The digestive tract of the spatangoid echinoid Echinocardium cordatum (Echinodermata): morphofunctional study
In: Acta Zoologica (Stockholm). Svenska Bokfoerlaget: Stockholm. ISSN 0001-7272; e-ISSN 1463-6395, meer
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| Abstract |
Echinocardium cordatum is a detritus-feeder. Its digestive tract has three functional parts: the mucus-secreting oesophagus where lubrication and sediment compaction occur, the protein-secreting stomach and gastric caecum where extracellular digestion occurs, and the absorptive non-secretory siphon, intestine, intestinal caecum and rectum. Each region harbours typical enterocytes. Acinar mucous glands occur in the posterior oesophagus prior to the incurrent aperture of the siphon; their secretions prevent sediment grains from entering the siphon and may be stimulated by epineural nerves running in the connective tissue layer. The siphon is a sucking organ, and its cytological features indicate that it is a site for absorption of dissolved organic matter. Enterocytes of the intestinal caecum show basal specializations, suggesting that it is a site of active exchange between coelom and gut. This caecum contains symbiotic sulphide-oxidizing bacteria. Collagen fibres are well developed in organs subjected to severe deformations (e.g., oesophagus and siphon); elastic-like fibres are conspicuous around haemal lacunae and at the attachment areas of gut-suspending mesenteries. Circular muscles are seen along the whole gut, whereas longitudinal muscles may be absent in areas such as the siphon. The bulky alimentary bolus that occurs in the ascending segments of the gut is transported by peristalsis; the scattered sediment grains that occur in horizontal segments are presumably moved by the contractions of the gut-suspending mesenteries. |
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