The hydrozoa present a “definite histological structure,” says Professor Huxley; “the body always exhibits a separation into at least two distinct layers of tissue, an outer and an inner.” The Hydras, or fresh-water polypes, which may be found in nearly every pond adhering to the duckweed, appears like tubes, and if touched will curl up into tiny knobs. But if let alone they will adhere to a glass by their single foot, or sucker, which can be moved at pleasure.
Fig. 832.—The hydra.
The foot, or sucker, is continued to a slender cylindrical stalk, from the end of which radiate a number of tentacles, or “feelers,” growing around the mouth, and serving to convey or attract food to the animal which is, so to speak, all stomach. There is no breathing apparatus, and what food it cannot digest is expelled from the mouth. The peculiarity which has given the hydra its name is, that no matter into how many pieces you cut this polype, the parts cut off will all develop into little polypes perfect as their parent.[40] But germination is carried on naturally by buds thrown out, and cast (by “gemmation”), or by the ordinary sexual production of ova.
The outer and inner skins of the hydra are called the ectoderm and endoderm, and the animal is quite capable of locomotion, walking, or rather moving, backwards, by raising and planting its sucker or foot, and by swimming. The prey is captured by the tentacles and by the darting out of tiny spears from the cells or “thread cells” which contain them on the surface of the body. The well-known “Portuguese man-of-war,” an ocean polype, has these “harpoons” greatly developed, and can inflict serious pain as of many stinging nettles; the sensation is exceedingly painful, and lasts some time.
Fig. 833.—Medusa.
The Medusidæ are known to the seaside visitor as the jelly-fish, and the other Acalephæ, the “hidden-eyed” medusæ, include the Portuguese man-of-war mentioned above, and many other umbrella-like animals. They have received the name of medusæ from Medusa, whose long, snaky locks the tentacles of the animals are supposed to be like. Some of these “floating umbrellas” are very dangerous, and will inflict severe stings upon any one in their vicinity. The tentacles or filaments extend for a long distance, and bathers should be cautious. We have often watched them, and they are beautiful to contemplate particularly at night, and in Kingstown Harbour, near Dublin, many exceedingly fine specimens have been obtained. The pulsation of the “umbrella” or bell, enables the animal to swim, and the even undulations of this beautiful covering are apparently caused by nervous contractions.
The jelly-fish have no resemblance to “fish,” and scarcely appear to exist; they are of no use to man, and when removed from the water dwindle by little and little to a tiny film and nothing more—they dissolve into air and water. Cases have been known and tales told of how farmers collected hundreds of these jelly-fish for manure, and when the cart reached the field, to the man’s astonishment, nothing was left but what appeared cobweb in the place of the load of fish.
The Cyclippe is a very common specimen, and moves by means of its cilia; Cestum Veneris—the zone or girdle of Venus—is another curious example. It appears like a glass ribbon about five inches wide and perhaps four or five feet long. The cilia when in motion are very brilliant in colouring, and the creature undulates through the water in a remarkable manner.