The gut structure and ultrastructure of Neoergasilus japonicus (Harada, 1930), a copepod from the family Ergasilidae (Copepoda, Poecilostomatoida) and a parasite of fish, were compared at different periods of the life cycle: in free-living specimens in October and after attaching to fish in January and June. Differences in the depth of the intestinal epithelium were prominent and other cellular characteristics appeared seasonally variable. We relate these to changes in the physiological activity. Preliminary data from stable-isotope analyses of attached specimens suggest nutritional contribution from parasitism. The possibility of a diapause in the life cycle, as well as the relationship between the morphology of the gut and early evolutionary parasitism of N. japonicus, are discussed

In this study, the structure and ultrastructure of the digestive system are compared in the early larval stages (nauplii I and II) of two copepod calanoid species, Acartia clausi Giesbrecht, 1889 and Hemidiaptomus roubaui Richard, 1888. The nauplii I of both species have no functional digestive tract, which is represented initially only by a blind esophageal slit and yolky endodermal cells, which fill the most part of the naupliar body, whereas at the nauplius II stage the differentiated digestive tract becomes functional. The resorption cavity corresponding to the future midgut is progressively formed in the endodermal mass during the premolt phase; it is surrounded by differentiating epithelial cells. In the ecdysial phase the foregut has associated labral glands, the midgut young R-, B-, and R'-cells of epithelium, and there is a short open hindgut.

 AbstractBaud, A., Cuoc, C. and Alekseev, V. 222_. Ultrastructure of the digestive tract of Diacyclops thomasi (Cyclopoida, Copepoda) during different stages of encystment during a summer diapause. -Acta Zoologica (Stockholm) 85 : 181-189Diacyclops thomasi (Forbes, 1882) has a life cycle comprising a summer diapause with whole-body encystment at the copepodid IV stage. Formed progressively, the cyst can indicate the extent of entry into diapause. In this study the ultrastructure of the midgut epithelium during encystment of CIV was compared with that of active CIV and females.In active individuals two well-differentiated epithelial cell types were observed: vacuolar cells (B-cells), and cells without vacuoles but with more densely packed microvilli and abundant rough endoplasmic reticulum and mitochondria (F/R cells). In encysted specimens a striking transformation of the midgut epithelial layer was noted. Only inactive nuclei with highly concentrated euchromatin and encircled by a thin rim of hyaloplasm were observed. Compartments of both urosome and especially cephalothorax displayed large lipid-rich lacunae. At the beginning of encystment, the midgut epithelium showed an intermediate state.Changes in ultrastructure observed in the midgut epithelium of D. thomasi clearly reflect different stages of diapause, and represent clear evidence of profound reorganization, which is progressively induced by diapause in the organism.