THE GASTROPODA 89 
Trochidae, in Neritina, in the Helicinidae, in Cyclostoma, in Thyca, in 
the Pyramidellidae, Eulimidae, Entoconchidae, and Coralliophilidae, 
in all the Toxiglossa, in the Heteropods, in <Actacon, Tornatina, 
Scaphander, Doridium, the Lophocercidae, Cymbuliopsis, Gleba, Clione, 
Umbrella, Doris, the porostomatous Doridomorpha, Tethys, the 
Elysiae, Gadinia, Amphibola, the Testacellidae, ete. 
(2) The radula is a sort of ribbon of greater or less width, formed 
of distinct and separate chitinous teeth, borne on a single supporting 
membrane. It is secreted ina ventral caecum (Fig. 74, A, 7), in which 
it is almost wholly contained, but its anterior extremity stretches 
out on the floor of the buccal cavity, where it forms a median 
projection (Fig. 74, A, p). The radular ribbon is supported by a 
system of paired cartilaginous pieces furnished with protractor and 
retractor muscles (Fig. 74, A, /, m), the action of which causes the 
radula to move to and fro and work like a rasp on the prey seized by 
the animal. The teeth are secreted at the bottom of the caecum or 
sheath of the radula, by a small number of matrix cells; in front of 
these is a transverse row of cells which secrete the basal membrane. 
The teeth are disposed in transverse rows, and in each row there is 
a median tooth called the “central” or rachidian tooth, on either 
side of which the remaining teeth of the row are symmetrically 
disposed. In the Euthyneura all the lateral teeth are generally 
similar to one another (Fig. 145), but in the Streptoneura, when 
there is more than one tooth on either side, they are divided into two 
clearly defined groups. The teeth nearest to the central are the 
“laterals,” properly so-called, and differ from the more elongated 
external teeth, which are known as the “marginals” or uncini (Fig. 
2, m). The number of teeth in any given transverse row is 
constant in any given species; it may, however, increase slightly 
with age, at any rate in various Aplysiidae and in the terrestrial 
Pulmonates. On the other hand, the number of teeth is variable 
from group to group, and generally is more considerable in the less 
specialised than in the more specialised groups. Thus, in the 
Streptoneura the Rhipidoglossa have numerous lateral teeth on 
either side of the central; the Taenioglossa have only three lateral 
teeth on either side, the Rachiglossa only one. Among the 
Opisthobranchs many teeth are included in each transverse row 
in Actaeon and the Pleurobranchidae, but there are only three in 
the thecosomatous “ Pteropods” and only one in the Elysiae. The 
number of successive transverse rows also varies from species to 
species, and consequently the total number of teeth in the radula 
is very different in different forms. There are sixteen (one tooth 
in each row) in certain Eolidae and Elysiae ; about two hundred and 
fifty in Buccinum wndatum; 1920 in Patella vulgata; 3500 in 
Lattorina littorea; 6000 in Doris tuberculata; 8343 in Limnaea 
stagnalis; 15,000 in Helix aspersa; 26,800 in Limax maximus ; 
