Abstract
To continue the investigation on the digestive system of polyplacophoran molluscs, a histological and ultrastructural study of the oesophagus, stomach and intestine of Chaetopleura angulata and Acanthochitona fascicularis was carried out. Stomach content examination revealed an omnivorous diet. In both species the epithelium of the whole digestive tract consisted mostly of elongated absorptive cells with an apical border of microvilli. Cilia were also frequently present. Mitochondria and electron-dense lysosomes were the prominent organelles in the region above the nucleus. The basal region was characterised by an association of mitochondria, peroxisomes and lipid droplets. In general, glycogen deposits were also abundant in absorptive cells. The ultrastructural features indicate that the absorptive cells of the digestive tract epithelium are involved in endocytosis, intracellular digestion and storage of reserves. Histochemical techniques showed that the secretory cells of the digestive tract contained proteins and polysaccharides in their secretory vesicles. The secretory cells with vesicles of low electron density were classified as mucous cells, and the ones with electron-dense vesicles were designated basophilic cells due to their staining by basic dyes in light microscopy. Additionally, basal cells that seem to correspond to enteroendocrine cells containing oval electron-dense vesicles were found along the digestive tract epithelium of both species. The thin outer layer of the digestive tract wall consisted of muscle cells and nerves embedded in connective tissue.
Highlights
Polyplacophorans, commonly known as chitons, are benthic marine molluscs characterized by their eight articulated dorsal shell plates surrounded by a flexible marginal girdle, which allows them to curl into a ball
The long intestine departed from the stomach and formed several
The long intestine departed from the stomach and formed several coils around the digestive gland (Figure 1A,B)
Summary
Polyplacophorans, commonly known as chitons, are benthic marine molluscs characterized by their eight articulated dorsal shell plates surrounded by a flexible marginal girdle, which allows them to curl into a ball. The broad and long radula used for feeding contains numerous hard teeth mineralised with magnetite and hydroxyapatite It is formed within the radular sac that can extend back from the buccal cavity to approximately one third of the animal length [1,15]. The oesophagus is relatively short and it is linked to two large pouches known as “sugar glands” as they contain polysaccharide digesting enzymes [17] These glands that were recently investigated by light and electron microscopy contain secretory cells and absorptive cells with many lysosomes [9]. Stomach content was analysed to obtain some information about the diet of these species
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