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Once more, crabs pass their earlier larval stages as free-swimming crustaceans, somewhat shrimp-like in appearance, and as agile as fleas: it is only by gradual metamorphosis that they acquire their legs and claws and heavy pedestrian habits. But once a year, as the land-crabs bask in the sun on St.
I had no fear of Aurelia, but when another free-swimming species of jelly-fish, Cyanea, or the blue-jelly, appeared, I swam ashore with all speed. This great jelly is usually more of a reddish liver-colour than a purple, and is much to be dreaded. Its tentacles are of enormous length.
The stationary oyster is of higher organization than the free-swimming medusa; and the cold-blooded and less heterogeneous fish is quicker in its movements than the warm-blooded and more heterogeneous sloth. But the admission that the several aspects under which this increasing contrast shows itself bear variable ratios to one another, does not negative the general truth enunciated.
These modifications do not concern us, as they depend on special adaptations, and do not affect the hereditary vertebrate type. When the free-swimming Amphioxus-larva is three months old, it abandons its pelagic habits and changes into the young animal that lives in the sand.
Among the echinoderms, notable for being covered with spines: starfish, feather stars, sea lilies, free-swimming crinoids, brittle stars, sea urchins, sea cucumbers, etc., represented a complete collection of the individuals in this group.
A perfectly similar appendage, "a most delicate tube or ribbon," was found by Darwin in free-swimming pupae of Lepas australis on the last joints of the "prehensile antennae."
Would he rise to the surface, he must flick his tail, and turn his toes and fingers upwards. Would he seek the bottom, he must depress them. Would he lie motionless, suspended in mid-water, he must point them straight outwards from his sides. It was the charm of a free-swimming existence that divorced him from a vegetarian diet.
These free-swimming cells, which apparently are formed only to reunite with each other, are called zoöspores, while the organism which results from their fusion is known as a zygospore. The zygospore thus formed slowly increases in size, until it in its turn develops a new generation of zoöspores.
The young clam, after having passed the free-swimming phase of his life, as well as the period of attachment to the person of a carp or similar fish, drops to the bottom and attaches himself loosely in the place and station in life to which he has been led; and he loyally sticks to his particular patch of ooze and sand through good fortune and evil.
The free-swimming embryo of an aquatic annelid, being ovate and not ciliated all over, moves with one end foremost; and its differentiations proceed in conformity with this contrast of circumstances.
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