(a) RASHI defines the Chomet as a "limace" (Fr.), or snail. (The word
"limga," with a Gimel, printed in our texts of Rashi appears to be a
printer's error; elsewhere Rashi consistently writes "limace," with a Tzadi)
Rashi in Chulin (121a, DH Halta'ah) says that if one looks at the shell of
the limace, one will see that the innermost of its spiral twirls (which
denotes the size of the Chomet at birth, since the shell grows as the body
of the snail grows) is the size of a lentil bean. Rashi's source for calling
a Chomet a snail is probably the Midrash which he quotes in Parshas Ekev
(Devarim 8:4) which says that the clothes of the Jews in the Midbar were
"like the shell of a Chomet, which grows as the Chomet grows."
(b) TOSFOS RID argues with Rashi and says that the Chomet mentioned here
must be a different type of Chomet than that mentioned in the Midrash. This
Chomet cannot be a snail, because the Mishnah in Shabbos (107a) says that
one who captures on Shabbos one of the eight Sheratzim mentioned in the
Torah is Chayav for Tzeidah, and one who punctures its skin and makes it
bleed is Chayav for Chovel.
How could the Gemara refer to the "skin of a Chomet" if it is a snail?
Snails do not have any skin! Moreover, how can one be Chayav for capturing a
snail? It is "already captured," since it cannot run away when a person
tries to pick it up, and thus it is like a blind grasshopper, or an infant
deer, for which one is not Chayav if he captures it on Shabbos since it does
not run away (Shabbos 106b, Beitzah 25a Rashi DH Bah).
The same questions apply to those opinions that maintain that the Chilazon
from which Techeles is procured is the Murex Trunculus snail. The Gemara
(Shabbos 75a) says that one is Chayav for Tzeidah if one captures a Chilazon
on Shabbos. (It is true that the Yerushalmi cited by Tosfos there says that
there is no Chiyuv of Tzeidah for capturing a Chilazon. However, the Bavli
argues and says that there is a Chiyuv of Tzeidah -- see Insights to Shabbos
75a regarding the Chilazon.)
Perhaps the answer is that anything that is *hard to find* falls into the
category of Tzeidah. Only if it does not require any effort in capturing it
*nor in finding it*, one is not Chayav for Tzeidah. If it requires effort
for either capturing it or for finding it, then one *is* Chayav for Tzeidah.
Therefore, one will be Chayav for capturing a snail, since it normally
buries itself beneath the ground or matches the colors of its surroundings
and is thus difficult to find. Alternatively, perhaps only the type of
animal that normally attempts to avoid capture can be considered "already
captured" if it is blind or weak. An animal that does not run away under any
circumstances, such as a snail, cannot be considered "already captured." It
is not logical to consider the normal state of an animal's roaming to be a
state of capture. Therefore, if one lifts up a snail he is Chayav for
Tzeidah. (M. Kornfeld)
Regarding the question how the Mishnah could say that one is Chayav for
puncturing the skin of a Chomet if a snail has practically no skin, it must
be that even the very thin skin of a snail is enough to be Chayav for.