246 Part III. — Tioenty -fifth Annual Report 



as is seen in a dead mussel, or extended into a long narrow band, 

 as is the case when the animal is forming a thread or dragging itself 

 along by means of it (fig. 43). The tip of the foot differs from the 

 remainder of the organ. It is possessed of a sucker property by which 

 it can take fii m hold of any hard foreign object. On the under surface 

 of the foot there is a groove (fig. 42) continuous with the byssus pit (by-p) 

 proximally, and distally opening out into a broad heart-shaped depres- 

 sion (de), a little short of the extremity of the foot. The tip of the 

 foot, that is, the part including and beyond or distal to this depression, 

 has the power of making vermiform movements, even after the foot is 

 cut out of the mussel. l"'he whole of this tip, including the depression, 

 acts as the sucker. The depression is simply a widening out of the 

 groove, to vanish gradually distally. 



The groove (fig. 42) consists of two cushion-like lips (1) and a narrow 

 inner part, which is closed in by the cushion lips to form a tube in which 

 the thread is moulded. 



The sides of the groove and its lips are crenate and vertically 

 laminate. When the foot is contracted the groove is a sinuous line, but 

 the more the foot is extended the straighter the groove and the smoother 

 the sides become. 



The foot is a mass of muscles and glands. On the outside the skin is 

 longitudinally groo\ ed, and when contracted it is circularly grooved. 

 The muscles of the foot enter it at its distal end and proceed right along 

 the foot to the tip, and are inserted all along the skin of the whole of the 

 foot on every side, in big bundles along ihe upper side and edges, single 

 fibres into the glandular areas. The only part where they do not appear to 

 reach the skin is the groove. They go to the lips of the groove. The 

 muscles are in bundles of various size, and between the bundles there 

 are lacunae, which give in cross section a spongy structure, except in 

 the glandular area. 



When the under surface of the foot is examined, it is seen that a 

 yellow-coloured band (y-g) borders the groove on either side, and is 

 continued right round the byssus-pit (fig. 45). This is the series of 

 glands which supply the secretion that forms the thread. There is a 

 separate gland to supply the depression in which the button that attaches 

 the thread to a foreign body is formed. This end gland (the purple 

 gland) appears to be different from the other, but that is possibly due 

 merely to it being differently set in the foot to supply this altered part of 

 the groove. 



Transverse Section of the Foot. (Fig. 51 ) 



In the transverse section there are shown first the groove, its cushion 

 lips, and on either side granular areas, viz., white glands, yellow when 

 seen from outside (y-g). The remainder of the section is taken up with 

 muscle bundles cut across (m), the so-called green (or purple violet) glands 

 (p-g) (fig. 55) and the vessels, of which three are prominent — a median 

 (m-v) and two lateral (1-v). 



The Skim. — The skin forms a rind of varying thickness ; it is 

 especially thick at the edge of the foot. Outside there is a thin clear 

 layer, the cuticle (ct) present all round except in the groove (fig. 54) ; 

 it is present on the lips of the same. Next that layer comes the pigment 

 layer, which is formed of large columnar cells (cl) containing more or less 

 pigment ; they are mottled with brown or violet colouring matter. The 

 columnar cells vary in size in different parts. Next the columnar cells 

 there is a fairly thick layer consisting of what resembles circular muscles. 

 On the inside of the rind come the longitudinal muscle bundles (m) that 



