The Gastropoda, or crawling mollusks, have a head, and are consequently animals of a higher organization than the Conchifera or bivalve class. Their mantle forms a vaulted chamber over the head and neck, and envelopes the foot or crawling-disk; all these the animal can protrude or draw in at pleasure. The head is of a globular form, with two or four exceedingly sensitive tentacles, arranged in pairs on each side of it, as in the garden snail, which has four, two long and two short. These tentacles, which the snail can push out and draw in at pleasure, are hollow tubes, the walls of which are composed of circular bands of muscle. The tentacles are pushed out by the alternate contractions of these circular bands, but they are drawn in again like the inverted finger of a glove by muscular cords proceeding to the internal extremity of the tentacle from the muscle that withdraws the foot. The structure of the tentacles is the same in all the crawling mollusks; they are most sensitive in the Helix or Snail family, but they are believed to be delicate organs of touch in all. The Gastropod mollusks never have more than two eyes, either placed on the tips, or at the base of one pair of tentacles; in the snail they may be seen as black points on the tips of the longest pair. In some of the higher Gastropods they are of great beauty, and appear to be perfectly adapted for distinct vision. Organs of hearing were discovered by Dr. Siebold at the base of one of the pairs of tentacles, consisting of vesicles containing a liquid and calcareous otolites, which perform remarkable oscillations due to the action of vibratile cilia. In the Snail and Slug group the number of otolites varies from eighty to one hundred.