6 Poe SiellU olleotovles Hamidbooks 
of the pulmonary aperture, the anus, or vent, 1s 
placed, appearing as a slit-like hole. 
Tue Dicestive System.—The mouth leads back 
into the buccal mass, or pharynx, a reddish muscular 
cavity, containing two or four true cartilaginous plates, 
which form an early example of an endoskeleton. 
(Macalister). On the floor of the pharynx is an up- 
growth of connective and muscular tissue, overlying 
which, is a chitinous sheet beset with teeth which 
point backwards. The whole of this apparatus is 
termed the odontophore, and the chitinous plate, with 
its teeth, the radula, or lingual ribbon. The odonto- 
phore is supported by a pair of cartilaginous plates, 
to the anterior and posterior surfaces of which muscles 
are attached, so that the radula may perform, by their 
alternate contraction and relaxation, a backward and 
forward motion. ‘The radula is then a rasping organ, 
and bites against one of the cartilaginous plates, which 
we have previously mentioned as existing in the 
buccal mass. In the Pulmonata, this plate, or jaw, is 
vertical, and placed on the upper pharyngeal wall ; 
but according to Macalister, in Paludina, there are 
opposed lateral jaws armed with flat plates, that are 
situated in a horizontal direction. The teeth are 
usually strengthened by a siliceous deposit, and, when 
worn away, are replaced by the development of some 
others from behind. 
The radula consists of a central or axial plate 
(rachidian teeth, rachis teeth, dentes) and a lateral plate 
on each side (wncini). The method of the formula 
for describing the numbers and position of these 
teeth, is as follows:—‘‘The central teeth of the 
