THE DIGESTIVE-TRACT OF THE SPATANGOID ECHINOID ECHINOCARDIUM-CORDATUM(ECHINODERMATA) - MORPHOFUNCTIONAL STUDY

Echinocardium cordatum is a detritus-feeder. Its digestive tract has t hree functional parts: the mucus-secreting oesophagus where lubricatio n and sediment compaction occur, the protein-secreting stomach and gas tric caecum where extracellular digestion occurs, and the absorptive n on-secretory siphon, intestine, intestinal caecum and rectum. Each reg ion harbours typical enterocytes. Acinar mucous glands occur in the po sterior oesophagus prior to the incurrent aperture of the siphon; thei r secretions prevent sediment grains from entering the siphon and may be stimulated by epineural nerves running in the connective tissue lay er. The siphon is a sucking organ, and its cytological features indica te that it is a site for absorption of dissolved organic matter. Enter ocytes of the intestinal caecum show basal specializations, suggesting that it is a site of active exchange between coelom and gut. This cae cum contains symbiotic sulphide-oxidizing bacteria. Collagen fibres ar e well developed in organs subjected to severe deformations (e.g., oes ophagus and siphon); elastic-like fibres are conspicuous around haemal lacunae and at the attachment areas of gut-suspending mesenteries. Ci rcular muscles are seen along the whole gut, whereas longitudinal musc les may be absent in areas such as the siphon. The bulky alimentary bo lus that occurs in the ascending segments of the gut is transported by peristalsis; the scattered sediment grains that occur in horizontal s egments are presumably moved by the contractions of the gut-suspending mesenteries.