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 Adaeon, Tarnatina, 

 Scaphander, Doridium, the Lophocercidae, Cymbuliopsis, Gleba, Clione, 

 Umbrella, Doris, the porostomatous Doridomorpha, Tethys, the 

 Elysiae, Gadinia, Amphibola, the Testacellidae, etc. 



(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 in a ventral caecum (Fig. 74, A, n), 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, /, in), 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 Eachiglossa only one. Among the 

 Opisthobranchs many teeth are included in each transverse row 

 in Adaeon 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 undatnm; 1920 in Patella vulgata; 3500 in 

 Littorina littwea; 6000 in Doris tuberculata ; 8343 in Limnaea 

 stagnalis; 15,000 in Helix aspersa; 26,800 in Limax maximus; 



