Hogg, on the Water- Snail. 
99 
into the interior of its shelly house for protection ; in this act 
it is greatly facilitated by the action of retractor muscles, 
having a strong tendinous attachment to the columella of the 
shell. The shell of the young animal, and thin portions of the 
older shell, viewed by polarized light on the selenite stage, 
are interesting and beautiful objects. In the young animal 
the growth of the membranous part is effected by the gradual 
expansion of the vascular and cellular tissues, and we are 
soon enabled to define the expanded foot. This is a fleshy 
disc, broader anteriorly and divided into transverse segments ; 
by a particular arrangement of the longitudinal muscular 
fibres it is enabled to perform a series of undulatory move- 
ments, by which means the animal glides smoothly along ; its 
under surface is likewise studded over with a number of small 
orifices, which assist in causing a vacuum to be formed, and 
thus it suspends itself in an inverted position from the surface 
of the water, moving about in any direction. The muscular 
fibres, by their interlacements, greatly assist the animal in its 
progression, and in the performance of rapid movements ; at 
the outer edge it is. turned over, or returned upon itself, 
forming a smooth and strong margin of condensed tissue and 
muscular fibres, w^hich take their course in broad fasciculi, and 
gradually taper off" to a thin tendinous attachment on the pillar 
of the shell. 
The mouth is situated at the under and fore-part of the 
head ; it is a muscular cavity, enclosing a dental apparatus, 
semicircular in shape and provided with transverse rows of 
projecting spines, or teeth of a horny structure, or, more 
correctly, alternating rows of incisor and canine teeth, each 
being pointed with silica, and accurately fitted to cut against 
each other ; they are thus admirably adapted for the scraping 
or stripping off" the cuticle from the blades of Vallisneria, 
which the animal does without killing the plant, and leaves it 
more accurately divided, than at all possible to obtain by the 
usual mode of splitting for microscopic observation. The 
gastric teeth are immediately joined to the oesophagus or 
gullet, and to this succeeds the gizzard, a strong muscular 
apparatus, a quarter of an inch in length, and having a rugose 
appearance, with transverse and longitudinal fibres, by means 
of which every movement requisite for the conversion of the 
food is effected, and passes into a small membranous sac, the 
stomach ; this is folded into longitudinal plicce, and from it 
arises the large intestine of considerable length, having much 
of the appearance of intestine in the higher order of animals, 
excepting in colour ; a narrow longitudinal band passes down on 
either side of the external coat, and internally it is apparently 
