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__ front margin of the shell entire. They are marine or fresh water 
. animals, mostly vegetable feeders. The River Snail (Paludina) 
gz and the common Periwinkle (Zittorina littorea) are examples of this 
a large tribe. The odontophore of the Periwinkle is a narrow ribbon, 
two inches long, and it contains about 600 rows of teeth, each row 
consisting of seven teeth, viz.: a central rachidean tooth, and three 
uncini on either side, hooked and dentated. Only about 24 rows 
of teeth are exposed for use. The remainder of the lingual ribbon 
lies coiled up on the right of the esophagus. In the Limpet the 
odontophore is no less than three inches long, and the greater part 
lies, folded up, but perfectly free, in the abdominal cavity. Zrochus 
and Haliotis have odontophores of striking beauty, the central 
_ teeth, which are numerous, but small, being flanked by rows of 
large "hooked teeth set very close together. 
___ Stphonostomata. This section embraces those univalve gastro- 
pods, which have the front of the aperture of the shell notched or 
_ produced into a canal. This serves to protect the siphon (formed 
_ by a prolongation of the mantle) through which water passes into 
_ the branchial chamber for respiration. The Stphonostomata are 
_ all marine animals; they are carnivorous, and most of them are 
_ prédacious. Their dental apparatus is combined with a very 
_ remarkable organ, by means of which they are enabled to attack 
_ and feed upon other molluscs, even upon those encased in a shell, 
_ which forms no protection against the formidable drill of these 
= -harmless-looking gasteropods. The common whelk ; the dog-whelk 
_ (Purpura lapillus) ; the whelk-tingle (saurex erinaceus) ; ‘and the 
pretty little Nassa reticulata all afford good illustrations of this 
- singular weapon of offence. It is a retractile proboscis which is 
_ ordinarily quite hidden away within the body, but, at the will of 
the animal, can be protruded through the oral orifice to a great 
_ length. This is effected by the action of the intrinsic circular 
muscles that form its walls.. The method by which it is drawn 
_ back into the body is by a number of longitudinal muscles, whose 
i ‘subdivided extremities at one end are Beebe to the interior of 
_ the body and, at the other, to the wall of the proboscis, the inser- 
tions of the muscles varying in position throughout the entire 
length of that organ. By their means the aniferior half of the 
_ proboscis is drawn up within the posterior half, in the same 
_ manner as the foot and lower half of a stocking, into which an 
_armis thrust, may be drawn up bodily (not turned inside out) 
within the main portion, that portion becoming turned back over 
the lower half. The proboscis is thus shortened, not only by the 
contraction of its muscular wall, but by the anterior half becoming 
an | inner cylinder, contained within the lar ger cylinder formed by 
f basal and _posterior half. Inside the inner cylinder is the 
so-called muscular tongue, armed with a series of siliceous teeth, 
those on the rachis being straight and sharply dentated, while the 
