The gut of Arenicola marina, is a long straight tube stretching the entire length of the worm. It can be divided into five regions:
(a) the proboscis, containing the buccal mass, pharynx and postpherynx,
(b) the oesophagus, divided into the jugular, muscular and glandular regions.
(c) the stomach, consisting of the cardiac and post-cerdiac regions,
(d) the intestine end
(e) the rectum.
Four layers are distinguishable in the gut wall, though the thickness and the types of cells found in the innermost one vary from region to region. They are:
(i) the outer peritoneal covering,
(ii) a longitudinal muscle layer,
(iii) a circular muscle layer and
(iv) an inner epithelium.
There is a system of ciliary tracts, which in the glandular region of the oesophagus is associated with the longitudinal ridges of the lining epithelium; and which consists of the ciliated ventral groove and lateral tracts arising from it in the post-cardiac region of the stomach, the intestine and the rectum. There are isolated patches of cilia on the walls of the cardiac region of the stomach. Movement of the cilia in these tracts is believed to keep the contents of the stomach mixed and in suspension, thus ensuring that the food particles are brought into contact with the entire epithelium. The cilia also assist the passage of food through the gut, although this is mainly brought about by the rhythmic contractions of the body wall impinging upon the latter. The oesophagus is capable of a considerable amount of muscular activity and initiates some of the gut movements.
Parts of the blood system of the animal lie in close proximity to the gut and as the blood system plays a part in digestion, its arrangement and the circulation of blood have been described.
Food consisting of beach sand passes through the trunk gut, i.e. the proboscis, oesophagus, stomach and intestine, to the rectum in about fourteen minutes in an actively feeding worm. Food is taken into the oesophagus from the base of the funnel of the burrow by the constantly everting and inverting proboscis, without any preliminary sorting. It is temporarily stored in the muscular region of the oesophagus, but eventually it passes to the stomach through the glandular region of the former, where mucus and a little enzyme material is poured onto it by the unicellular glands in the walls. At the junction of the oesophagus and the stomach there are the openings of the oesophageal pouches, the secretion from which contains mucus and Qestive enzymes. While in the stomach food particles in the sand are digested both intra- and extra-cellularly, the epithelial cells engang the food particles thus leaving the lumen of the gut filled with the large residual mass of indigestible, mostly siliceous, material. These food particles are taken from the epithelial cells by amoebocytes, which while digesting them, wander to all parts of the body in the blood and coelomic fluid. The indigestible remains of these particles are deposited by the amoebocytes in the epidermis, peritoneal cells, intravasal tissue, coelom and in the lumen of the gut.
pH values are given for different parts of the gut, only that of the stomach (pH 5·4–6·0) differing appreciably from neutrality. Changes in pH affect the viscosity of the mucus, which is least viscous in the stomach where the pH is lowest. This aids the cilia and the movements of the gut in keeping the contents of this region in suspension. Elsewhere in the gut, the mucus is more viscous and acts as a lubricant, protecting the gut wall from the abrasive action of the sand grains.
An actively feeding worm defaecates at intervals of approximately forty-five minutes. During each defaecation cycle, the intestine absorbs 2·67 gm. of water. This is a considerable quantity for a worm weighing about 6 gm. (excluding tail). The reason for this absorption of water from the gut contents is not known, but it may be linked with the necessity to produce coherent faeces, which can be ejected well outside the exit of the burrow and so not interfere with the respiratory and feeding activities of the worm.
High ambient temperatures (water temperature 28°C.) do not affect the rate of defaecation, and, therefore, neither do they presumably affect that of feeding. These rates are influenced to a considerable extent by the water content of the sand in which the burrow of the worm lies.
The gut of Arenicola marina is adapted anatomically and physiologically to obtain the maximum extraction of the very small proportion of organic matter from the beach sand, which is its diet. The earthworm resembles Arenicola in that its diet also contains a large inorganic indigestible portion and as is to be expected, there is considerable similarity in the physiology of digestion in the two animals. In as far as the digestive processes of other sand eating animals are known, e.g. other oligochaetes, the spatangoids, and holothurians they all appear to share with A. marina the characteristic of a large amount of digestion occurring intracellularly in wandering amoebocytes, thus leaving the gut lumen free to deal with the vast quantities of indigestible siliceous material.