UNIVALVES. 49 



portion of this covering, either in the form of a membrane or a 

 cellular layer. Every part of the mantle can secrete shelly matter, 

 but the work is mostly done by the margin, where the colour cells 

 also lie. Occasionally the mantle edges turn up over the shell 

 sufficiently far as to meet, and then the shell becomes internal. In 

 the slugs this is shown in an advanced stage ; in Avion the shell has 

 disappeared all but a few granules; in some families it has 

 disappeared altogether. 



Nearly every form of spiral is represented in the class, from the 

 flat disc to the narrow tube with almost parallel sides, and from whorls 

 as close as those of a paper spill to whorls that nowhere tench each 

 other ; the majority right-handed, but many left-handed, and some 

 with every intermediate stage between right and left, showing how 

 the reversal has come about. The shell grows with the animal, 

 who sometimes moves downwards from the upper whorls and leaves 

 them decollate, as it is called, that is to decay and drop off. As it 

 grows the periods of rest are marked by the lines of growth, and in 

 some cases by either the thickening of the lip, which afterwards 

 varies the surface with what is known as a varix, or by the row of 

 spines which once fringed the mouth. 



When young the lip is thin, but with age it usually thickens into 

 a rib or is toothed or curved. The mouth is of every intermediate 

 shape between a parallel gash and a circle, and is either with or 

 without a notch or canal. When there are two of these notches or 

 canals, that on the anterior side generally takes the siphon, while 

 that on the posterior side carries the vent. When the mouth is not 

 notched it is said to be entire, and this in most cases shows the 

 animal to be a plant-eater, the notches or canals being generally 

 distinctive of the carnivorous forms. 



The mouth of the animal affords a similar guide to habits, 

 although not so frequently, the vegetarians having the mouth on the 

 surface of the head, while the flesh-eaters carry it on a proboscis ; but 

 there are many exceptions to this. This mouth not the mouth of 

 the shell is furnished with lips which in many cases are extensile. 

 In the bivalves it opens at once into the gullet, but in the rest of the 

 class it leads into a pharynx, which is fitted with jaws for biting the 

 food, and with a lingual ribbon, otherwise known as a radula or 

 odontophore, with which the food is scraped into a triturated mass 

 before it passes down the gullet into the stomach. The jaws are 

 distinctive, not only of the genus but of the species ; those of the 

 freshwater pulmonates are recognisable at once by their pair of 

 accessory side plates. 



The radula consists of a series of hook-like teeth, made of chitin, 

 the same substance as that of which the ligament of the bivalves is 

 composed, and it is generally silvery-white, tipped occasionally with 

 red or yellow. As it wears away in front it is pushed up by the new 

 rows of teeth forming behind. Sometimes the worn-out teeth in 

 front do not drop off, but are preserved in a special sac, the askos, 

 from which the sub-order to which their species are assigned derives 

 its name of Ascoglossa. In Eulima, Stilifer, and Odostomia, among 

 others, the lingual ribbon is missing, and hence their genera are 

 grouped as Gymnoglossa. The radula is also missing in the sea- 

 hares and other nudibranchs, and a few more. 



