THE FEEDING MECHANISM, FUNCTIONAL MORPHOLOGY AND HISTOLOGY OF THE ALIMENTARY CANAL OF TEREBELLA LAPIDARIA L. (POLYCHAETA)

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Abstract
Summary.: Food ingestion in Terebella lapidaria has been described. The tentacles form the food collecting apparatus. Food trapped by the mucus which they secrete, is transferred to the upper lip whence it is conveyed by cilia to the mouth. The lower lip is of little importance in food ingestion. The structure and functions of the alimentary canal have been studied. The upper and lower lips, particularly the latter, are associated with a complex musculature. Also internally, are to be found three coelomic cavities, those of the upper and lower lips and of the thoracic cavity. The last two are separated by a delicate elastic diaphragm. Lip movements are effected by two agencies acting in combination: the variations in pressure of the coelomic fluid within the three coelomic cavities. the muscles of the upper and lower lips. The upper lip is concerned with the ingestion of food and with the closure of the mouth. The lower lip similarly, is involved in the closure of the mouth, and in addition assists in tube formation by virtue of the secretion of cement and mucus. The cibarium other than the zone containing the cement glands, the bucco‐pharynx and the oesophagus are the loci of a complex type of mucous secretion in which both epithelium and a glandular subepithelial layer are involved. The stomach divided into glandular and muscular regions, contains no peritrophic membrane. Pore canals similar to those described by Baker (1942) have been observed. Two types of enzyme‐secreting cells and their secretory cycles have been described. Except for the replacement cells all the cells of the glandular stomach are capable of both secretion and absorption. All regions of the intestine secrete mucus. The process is similar to that mentioned above. The anterior intestine is primarily absorptive; apart from mucus secretion the posterior intestine appears to perform no recognizable function. The movement of food along the alimentary canal has been studied and can be attributed to two agencies:— peristalsis and antiperistalsis induced by the body wall musculature. the cilia of the cibarium, bucco‐pharynx and oesophagus and of the intestine. The classification of the Terebellomorpha has been discussed. Dales' (1955) classification as it stands, is thought to be oversimplified. It is suggested that there are two parallel lines of evolution in the Terebellidae, sand dwelling genera which have developed the tube‐building habit and to a lesser extent the tentacles, and those which live in rock crevices and have greatly developed the tentacles and to a much smaller extent the tube‐building habit. An attempt has been made to indicate the means by which the alimentary canals of the Onychophora, Myriapoda and Insecta could have evolved from that of an ancestral polychaete. It is suggested that the buccal cavity, pharynx and oesophagus of the Polychaeta are homologous with the foregut of the Arthropoda cited above. Similarly the polychaete intestine is to be regarded as homologous with the arthropod hindgut. Regions secreting the cuticle and peritrophic membrane are regarded as being derived from those productive of mucus in the ancestral polychaete.

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