Fig. 78.—The Squirrel’s-tail Sea Fir (Sertularia argentea), with a portion enlarged
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.
The higher cœlenterates differ in certain particulars from the lower forms just referred to. Thus, they frequently have a large number of tentacles around the mouth, often arranged in several distinct whorls. They have also a stomach separate from the general body-cavity, but communicating with the latter below; and the body-cavity is divided into compartments by a number of radiating partitions. Some, also, develop a hard, stony skeleton by secreting carbonate of lime obtained from the water in which they live.
Fig. 79.—Sertularia filicula
We often see, when collecting on the beaches of rocky coasts, and especially after storms, a number of vegetable-like growths, of a greyish or brownish colour, each consisting of one or more main stalks bearing a number of delicate branches. Some of them, by their peculiar mode of growth, have suggested the name of sea firs, and a few of these, together with other animals of the same group, may readily be recognised by the accompanying illustrations. They are the objects already referred to as being commonly included in collections of sea-weeds by young naturalists, but they are in reality the horny skeletons of colonies of cœlenterates of the simplest type, belonging to the division Hydrozoa.
Fig. 80.—Sertularia cupressina