female also differs in having its two dorsal arms enlarged at the end to form a veil or mold with which she secretes a shell. In a vast number of species, the males are characterized by having one of the arms modified to form a copulatory organ. This arm is known as the hectocotylus. In certain octopods, including the Argonauta, this arm is broken off and left in the female to fertilize the eggs. In all the other groups the hectocotylus is simply held inside the female until copulation is complete. It is interesting to note that more than 2000 years ago Aristotle recorded the presence of the hectocotylus arm in the octopus and correctly associated it with its sexual purpose.
In the males the sperm is gathered into large sacs or spermatophores of several inches in length. These sacs find their way in some unknown manner into the hectocotylus arm. Each sac contains a tiny, coiled, spring-like filament which spews the ripe sperm out of the sac.
The eggs of the cephalopods are laid in various ways. They may be single and floating in the pelagic species, such as Oegopsida, congregated together in a shelly nest as in Argonauta, laid in jelly tubes as in the Loligo squid, or anchored in grape-like bunches under rock ledges as in the Octopus. The embryo emerges from the egg fully developed and does not have a free veliger stage. With the aid of a lens it is possible to see the beautiful splotches of bright chromatophores in the skin of the tiny young even before they hatch. About a hundred eggs are laid at one sitting by the octopus; the squid egg strings from one female may contain over 40,000 eggs. Some species of octopuses take pains to watch over their brood of eggs and from time to time may carefully go over them with their tentacles to remove dirt.
THE CHITONS—AMPHINEURA
Amateurs and professionals alike have found the chitons or coat-of-mail shells an extremely interesting and fruitful field of study, and no collection is complete without at least three or four representatives of this strange group of mollusks. The chitons closely resemble the gastropods except that they bear eight shelly plates. For those who wish to excel in a more serious study of a relatively small class of mollusks, no more inviting series of species awaits the collector than our chitons of the rocky shores. They are dealt with in this book in some detail, for no popular shell book has hitherto attempted to open the doors to this supposedly “difficult and poorly known” group.
There are nearly fifty species in our Atlantic waters and perhaps twice that number on the Pacific Coast, and yet this represents fewer species than are found in the single family of Wentletraps or Epitonium. Some private collectors, such as the late Dr. R. B. Bales, were able in a few years to make larger and finer collections of Florida chitons than are found even
