Some Observations on the Pedal Gland of Milax

Abstract

1. The pedal gland of Milax is a compact mass of secreting cells traversed by a tube, the excretory canal, "which is free from the gland for about one-third of its length. The gland lies free in the body cavity, held down to the foot by small muscles. The pedal artery runs along the top of the gland. The canal has a projection from the roof into the lumen in the posterior part, and the floor shows a groove and two humps covered with cilia, forming three ciliated tracts running the whole length of the gland. The cilia in the groove are longer, and have a slower movement, than those on the humps. 2. The gland in Milax differs in position and structure from that in Limax : (a) It is not embedded in the foot. (b) It has no muscle fibres among the glandular cells. (c) A part of the canal is free from the gland at the posterior end. 3. The excretory canal is not emptied by muscular contraction (as in Limax) but (a) by the movement of the cilia ; (b) by the fact that the slime is very tenacious and, therefore, when the slug touches the ground and then moves forward, the slime is drawn out in a long string from the orifice of the canal. 4. The main uses of the mucus secretion are : (a) To provide a smooth track on which the pedal waves of the foot can function in forward movement. (b) To enable the animal to adhere firmly to the substratum. (c) To form a slime-string by means of which the animal can descend from trees to the ground : this is also used to suspend the two slugs during the act of copulation. (d) To assist the mucus glands of the skin in keeping the body moist. 5. The animal cannot progress in the usual way without the slime-track, i.e. if the gland is cauterized the animal moves, not rhythmically, but after the manner of a looper caterpillar. 6. The chief function of the pedal gland is to supply mucus to form the slime-track on which the animal moves. It is also probable, as Cuénot suggested, that the posterior part of the roof of the canal in this gland has an excretory function.