THE GASTROPODA gl 
36,000 in Zritonia hombergi; 40,000 in Helix ghiesbrechti ; 75,000 
in Susania tuberculata; and as many as 750,000 in Umbrella. It 
follows that the length of the radular ribbon varies very much ; it 
is considerable in Cyclostoma and Patella (Fig. 88, 7), in which it 
exceeds the length of the body, and in the Littorinidae, in which 
it is coiled into a spiral so as to occupy less room; in one species, 
Tecturius, it attains to seven times the length of the body. 
The form of the teeth is also constant in a given species, but 
varies from group to group, and is therefore, when taken in 
conjunction with their number, of considerable assistance in 
characterising the divisions of the Gastropoda, especially of the 
Streptoneura ; hence the importance of the radula in systematic 
works. But occasionally the radula may vary in the individuals of 
the same species, as, for example, in the Buccinidae ; and, on the 
other hand, groups tolerably far apart from one another may 
exhibit analogous features in the radular teeth. Further, it has 
been shown that the number of teeth in a transverse row varies in 
all the groups founded upon this character. Among the Taenio- 
glossa, in which the radular formula is 2.1.1.1.2, the two marginals 
are absent in Lamellaria and Jeffreysia ; and contrariwise, there are 
more than two marginal teeth in certain species of Zurritella, in 
Struthiolaria, and Triforis. A still larger number of teeth, but no 
median tooth, is found in Solarium, Scalaria, and Janthina. In the 
Rachiglossa, characterised by the formula 1.1.1, the central tooth 
is reduced in Columbella, and the laterals absent in the Marginellidae 
and in certain Volutidae and Mitridae, and in the young Harpa, the 
adult in the last-named genus being devoid of a radula. Finally, 
although the radular formula of the Toxiglossa is given as 1.0.1, 
there is a central tooth, and more than one lateral in sundry Pleuro- 
tomatidae (Spirotropis: 1.1.1.1.1). The radula is absent in a few 
genera only, and those are generally parasitic or suctorial forms, 
such as Thyca, the Eulimidae, Pyramidellidae, Coralliophyllidae, and 
certain Terebra among the Streptoneura, and in the Tornatinidae, 
Cymbuliopsis, Gleba, the Doridiidae (in which a vestige of the radular 
caecum is still retained), the porostomatous Doridomorpha (Dov7- 
dopsis, Corambe, Fig. 164, Phyllidea), and the Tethyidae. 
The buccal opening of Gastropods is furnished with glands, 
often in considerable quantity (Bullidae, Nudibranchs), and in many 
stylommatophorous or terrestrial Pulmonates these glands are 
so highly developed as to form lobulated masses known as the 
“organs of Semper.” But in all Gastropods, with very rare excep- 
tions, the salivary glands proper open into the interior of the buccal 
cavity on either side of the radula. These organs are generally 
simple mucous glands, without any digestive action, but in certain 
forms—Doliwm galea is an instance—their secretion contains as 
much as 4 per cent of sulphuric acid, which serves to dissolve the 
