carnivorous species.

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Comparative structural, ultrastructural, and cytochemical studies were made of the midgut and hepato pancreas of five species belonging to the two suborders of the order Mysidacea, the Lophogastrida and Mysida, under different feeding conditions to investigate their role in digestive processes. Nutritional experiments were also carried out to complete the study. The midgut shows structural differences between the two suborders. The epithelium is composed of a single cell type that is implicated in absorption and production of peritrophic membranes. The intestinal epithelium lies on a characteristic three-layered basal lamina, the thickness of which would increase the strength of the intestinal wall for peristaltis and for transport of fecal pellets. The hepatopancreas displays marked structural differences between the suborders, and the cell distribution is better ordered in Mysida. Absorptive R-cells play an important role in lipoprotein metabolism. B-cells are implicated in intracellular digestion, as is attested to by the presence of a typical vacuolar apparatus. The total absence of the F-cell, considered to secrete enzymes in the Decapoda, raises the question of the origin of the enzymes necessary for extracellular digestion. A review of the cell affiliations of the hepatopancreas in the Mysidacea is required.