MOLLUSC A. 



49 



ward again, the slanting teeth are cleaned of whatever 

 they had on them, and leave it to be swallowed by the 

 throat. The teeth are worn out very fast by this oper- 

 ation in front, and the band of membrane carrying 

 them moves forward, bringing up fresh teeth. In order 

 to keep up this supply, there are new rows being con- 

 tinually formed in the rear. These grow in a sort of 

 pocket behind the tongue, and around a core, which is 

 figured in Fig. 26, co. This figure shows below at rd 

 the inner side of a number of rows, which are formed 

 or forming lower down out of the surface membrane 

 of the core. 



The lips on either side of the mouth, and the deep 

 incision between the head and the crawling part of the 

 foot, are also noticeable. 



Into the latter opens {gdo) the long glands of a 

 yellow color (^^/), which secrete a large portion of the 

 mucous used by the animal to smooth its path over all 

 surfaces, and, as a lubricator, to render its motions 

 easier by excluding the air more effectually. Some 

 snails and slugs have other openings in the foot for 

 excreting mucous, which dries so rapidly, and becomes 

 so tough, that they can suspend or lower themselves 

 from the branches of trees by its means, as the spider 

 does with its thread, which is also only hardened 

 mucous. 



Those who wish to go beyond mere externals will 

 find it necessary to put some of the killed specimens 

 into very dilute acetic acid. This will act upon and 

 dissolve all the exposed parts of the shell, and leave 

 the brown, horny skin entire, if the acid has not been 

 too strong, and effervescence too violent. 



