THE GASTROPODA 91 



36,000 in Tri/onia hombergi; 40,000 in Helix yhiesbrechti ; 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 Cydostoma and Patella (Fig. 88, r), 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, 

 Tectarius, 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 Turritella, in 

 ^fnitjtiolaria, 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 Thi/ca, the Eulimidae, Pyramidellidae, Coralliophyllidae, and 

 certain Terebra among the Streptoneura, and in the Tornatinidae, 

 Ci/mbuliopsis, Gleba, the Doridiidae (in which a vestige of the radular 

 caecum is still retained), the porostomatous Doridomorpha (Dori- 

 clopsis, 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 Dolium galea is an instance their secretion contains as 

 much as 4 per cent of sulphuric acid, which serves to dissolve the 



