Quarterly Journal of Microscopical Science, Vol s3-95, 251-270, Copyright © 1954 by Company of Biologists

The Mechanism of Proboscis Movement in Arenicola

¹ Department of Zoology, University College, London

The mechanism of proboscis movement is analysed in detail in Arenicola marina L. and A. ecaudata Johnston, and discussed in relation to the properties of the hydrostatic skeleton.

Proboscis activity is based on the following cycle of movements in both species.

Stage I. The circular muscles of the body-wall and buccal mass contract; the head narrows and lengthens.

Stage IIa. The circular muscles of the mouth and buccal mass relax; the gular membrane (or ‘first diaphragm’ of previous authors) contracts; the mouth opens and the buccal mass emerges.

Stage IIb. The longitudinal muscles of the buccal mass and body-wall contract; the head shortens and widens and the pharynx emerges.

Stage III. As Stage I.

The two species differ anatomically and in their hydrostatic relationships. In ecaudata, the forward movement of body-fluid which extrudes and distends the proboscis is largely due to the contraction of the gular membrane and septal pouches. In marina, the essential mechanism is the relaxation of the oral region which allows the general coelomic pressure to extrude the proboscis. The gular membrane of marina contracts as that of ecaudata does, but its anatomy is different and it appears to be a degenerating structure as far as proboscis extrusion is concerned.

Withdrawal of the proboscis may occur while the head is still shortening and widening in Stage IIb, or while it is lengthening and narrowing in Stage III. The proboscis is used both in feeding and in burrowing; in the latter case nothing enters through the mouth; the difference is largely caused by variation in the timing of withdrawal relative to the 3-stage cycle.