1 and 2. Retracted filaments; 3. Partly protruded; 4. Fully protruded. Magnified × 600. (Warne.)
Polypomedusæ.—Among the higher development of the stinging group is the jelly-fish. The Siphonophora, as represented by the Portuguese man-of-war, are in their turn the highest development of swimming-bells, and exhibit many modifications and combinations of individuals. The tentacles of the Physalia, the best known, are stiff with batteries of stinging-capsules, the sting of which is more like the shock of the electric current. The Challenger soundings brought to light some remarkably interesting forms, and these have furnished much work for the microscope, as all their larval forms are extremely curious. Among the Hydromedusæ there are many different life histories. Take the jelly-fish, the eggs of which have given up forming stocks, and are hatched out at once as Medusæ. There are others, the eggs of which form stocks; others, again, in which the sexual individuals do not swim away as jelly-fish. The last were at one time described under a new name, because of one or two curious forms being taken creeping on the ground. This creeping Medusa (Clavatella prolifera) has six arms, the tips of which are provided with true suckers, and on these it walks as on stilts, while from each arm a short stalk arises, the swollen end of which is beset with stinging capsules. It has an extensile mouth-tube, and feeds upon small crustaceans found on seaweeds.
Fig. 348.—Plumularia primata. Doris tuberculata seen clinging to a fucus.
Among the forms that swim away as jelly-fish a very curious example is presented in Corymorpha mutans. These swim about for a time, and then firmly attach themselves by numerous thread-like appendages, forced into the sand, and where the young prepare for their next metamorphosis. As an example of the stocks of those representatives which do not swim away as jelly-fish, take the beautifully-feathered, plant-like creatures found erect along the seashore, the Sertularia ([Fig. 358], No. 12) and Plumularia. Plumularia primata, [Fig. 348]. Other members of these groups will be found in [Plate IV]., Nos. 95-99.
Fig. 349.—Group of female stock of Hydractinia echinata.
a, a. Nutritive individuals; b, b. Female individuals and groups of eggs. Highly magnified.—(Warne.)
In addition to the nutritive individuals, there are the egg-bearing; these do not become free-swimming individuals. One small family is neither branched nor feathered—the Hydractinia echinata, found in the North Sea and on the Norwegian coasts, where it attaches itself to the shells of gastropods, selecting those inhabited by hermit crabs. The part of the stock common to all the individuals is the skin-like portion which adheres to the surface of the shell. In some spiny processes are produced, and the nutritive canals running down the stems of the polyps are continued into the membrane belonging to the stock, as seen in [Fig. 349].
The nutritive individuals are distinguished by long tentacles, mouths, and digestive canals. The females have no mouths, and are supplied with food through the system of canals running to them from the nutritive males. These reproductive members are furnished with stinging threads instead of tentacles for the protection of their ova. The ciliated larvæ, in a very short time, swim off to found new colonies.