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 



