GENERAL DESCRIPTION OF MOLLUSCS. 



613 



The cartilages, too, which are attached by one end to the floor of the mouth, 

 can be worked backwards and forwards by special muscles, and so bring the 

 teeth into play. The action of the tongue thus formed can be well seen in a 

 common pond snail, as it slowly crawls up the glass-side of an aquarium, 

 clearing off the microscopic plant-growths as it goes. Of course the teeth in 

 front soon wear away and drop off, so that further and further portions of 

 the radula have to be brought into use as required. The portion of the 

 radula held in reserve for this purpose is stowed away in a kind of pocket 

 (the radula sac), situated at the back of the mouth and immediately under 

 the oasophagus. At the extreme end of this radula sac a special set of cells, 

 in the substance of its wall, are engaged in forming fresh teeth, so that a 

 constant supply is being manufactured ready to be moved forward in place 

 of the worn-out ones. When the reserve supply needed is very great the 

 radula is of great length, and the radula sac, correspondingly long, has to be 

 stowed away amongst the other organs of the body. This is especially the 

 case in the limpet and the periwinkle, which last has proportionately 

 the longest known radula of any mollusc. As a rule, in each transverse 

 row of teeth there is a symmetrical central one called the median or 

 rachidian tooth ; on either side of this the teeth are unsymmetrical, 

 and their shape changes as they are traced outwards from the centre, 

 but each one pairs exactly with the corresponding tooth on the opposite side 

 of the median one. Generally the first few near the median more or less 

 resemble each other, and are succeeded rather abruptly by smaller ones, so 

 that the whole radula appears divided into three longitudinal tracts. The 

 middle tract is then termed the rachis and the teeth on it the median and 

 admedian, whilst the outer tracts are styled pleurae, and their teeth uncini. 

 Sometimes between the admedian and the uncini there is a conspicuous tooth 

 differing from either, which may be termed the lateral or capituliform tooth. 

 The shapes and patterns 

 of these teeth (Fig. 1) 

 and their arrangement 

 are so characteristic in 

 the different genera as to 

 form valuable aids to 

 classification, though, 

 since they are apt to vary 

 in the young and adult 

 state of the same in- 

 dividual, they do not 

 furnish an unerring 

 clue to the identity 

 of species. A formula 

 has been invented for 

 expressing the number 

 of teeth in each trans- 

 verse row thus : 



I'l'l signifies that 

 there is a- median, with 

 a single lateral on either 

 side ; 2'11-1'2 has in 

 addition 2 uncini on either side. When the uncini are very numerous 

 the sign oo (= infinity), or, better still, x is employed, thus oo 'l'4'l'4*l*oo, 



Fig. 1. 



A, Docoglossa (Patella vulgata). 



B, Rhipidoglossa(7Voc/ittS cinerarius). 



C, Tamiojrlossa (Cyprcea Europcea). 



D, Rhachiglossa (Buccinum undatum) Whelk. 



E, Achatina fulica. 



