606 K. J. HARVEY GIBSON ON THE 



connection with the physiological section of this research. With that intention 

 he has omitted certain histological details, more especially into the minute 

 structure of the crop, the openings of the bile ducts, &c, with regard to which 

 it may be possible to obtain some information when the physiology of these 

 parts comes to be investigated. 



Considering the alimentary canal first of all macroscopically. The buccal 

 cavity is entered by an oval or kidney-shaped opening on the oval disc. 

 Immediately within the circular puckered outer lip which guards the opening 

 is a shallow cavity, which is closed posteriorly by a pair of inner lips. These 

 inner lips are two stout flaps of muscle which rise vertically from the floor of 

 the buccal cavity, and project almost to the roof. They leave between them a 

 narrow vertical slit, through which the odontophore may be seen (PI. CLII. 

 fig. 54, a). Within these lips a large irregular chamber is found, which may be 

 termed the pharyngeal chamber. Its form may best be understood by following 

 the reflections and foldings of the pharyngeal mucous membrane (PI. CLIII. 

 fig. 62). Ventrally the mucous membrane, starting from the inner aspect of the 

 inner lips, runs along the floor of the chamber for a short distance until it has 

 reached the middle of the odontophore. There it bends anteriorly, and is 

 reflected up over the anterior part of the odontophore. Laterally the mucous 

 membrane follows a similar course, being there reflected over the sides of the 

 odontophore. The mucous membrane then passes beneath the radula, forming 

 the subradular membrane. At a short distance from the centre of the dorsal 

 surface of the odontophore the membrane unites above the radula, — forms in 

 this way a bag for its reception. Indeed, the radula is an epidermic modifica- 

 tion of the pharyngeal mucous membrane. The radula lies folded in this bag, 

 from the wall of which it is developed. The bag is suspended among the 

 viscera, usually to the left side, sometimes, however, on the right, more rarely 

 still in a spiral coil on the floor of the visceral cavity. 



Returning to the pharyngeal mucous membrane, dorsally it springs from 

 the palate, and when it has become free from the body or neckwall, and uniting 

 with the lateral portions of itself, passes backwards over the dorsal surface of the 

 odontophore. After passing the origin of the radular sheath, it unites with 

 the ventral portion, and becomes the pharynx proper. 



On the palate and on the floor of the pharyngeal chamber, there are two 

 structures which require mention. These are two plates which protect the 

 subjacent tissues from injury from the teeth of the radula. The palatal plate 

 (PL CL. fig. 17, and PI. CLIII. figs. 63 and 64) is sunk in the tissue of the 

 palate, and like the radula and the ventral plate, is a development of the 

 pharyngeal mucous membrane. Looked at from below, it presents a central 

 triangular area, which is of a brown colour, and from which posteriorly (apex) 

 project two large almost colourless flaps; from the (base) anterior end also 



