The /Amphineura/, as already observed, approach most closely to the ideal mollusk just described. They are bilaterally symmetrical. This fact is so important that it constitutes them a class, notwithstanding the fact that in respect to the foot (the basis of division into classes) they would be included with the /Gasteropoda/, for (barring some exceptional instances) they creep along upon a foot quite as our ideal mollusk would, and as the /Gasteropoda/ do. The head carries no tentacles, thus essentially differing in this respect from the ideal form. The mantle is extended down in front, completely covering the head. The branchiæ are confined to a few pairs of /ctenidia/, or plume-like gills, within the mantle groove or cavity, and are arranged upon each side of the excretory opening like small feathers. There are two orders of the /Amphineura/, the /Polyplacophora/ and the /Aplacophora/ (or /Solenogastres/), the one name meaning "bearing many plates," and the other "without plates," the word "plate" in this sense being synonymous with "shell." The shell of the first order consists of eight calcareous disks arranged in a longitudinal row along the back or dorsal side of the animal, which overlap like shingles on a roof and admit of great variation of form in the various families. [pg322] It will be observed from the examples given that the shell plates do not entirely cover the mantle, but that a considerable portion of the upper mantle surface is left exposed along the side of the animal. This exposed portion of the mantle is called the /girdle/. It is always very much thickened and is often exceedingly tough. In some species it is covered by a great number of calcareous spicules or granules which are embedded in the leathery mantle tissue. Strangely enough, the /Chitonidæ/ (a large family of the /Polyplacophora/) possess a great number of eyes scattered over the shell plates like minute dots. [Illustration: /Chiton spinosus/, dorsal view.] The members of this order are to be found in all parts of the world, but their greatest development is upon the west coast of America. In California the student of marine life has a splendid opportunity to examine the chitons. They are generally to be found adhering to the under surface of stones about low-tide mark. They are very sluggish in their movements and may readily be removed from their resting-places by suddenly thrusting a knife-blade under the foot and prying them off. When thus detached they have a habit of rolling up into a ball and so remaining indefinitely; to prevent this they should be placed at once in a jar of sea-water, where they will resume their natural position. For preservation as cabinet specimens, chitons should be tied down between two boards or flat substances, to hold them firmly extended until dried; otherwise they are apt to contract out of shape. [Illustration:/Chiton/, ventral view: /an/, anus; /cten/, ctenidia; /ft/, foot; /mant/, mantle-edge; /mo/, mouth.]