564 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
The fibres are inserted at a considerable angle, and 
from the shell, they pass first as a. flat band and then, 
becoming circular in section, acrass the dorsal surface of 
the adductor, directly towards the base of the foot. This 
brings the Retractor under the pericardium and the 
digestive gland until it reaches the visceral mass, through 
which it plunges, just at the junction of the digestive 
gland and reproductive organ, lying enclosed in a fairly 
definite blood space. Transverse sections through the 
muscle close to the base of the foot show (fig. 47, B. g.) 
that the muscle has taken a tube-like form, enclosing the 
byssus gland. The muscle fibres pass around the gland, 
which has the form of compressed pouches separated by 
lamellae of connective tissue, and ultimately they become 
lost in the tissue of the foot. 
The Intrinsic Muscles of the foot make up the bulk 
of the tissue in this part of the body (fig. 8). They run 
chiefly in two directions. There is a definite layer of 
circular muscle fibres underlying the surface and 
extending all round the foot, more internal still is a 
series of longitudinal muscles running along the axis of 
the foot. In addition there are many fibres diverging 
radially from the centre, and also scattered fibres passing 
in various directions. 
Other intrinsic muscles are to be found in the visceral 
mass in the reproductive region. ‘There is a layer of 
transverse muscles passing round in the connective 
tissue sheath which encloses the visceral mass, and con- 
nected with this sheath are scattered muscle bundles 
running across from one side to the other and serving to 
strengthen and form a support for the alveoli of the 
gonad. 
The Ctenidial Muscles (fig. 45, Br. m., Br. m', 
Br. m.") ave arranged as follows:—There is first a layer 
