610 R. J. HARVEY GIBSON ON THE 



cast-off epithelial cells. At the palatal plate, and also at the ventral plate, the 

 epithelium becomes many layers deep (elsewhere it is in a single layer, save in 

 the glands), and the superficial cells secrete a very thick cuticle which becomes 

 modified into the distinct and separable palatal plate in the one case, but in the 

 other remains attached to the cells (PI. CLII. fig. 56). Beneath the edges of 

 the ventral plate the connective tissue forms two soft pads. 



The radula, which lies in a long sac formed by an out-pushing of the 

 pharynx, from the Avail of which it is developed, is a ribbon, expanded at the 

 anterior end into a flat plate, which is wrapped round the anterior cartilages, 

 and is continuous with the pharyngeal mucous membrane, viz., with that 

 reflection of it which covers the under surface of the buccal mass. Posteriorly 

 the ribbon is bent on itself in its sac and ends in a soft clubbed end. The teeth 

 are not developed in the latter part of the ribbon, though their general position 

 is mapped out on the membrane. The various ridges are then strengthened 

 and developed by deposition of particles of chitinous matter with which the 

 sac is plentifully supplied. When completely formed (PI. CLII. fig. 60) the 

 radula is seen to be composed of a tough chitinous band, from which spring a 

 number of teeth. The teeth are arranged in curved rows, the concavity of the 

 row being anterior. The radula tends to tear in that way. Each row consists 

 of ten teeth. The four central teeth (4, 5, 5, 4) are similar in shape, although 

 the middle two are slightly smaller than the exterior two. Each is composed 

 of a yellowish root and neck, succeeded by a brown band or collar, and 

 terminated by a black crown, which is in shape like a bird's claw, the claw 

 having its convexity directed towards the mouth. Next to the four central 

 teeth, and placed a little in advance of them, is, on either side, a tooth slightly 

 larger than any of the central teeth, but similar to them in structure, save that 

 it has three claws instead of one (3.3). Most external of all, there is a pair of 

 teeth on either side which are flat, faintly yellow in colour, without claws ; the 

 ends are, however, slightly curved upwards and backwards. These teeth are 

 also slightly in advance of the last mentioned, and the outer of the two is 

 slightly beneath and in advance of the inner. The form and relationships of the 

 teeth will be much better understood by reference to the figure (PI. CLII. 

 fig. 60). 



The cartilages of the odontophore are composed of the usual elements. The 

 cells are large, and the matrix (cell walls) small in amount, so that the cartilage 

 is spongy in texture. The cells are smaller as they approach the perichondrium. 

 The muscle fibres of the odontophore are nucleated, and are similar to those 

 fibres found in the heart {q.v.). 



The salivary glands are compound tubular glands. The walls of the tubules 

 are composed of cubical epithelium with yellow granules in the cells. The 

 outlines of the cells are difficult to make out. The salivary ducts likewise are 



