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The free-swimming larva sinks to the floor of the sea, abandons its locomotive habits, and attaches itself to stones, marine plants, mussel-shells, corals, and other objects; this is done with the part of the body that was foremost in movement. The attachment is effected by a number of out-growths, usually three, which can be seen even in the free-swimming larva.
When the Ascidia-larva has attained this stage of development it begins to move about in the ovolemma. This causes the membrane to burst. The larva emerges from it, and swims about in the sea by means of its oar-like tail. These free-swimming larvae of the Ascidia have been known for a long time. They were first observed by Darwin during his voyage round the world in 1833.
This completes the life-circle of the remarkable and instructive animal. If we compare these permanent blastulae with the free-swimming ciliated larvae or blastulae, with similar construction, of many of the lower animals, we can confidently deduce from them that there was a very early and long-extinct common stem-form of substantially the same structure as the blastula.
The thought of their life history brings to mind how sadly they retrogress as they grow, hatching as minute free-swimming creatures like tiny lobsters, and gradually changing to this plant-like life, sans eyes, sans head, sans most everything except a stomach and a few pairs of feathery feet to kick food into it. A few pitiful traces of nerves are left them.
The Medusa really descends from one of the plant-like animals of the early Archaean period, but it has abandoned the ancestral stalk, turned upside down, and developed muscular swimming organs. Its past is betrayed in its embryonic development. As a rule the germ develops into a stalked polyp, out of which the free-swimming Medusa is formed.
Hence the formation of a medullary tube out of the outer skin takes place in the naked dorsal surface of the free-swimming larva of the Amphioxus in just the same way as we have found in the embryo of man and the higher animals within the foetal membranes.
"Because the enemies of fish are numerous and free-swimming," was the answer, "and also because fish produce an enormous amount of eggs. Oysters do also, but fertilization is so largely a matter of chance that but a few of the tens of thousands of eggs ever really have a chance to become young oysters.
These sea-anemones themselves are mysterious evidences of the gradual advance of organisms from the slime to the poem. They are animals, and attach themselves by a muscular base to the rocks or shells, or are as free-swimming as perch. I saw them two feet in diameter, seeming all vegetable, some like chrysanthemums and some resembling embroidered pin-cushions.
"Well, Paul," the boy answered, "the young of sponges are larvæ which swim in the water by threshing with short hairs until they find something suitable to stick on. Lots of animals which become fixtures are free-swimming when young, oysters, for instance." "Then a sponge doesn't seed itself, like a plant?" "No, Mr.
On the rocky, volcanic seafloor, there bloomed quite a collection of moving flora: sponges, sea cucumbers, jellyfish called sea gooseberries that were adorned with reddish tendrils and gave off a subtle phosphorescence, members of the genus Beroe that are commonly known by the name melon jellyfish and are bathed in the shimmer of the whole solar spectrum, free-swimming crinoids one meter wide that reddened the waters with their crimson hue, treelike basket stars of the greatest beauty, sea fans from the genus Pavonacea with long stems, numerous edible sea urchins of various species, plus green sea anemones with a grayish trunk and a brown disk lost beneath the olive-colored tresses of their tentacles.
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