The posterior esophagus of Bulla striata, running from the gizzard to the stomach, was investigated with light and electron microscopy to obtain new data for a comparative analysis of the digestive system in cephalaspidean opisthobranchs. In this species, the posterior esophagus can be divided into two regions. In the first, the epithelium is formed by columnar cells with apical microvilli embedded in a cuticle. Many epithelial and subepithelial secretory cells are present in this region. In both, electron-lucent secretory vesicles containing filaments and a peripheral round mass of secretory material fill the cytoplasm. These acid mucus-secreting cells may also contain a few dense secretory vesicles. In the second part of the posterior esophagus, the cuticle is absent and the epithelium is ciliated. In this region, epithelial cells may contain larger lipid droplets and glycogen reserves. Subepithelial secretory cells are not present, and in epithelial secretory cells the number of dense vesicles increases, but most secretory cells still contain some electron-lucent vesicles. These cells secrete a mixture of proteins and acid polysaccharides and should be considered seromucous. The secretory cells of the posterior esophagus are significantly different from those previously reported in the anterior esophagus of this herbivorous species.

Histochemical tests were done on newly excysted metacercariae and worms recovered from an abnormal host (rat) and the definitive host (dog) for some oxidoreductases, phosphatases and glycosidases. The results demonstrate that rat worms have enzymatic distribution and intensities more similar to those of metacercariae than to adult worms from dogs. Ultracytochemical examination of acid and alkaline phosphatase and Mg-ATPase activity was also carried out. Acid phosphatase activity occurred exceptionally in the excretory bladder and caeca of dog worms. No activity was observed in rat worms except for lysosomal granules in the tegument. Alkaline phosphatase activity was exhibited in the excretory bladder in both dog and rat worms. Mg-ATPase activity occurred in the tegument and parenchymal cells in dog worms and in the excretory bladder in rat worms. In metacercariae, little or no reaction for these enzymes was present except for Mg-ATPase activity on the excretory ducts. These observations, together with the histochemical results, indicate that metabolic activity in rat worms is higher than in metacercariae although it is strongly reduced compared with dog worms.

The distribution of different classes of acetylcholine (ACh) receptor-like molecules in sperms of different invertebrate and vertebrate species is described. ACh receptor molecules belong to one of two classes: muscarinic receptors (mAChRs), associated with signal transduction mechanisms in the inner domain of the cell, and nicotinic receptors (nAChRs), capable of opening Na+ channels when activated by the ligand. Molecules immunologically related to mAChRs and to ACh can be identified by specific antibodies, and revealed by immunofluorescent or immunogold staining; the nicotinic receptor-like molecules are localised as curare-sensitive affinity sites for α-bungarotoxin. In all species studied, both classes of receptors were found, with a similar distribution. Muscarinic-like molecules were found mainly in the sperm head regions of most species; such a localisation may be correlated to a function in sperm–egg interaction, for instance in the regulation of the block to polyspermy. Nicotinic-like molecules are present mainly in the tail and in the post-acrosomal region of most animals, thus confirming their function in the regulation of sperm propulsion, but are also present at the acrosomal region of most species. The distribution patterns of the different classes of molecules indicate that both may be in sperm–egg interactions, in addition to their known function in the regulation of sperm propulsion.

The aim of this report is the synthetic presentation of observations related to ultrastructural aspects of the lipid metabolism during the ontogenesis of trophic mechanisms in three teleost species, the sea bass, Dicentrarchus labrax, the sea bream, Sparus aurata and the pike-perch, Stizostedion lucioperca. The results have shown the respective roles of the vitelline syncytium, the intestine, the liver and the exocrine pancreas during the change from an endogenous lipid source, the yolk vesicle, to an exogenous lipid one, food. They make it possible to stress the close correlation between the development of the biliary function, that of the exocrine function of the pancreas and that of the intestinal absorption of lipids. These activities established during the endotrophic stage are slight at the beginning of trophic life and then increase at the end of the endo-exotrophic stage. Analysis of the results suggests the occurrence of four physiological phases characteristic in early lipid metabolism establishing the main heads in acquisition of the trophic autonomy according to a pattern similar to that of mammals.

In this study, glycogen distribution during the development of larval salivary glands in two Drosophila species (D. melanogaster and D. auraria) was examined using the osmium tetroxide–potassium ferrocyanide technique at the ultrastructural level. Glycogen particles are found in big clumps in the salivary gland cells of D. melanogaster during the early third instar, mainly at the basal part of the cells; they are also found as scattered particles in the cytoplasm and very close to the apical plasma membrane during the secretory activity at this developmental stage. During the middle third instar, where the production of ‘glue’ secretory granules begins, glycogen particles are mainly scattered and they are seen near the Golgi complexes and secretory granules. No glycogen particles are seen during the late third instar and secretion of ‘glue’ secretory granules. Re-formation of glycogen begins after spiracle eversion. In the larval salivary gland cells of D. auraria, glycogen particles (scattered and in clumps) are seen in every developmental stage. In the larval fat body cells of both species, many huge clumps of glycogen particles are seen, independent of the developmental stage. These results indicate that glycogen provides an important source of energy during development of Drosophila salivary glands, but different species use glycogen at different metabolic pathways, according to their needs.