ANATOMY OF THE PEART, OYSTER 47 



section when in a state of contraction (Plate III., fig. 21). It becomes flattened in 

 the distal third when the organ is extended (fig. 20). In oysters ot 2 years of age and 

 upwards the dorsal surface and the sides of the foot are usually so thickly covered 

 with dark chestnut pigment speckles as to appear quite brown. The speckles become 

 less numerous on the lower parts of the sides as they approach the prominent white 

 edges of the venh'al or pedal groove, while the floor of the locomotor or anterior part 

 of the pedal groove is usually pigmented similarly to the exposed part of the foot. 

 Occasionally the pigment is purplish-brown, dark drab, or even dark orange. 



The foot in 3-year-old oysters is capable of extension to over 1^ inches when fully 

 turgid ; in contraction the length is i inch or even less. In an older oyster it may 

 be extended to over 2 inches. Oysters 6 to 9 months old can extend the foot to a 

 length of 1 inch, contracting it to about ^ of an inch. 



A deep pouch-like pit, the byssus gland or organ, is lodged at the proximal end of 

 the foot upon the ventral aspect. The wide mouth of the pit (Plate III, figs. 13, 14, 

 15, 22, &c.) is a little way anterior to the junction of the foot with the visceral mass, 

 while the pouch itself penetrates deeply backwards and slightly downwards into the 

 central portion of the visceral mass (Plate VI., fig. 2). The axis of the mouth of the 

 byssal gland coincides with the longitudinal axis of the foot. The byssal gland 

 lodges the common "root" of a bundle of stout, laterally-flattened, bronze-green 

 fibres, the byssus, which by means of a discoid attachment at the distal extremity of 

 each fibre anchors the pearl oysters to rocks and other objects (Plate III., 

 figs. 16, 22, 22, 23). The anterior edge of the mouth of the byssal gland passes into 

 a groove, the pedal groove, extending medially along the whole of the remaining 

 length of the ventral surface of the foot (Plate III., fig. 20, and Plate VIII., figs. 3 

 and 4). 



The pedal groove comprises two regions functionally distinct, a distal or locomotor 

 and a proximal or secretory. Both are, indeed, lined with a layer of secreting glands, 

 but while this layer is of great thickness in the proximal region, in the distal it is 

 thin, and the secretion is of a different character (Plate VIII., figs. 3, 4). When the 

 foot is contracted, the edges of all parts of the pedal groove are approximated. 



The hinder or secretory part of the groove has two regions, an anterior part which 

 is cup-shaped when in use, and a hinder which is nearly always tubular through the 

 approximation of the lips of the groove. The anterior cup-shaped part is about 

 mid-way between the base and the tip of the foot, and may be termed the " disc-pit." 

 The tubular byssal groove connects this with the byssal gland. Figs. 3 and 4, on 

 Plate VIII. , represent the structure of the foot as seen in transverse section ; both 

 show the numerous muscle bundles, longitudinal, circular, and oblique ; fig. 3 has the 

 open locomotor pedal groove ventrally, and fig. 4 the closed byssal groove which 

 becomes an open canal further out. 



Functions of the Foot. The functions of the foot are threefold : the distal ventral 

 surface subserves locomotion ; the median and posterior parts effect attachment by 



