THE MOLLUSCA 7 
the Lamellibranchs, various isolated forms of Gastropoda, and the 
Cirrhoteuthidae among Cephalopods. The radular apparatus has 
the form of chitinous teeth, disposed in transverse rows; the 
number of teeth in each row is sub- 
ject to variation, but they are always 
arranged symmetrically on one side 4 
and the other of a central tooth 
(Fig. 2). 
Thus there are eight lateral teeth 
on either side of the central tooth in 
Polyplacophora (8.1.8); two laterals 
on either side of the central in ‘ . 
Scaphopoda (2.1.2); three laterals _ ws 
on either side of the central in almost * fie am 5 = 
all Cephalopods (3.1.3). In the ao : oe 
Gastropods the number of teeth in 
each row varies considerably in the Ket Lays 
different sub-groups. The radular “rene 
ribbon issues from a_ pharyngeal \ 
caecum, in which it is secreted, and es 
is applied to the surface of paired — qyansverse rows of the radula. A, 
cartilaginous pieces situated on the Porochiton ruber. B, Natica clausa: C, 
. se 7 gonum ,; >» 4v0SSia giau 
floor of the buccal cavity. These copis; all much enlarged. c, central 
: tooth; J, lateral teeth; m, marginal 
so-called cartilages have a charac- feeth.’ (After G. 0. Sars.) = 
teristic vesicular structure quite dif- 
ferent from that of ordinary cartilage such as is found in Cephalo- 
pods and Opisthobranchs. Applied to these cartilaginous pieces 
the radula, by the action of special muscles, executes backward 
and forward rasping movements. 
The cuticular lining of the stomach is specially developed in 
Lamellibranchs and in certain Gastropods in which it is sometimes 
differentiated into masticatory plates provided with special muscles. 
The buccal cavity or anterior dilatation of the alimentary tube 
receives in the Amphineura, Gastropoda, and Cephalopoda, the 
secretion of the so-called salivary glands, of which one or several 
pairs may be present. The oesophagus may present various 
forms of accessory dilatations, known as “gizzards,” glandular 
regions, ete. - 
The secretion of an important and voluminous digestive gland 
is poured into the second primary dilatation of the gut or stomach : 
this gland is called the liver, but this name must not be taken to 
imply a physiological identity with the liver of vertebrates. The 
liver is an acinous glandular organ, the epithelial cells of which are 
all very similar to one another in Polyplacophora, but in other 
forms they are generally differentiated into ferment cells and 
excretory cells. From the physiological point of view this gland is 
