ANATOMY OF THE SNAIL. 241 
is, in those species inhabiting a spiral shell, asymmetrical 
and wound in a spiral, the visceral mass extending into the 
apex of the shell. In the Nudibranchs (Figs. 190, 192), and 
the slug, the body being naked is symmetrical 
on each side. 
The digestive tract is doubled on itself, the 
vent ending on one side of the mouth. In 
some Nudibranchs the intestine has numerous 
lateral offshoots, or gastro-hepatic branches, 
which resemble similar structures in the Plana- 
rian and Trematode worms. A heart is always 
present, except in the parasitic Hntoconcha, 
and sometimes, as in Chiton, Neritina, and 
Haliotis, it is perforated by the intestine. In 
some genera there are two auricles to the heart, 
but as a rule but one is present. The Gastro- gf '8 (Siimitoe 
pods breathe by gills either free, or contained snared twice— 
in a cavity in the mantle, while in the land- 
snails (Pulmonata) the air is breathed directly by a lung-like 
gill in a mantle-cavity. The kidney is single. The sexes 
are either distinct or united in the same individual. 
An excellent idca of the structure of a typical Gastropod 
may be obtained by a dissection of Natica (Lunatia) heros. 
This is a large mollusk, common between tide-marks from 
Labrador to Georgia. On taking it up the student will 
notice the large, round, swollen, porous foot, from which 
the water pours as if from the ‘‘rose” of a watering-pot. 
The shell is large, composed of several whorls, with a small 
flattened spire or apex. The aperture is large, lunate in 
shape, and can be closed by a large horny door or oper- 
culum. (In some mollusks, Natica, Turbo, etc., the oper- 
culum is of solid limestone, and small ones are used as “‘ eye- 
stones,” being inserted in the eye and moved about by the 
action of the lids, thus cleansing the eye of irritant particles 
of dust, etc.) 
The animal should then be placed in a dish of salt water, 
and its movements observed. There are but two short, 
broad, flattened tentacles, situated on a flap or head-lobe 
(prosoma) of the mantle or body-walls. No eyes are present 
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