One of the most interesting groups of marine life is that including jelly-fishes and anemones. In it are the pretty little sea firs, so often mistaken for sea-weeds by the youthful admirers of these plants, who almost always include them in their collection of marine _algæ_; the transparent, bell-shaped jelly-fishes, which may often be seen in thousands during the summer, carried by the tides, and swimming gently by graceful contractions of their bells; and, most beautiful of all, the lovely anemones--the ‘sea flowers’ of the older naturalists, by whom they were regarded as forms of vegetable life. [Illustration: FIG. 77.--THREAD CELLS OF A CŒLENTERATE, MAGNIFIED 1. Thread retracted 2. Thread protruded] The simplest animals of this group are minute jelly-like creatures, of a more or less cylindrical form, usually fixed at one end, and having a mouth at the other. The body is a simple hollow cylinder, the wall of which is made up of two distinct layers, while the cavity within serves the purpose of a stomach. The mouth is surrounded by a circle of arms or tentacles by means of which the creature is enabled to capture its prey. These arms are capable of free movement in every direction, and can be readily retracted when the animal is disturbed. They are also armed with minute oval, hollow cells, each of which has a slender filament coiled up into a spiral within its cavity. Each filament is capable of being suddenly protruded, thus becoming a free whip-like appendage, and these are so numerous as to be very effectual in seizing and holding the living beings on which the animal feeds. This would undoubtedly be the case even if they were capable of mechanical action only, but, in many instances at least, they seem to be aided by the presence of some violent irritant, judging from the rapidity with which the struggling prey is paralysed when seized, especially in the case of some of the larger members of the group. The simple forms referred to increase by a process of budding, the buds appearing first as simple swellings on the side of the parent creature, and afterwards developing a mouth and tentacles, thus becoming exactly like the adult form. Clusters of eggs also are developed in the outer layer of the body-wall, and these are set free at intervals, and produce new individuals. These animals possess no blood system of any kind, and have no special organs for respiration, but the nutrient matter absorbed from the body-cavity permeates the soft structures of the flower-like body, and the oxygen required for respiratory purposes is readily absorbed from the surrounding water. [Illustration: FIG. 78.--THE SQUIRREL’S-TAIL SEA FIR (_Sertularia argentea_), WITH A PORTION ENLARGED]