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Lives of the Other Mollusks 53
specimens in warm water for five or ten minutes will usually afford sufficient
information. The upper surface as well as the under surface of each valve
has characteristic areas which aid in identification.
In one family of chitons, the Chitonidae, the upper surfaces of the valves
of some species bear microscopic eyes which consist of an eye capsule,
cornea, iris, lens, retina and optic nerve, but they are probably useful only
in sensing changes of light intensity and passing shadows.
The girdle is the leathery rim which encircles the eight valves. In
some species the girdle entirely or partially covers the valves. The surface of
the girdle may be covered with beautiful little scales or with spines, hairs or
tufts of bristles. Unfortunately these characters vary among individuals
and cannot always be used to separate species, although the general types
are fairly reliable in distinguishing genera.
The radula or ribbon of teeth is very long, and is composed of thick
and dark amber-colored teeth. There are usually about seventeen teeth in
each transverse row, in the following order reckoned from the center: one
simple, small central; flanked on each side first by a translucent minor lateral
and then by a major lateral which bears a conspicuous black cusp; next, two
boss-like uncinal plates; then a twisted spatulate uncinal; and, finally, three
scale-like external uncini. The radula of the chitons have not been demon-
strated as useful characters in separating species because of their great vari-
ability, although some workers claim that the major laterals are useful.
The sexes are separate in the chitons. Some species lay eggs in a glu-
tinous, indistinct mass. There may be a free-swimming veliger stage in some
species. In other species the young live under the mantle edge of the mother
for protection.
THE TUSK-SHELLS—SCAPHOPODA
To our Northwest Pacific Indians and our early pioneers the tusk-shells
were a familiar form of wampum, but today few Americans would recog-
nize one on sight. The 200-odd known living species are for the most part
inhabitants of deep water, although a few of our American species live in rela-
tively shallow water and are frequently washed ashore. The shells resemble
miniature elephant tusks open at both ends, and the sluggish creature lives
embedded obliquely in sand and mud, with only the small end of the shell
projecting above the surface of the substrate.
Like many gastropods, the scaphopods possess a single shell and a set
of radular teeth but, like the bivalves, they have a nonlobed velum in the
larval or veliger stage, and in adulthood have a wedge-shaped foot and lack
a definite head. They lack gills but absorb oxygen from the sea water
through the tissues of the mantle. Water is first taken in through the small