ANATOMY AND PHYSIOLOGY OF A SNAIL. 17 



thrown off by the pressure of the foot. Hammersten finds that 

 the foot of some snails, as H. pomatia, excretes mucus as well as 

 the mantle, and possibly this excretion also helps in the disin- 

 tegration of the epiphragm. The name of epiphragm was first 

 given to this structure by J. P. R. Draparnaud, to whose memory 

 this present little volume of conchology is inscribed ; old Martin 

 Lister (born 1638 (?), died 1712), naturalist and court physician, 

 who thought he had done everything in natural history that could 

 be done, called it the operculum saliva confectum ; Miiller termed 

 it the operculum hybernum. The operculum proper is a horny or 

 shelly plate attached to the back of the foot in many water- snails, 

 as Paludina, Neritina, Bythinia, and Valvata, and in a few land 

 snails, as Cyclostoina and Acme, It contains more conchiolin in 

 its composition than the shell whose mouth it closes, and not 

 less conchiolin and more chitine, as Dr. Henry Woodward has 

 recently supposed. Those Gastropods which have no operculum 

 are spoken of collectively as the inopercular univalves ; those with 

 an operculum, on the other hand, as the operculated univalves. 

 No operculum of any shell, whether of fresh-water, land, or 

 marine habitat, exhibits an annular form. In a marine form, 

 Lithedaphus ( Calyptraa) equestris, an operculum is attached to the 

 whole length of the foot, so that on first appearance the shell 

 might be taken to be a bivalve. But as Professor Owen has 

 remarked that upon a comparative study of the operculum it gives 

 characters of secondary importance since it " sometimes varies 

 in structure in species of the same genus, as it is present in some 

 Volutes, Cones, Mitres, and Olives (marine forms), and absent in 

 other species of those genera, and as some genera in a natural 

 family, as Harpce and Dolium among the Buccinoids, are without 

 an operculum, whilst the other genera of the same, family possess 

 that appendage." The operculum grows by the addition of matter 

 to its circumference, its youngest part being called the nucleus, 

 which may be excentric, as in Paludina vivipara, or central, as 

 in Bythinia Leachii. The clausilium is characteristic of the genus 

 Clausilia, and differs from the operculum in not being attached 

 to the animal. It may be best seen by taking a Clausilia laminata 

 and breaking away the outer part of the body-whorl, when it will 

 be found to consist of a shelly plate attached to the columella by 



