Feeding and gut structure in Nerilla antennata (Annelida: Archiannelida)

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Abstract
The littoral and sublittoral marine archiannelid Nerilla antennata Schmidt 1848 browses selectively on organic debris and micro‐organisms (algae, protozoa and bacteria) resident on the substrata. The gut consists of a mouth, a buccal cavity with a posteriorly‐situated muscular pharynx, a proventriculus, stomach, intestine and anus. The alimentary canal is ciliated throughout its length, except for the muscular pharynx. The selected food is abraded and fragmented by the “to and fro” action of the muscular pharynx and this organ, aided by the anterior, ventral ciliary sheet, directs the food into the buccal cavity. Mucus from the posterior lips of the mouth and granules from the salivary glands are mixed with the food particles as these are concentrated in the proventriculus. The particles are then drawn, by a peristaltic wave, into the stomach. Regurgitation is prevented by posteriorly‐directed cilia which line the constriction separating the proventriculus from the stomach.The stomach wall consists of cells which vary in shape from columnar (25 μm in height) to cuboidal (12 pm) and there is an anterior ring of strongly basophilic cells surrounding the entrance of the proventriculus into the stomach. The distal regions of these cells contain numerous proteinaceous granules and bear a somewhat reduced number of cilia.Pyriform gland cells occur laterally beneath the cells of the stomach wall. Their necks penetrate between the sharply delineated stomach wall cells, to discharge a proteinaceous mucoid material into the gut lumen. Sections of the stomach stained with toluidene blue differentiated two types of non‐glandular cells, one positively staining and the other negative ly staining. The anterior stomach contains a predominance of negatively‐staining cells, whilst posteriorly the proportion of positively staining cells increases until they form a ring surrounding the posterior stomach lumen.Lipid droplets were localised mainly in the epidermis and in the walls of the stomach and intestine. Following a short period of starvation a lowered concentration of lipid droplets in the distal region of the stomach and intestinal cells was noted.