OPINIONS: The Gemara says that the minimum size of a Sheretz which is Metamei is the size of a lentil bean, because the smallest, whole Sheretz (when it is young) is the size of a small lentil. This small Sheretz is called the "Chomet." What is a "Chomet?"
(a) RASHI defines the Chomet as a "limace" (Fr.), a snail or slug (see SEFER LEKACH TOV of Rav Chananel Tuvya Dayan (5749, Bnei Brak), who discusses the exact definition of this word). (The word "limga" with a Gimel, printed in our texts of Rashi, appears to be a printer's error, because Rashi elsewhere consistently writes "limace" with a Tzadi.) Rashi in Chulin (121a, DH Halta'ah) says that upon examination of the shell of the limace, one will notice that the innermost of its spiral twirls (which indicates 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 that a Chomet is a snail seems to be the Midrash he quotes in Parshas Ekev (Devarim 8:4) which says that the clothes of the Jews in the desert were "like the shell of a Chomet, which grows as the Chomet grows."
(b) The TOSFOS RID disagrees with Rashi and explains that the Chomet mentioned here is not the same as the Chomet 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 liable for the Melachah of Tzeidah, and one who punctures its skin and makes it bleed is liable for the Melachah of Chovel.
If the Chomet is a snail, why does the Gemara refer to the "skin of a Chomet"? Snails do not have skin! Moreover, one cannot be liable for "capturing" a snail; it is "already captured" since it cannot run away when a person attempts to pick it up. It is like a blind grasshopper or an infant deer, for which one who captures it on Shabbos is not liable because it does not run away (Shabbos 106b, Beitzah 25a, and Rashi there, DH Bah).
The same questions may be asked on the opinion which maintains that the Chilazon creature from which Techeles is procured is the Murex Trunculus snail. The Gemara in Shabbos (75a) says that one is liable for Tzeidah if he captures a Chilazon on Shabbos (see Insights to Menachos 44:1 and Shabbos 75:1). (Although the Yerushalmi cited by Tosfos there says that one is not liable for Tzeidah if he captures a Chilazon, the Bavli argues and maintains that one is liable.)
Perhaps the answer to these questions is that the Melachah of Tzeidah includes capturing a creature which is hard to find. Only when no effort is required in order to capture it or to find it is one not liable for Tzeidah. If effort is required either to capture it or to find it, then one is liable for Tzeidah. Therefore, one will be liable for capturing a snail, since it normally buries itself in the ground or matches the colors of its surroundings and thus is difficult to find.
Alternatively, perhaps only an animal which normally attempts to avoid capture can be considered "already captured" when it is blind or weak. An animal which 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, one who lifts up a snail is liable for Tzeidah. (M. KORNFELD)
With regard to why the Mishnah in Shabbos says that one is liable for puncturing the skin of a Chomet if a snail has no skin, perhaps the very thin skin of a snail is considered skin such that one is liable for puncturing it on Shabbos.