64 W. N. F. WOODLAND 



a bundle of gland ducts, can only be associated with some 

 special function of the proboscis. The proboscis, as we have 

 seen, is not an organ of attachment — a sucker — but it is, on 

 the other hand, a very efficient organ of penetration. Amphi- 

 lina, as already related, normally bores its way through the 

 tough body-wall of the fish in order to hberate its larvae to 

 the outside world, and we know that it also occasionally bores 

 through nmscles, the wall of the gas bladder, the kidney, 

 and other organs and tissues. But in order to overcome 

 resistance a penetrating organ must possess (1) some degree 

 of rigidity, (2) some instrument with which to tear tissue, 

 and (3) a powerful muscle to work the apparatus. The large 

 proboscis muscle — which I shall in future term the boring 

 muscle (bm) — fulfils requirements (1) and (3), and the 

 serrated cuticle covering the outside of the protruded proboscis 

 fulfils requirement (2). The anterior thickness of the boring 

 muscle not only supphes the proboscis with a dense more or 

 less rigid core with which the animal can push its way into 

 resistant tissue, but the posterior extension and firm attach- 

 ment of the muscle to the large anchor-cells firmly embedded 

 in the axial parenchyma (and placed at right angles to the 

 length of the fibres) enables the worm to ' put its whole weight ' 

 into the boring process and to draw the hind portion of the 

 body through the perforation or path made by the proboscis. 

 The slight twisting of the fibres of the boring muscle in and 

 below the proboscis also doubtless serves for a semi-rotary 

 movement of the proboscis, enabling the serrated cuticle to 

 saw its way through the tissue. The proboscis muscle thus 

 serves as a boring nuiscle, as a notochord for the anterior end 

 of the lx)dy and as a means of dragging the hind half of the 

 body along the path excavated by the proboscis, and these 

 important functions amply account for its huge size. Eetrac- 

 tion of the proboscis is effected by the retractor muscle, and 

 protrusion of the proboscis probably results merely from the 

 slackening of the retractor (the stiff boring muscle naturally 

 projecting fcjr wards in a position of rest) but possibly also 

 from the contraction of the longitudinal muscles attached to 



