contact with some solid body upon the bed of the ocean, it dies. With the discovery of a solid resting-place its purpose is accomplished: it loses its cilia, and, cementing its body fast, it elongates and becomes converted into a root, from which the bud which is to form the first feeding hydra soon arises, and, acquiring a mouth and tentacles, begins to accumulate food and to provide for the growth of a new colony. In some other species the planula becomes a feeding hydra instead of a root, and the history of the various hydroids shows clearly that the root is directly comparable with a hydra and is a member of the community which, like the others, is specialized for a particular purpose.
On a sea-beach there are few hard solid bodies except the shells of mollusks, and these are therefore the only available resting-places for the planulæ, but a colony which is founded upon the shell of a living mollusk has no chance of prosperity, for the mollusk is sure to soon plow its way under the sand or into the mud, and a delicate hydroid can not survive such rough usage, nor is the case of a planula which finds an empty shell any better, for the first storm will either bury it under the sand, or toss it high and dry above low-tide mark, or sweep it off into some deep channel to be buried under the mud and sediment.
Everything is favorable to the new colony which is started on a shell which a hermit-crab has selected for a house, and the chances are that it will grow and prosper and soon become a vigorous, flourishing settlement; for the crab does not creep like a snail, but trots around on the tips of his claws with the shell held well up above the sand, and he is far too intelligent and wide awake to permit himself to be stranded on the beach, or swept away into muddy quicksands. As the gentle waves ebb and flow on the shore he follows them back and forth, keeping close to the edge, where the food which is washed out of the sand is most abundant and the aeration of the water most perfect. As long as the sea is calm he may be trusted to carry his load of hydroids into the places which are most favorable for them, and as soon as a storm approaches he trots off with his charge to a safe shelter in deeper water and waits until it has passed. A colony which is founded on his shell is sure to flourish and increase, for this location affords all the elements of prosperity, and, while small colonies are often found in other places, the most vigorous and largest ones are, as a rule, found only in this peculiar habitat.
In the following diagram I have attempted to exhibit at one view all the phases in the remarkable life-history of Dysmorphosa. The sign of equality between two stages indicates that the one on the left becomes transformed into the one on the right, without multiplication and without loss of identity; the sign x indicates
