An investigation of the morphology, histology and functioning of the food canal and the associated haemal canals in Echinus esculentus has been carried out.
Two kinds of secretory cell have been identified in the lips and pharyngeal wall which produce mucus and an acid secretion respectively. The oesophagus is lined with mucous cells. The stomach has an inner epithelium containing chains of fragile granules which are discharged from its free surface to provide extracellular enzymes. A layer of vacuolated cells which may assist absorption of liquids lies at the base of the epithelium. Amoebocytes of all kinds are present within the stomach wall and in the haemal channels. The siphon, intestine and rectum are lined by a cuticularized epithelium. The whole of the outside of the gut is covered with a ciliated peritoneum.
Ingestion in Echinus takes place either by the action of the tube feet holding and presenting such food as sea-weed to the mouth or by the teeth scraping food from the surface immediately below them. The co-ordinated action of many muslces of Aristotle's lantern results in the opening or closing of the teeth.
The pH of the gut graduates from 5·9 at the mouth to 6·3 at the stomach and 6·9 in the intestine.
Water circulates through the mouth, pharynx, oesophagus, siphon and intestine, particularly during periods when the animal is not feeding, and probably serves a respiratory function.
Experimental feeding of Echinus with iron saccharate shows that the substance is taken up by agranulocytes within the lumen of the stomach, transported through the stomach wall to the haemal spaces where association takes place with granulocytes so that clumps are formed. Migration of iron granules associated with amoebocytes takes place through the haemal canals to the radial and gonadial regions. There is no evidence to support Oomen's hypothesis that the granulocytes burst within the gut to provide digestive enzymes.
The haemal system acts as a pathway for the migrations of the nutritive granulocytes as well as providing a system for the possible diffusion outwards from the gut of dissolved substances.
The use of amoebocytes in the lumen and epithelium of the gut for carrying out some intracellular digestion and absorption is correlated with a poorly developed haemal system which lacks a pumping mechanism, with omnivorous feeding and a slow passage of food through the gut.