KLEDONE. 499 
muscular mass enclosing it\is known as the Buccal Mass 
(fig. 17, B.M.). The buccal mass is surrounded, and 
therefore concealed, by the muscular bases of the arms 
(Pl. III, fig. 14, B.MW., arm). ‘The pharynx is furnished 
with two powerful chitinous jaws, whose shape curiously 
resembles that of a parrot’s beak, and which are placed 
dorsally and ventrally. Unlike the parrot, however, the 
ventral jaw of Hledone, which bites outside the dorsal, is 
the larger and wider (PI. IV, fig. 27, J, and J.). These 
jaws bite vertically with great force, tearing up the food 
captured and held by the suckers before it is passed on to 
the rasping action of the radula. The anterior edge of 
each jaw is thick, and dark brown in colour. The 
trenchant border is sharp, and a raised ridge some 
distance behind this gives attachment to the muscles 
working the jaws (fig. 2/7, r.). This part of the jaw is 
exposed by cutting away the lip (Pl. IV, fig. 24). 
Further in, they decrease in thickness, and their colour 
lightens, and posteriorly they are thin, colourless, and 
semi-transparent. On the floor of the pharynx, slightly 
anterior to the middle point, is a muscular outgrowth— 
the tongue (fig. 24, ¢.). This forms the anterior wall of 
the Radula sac, at the base of which is the growing point 
of the radula (fig. 24, rad.). The Radula is a broad 
chitinous ribbon which, issuing out of its sac, runs over 
the upper and anterior surface of the tongue, which is 
responsible for the rasping action of the radula, as it 
works forwards, backwards and laterally. The tongue is 
strengthened internally by two small cartilaginous strips, 
which give it rigidity and also provide attachment for its 
motor muscles. Here, then, the food cut up by the jaws 
is further triturated. The teeth of the radula are large, 
and each row consists of three on each side of a central 
large tooth. The innermost of the three is the smallest, 
