In general structure the Alcyonium resembles the sea anemone, but the firm body-wall of the colony is supported and protected to some extent by the presence of minute spicules of carbonate of lime; and it is interesting to note that while the tentacles of anemones and corals make up a number that is a multiple of either five or six, those of the Alcyonaria and the allied ‘Sea pens’ are always in multiples of four.
CHAPTER X
STARFISHES, SEA URCHINS, ETC.
Still passing up the scale of animal life, we now come to the Echinodermata—the other sub-kingdom which we have already referred to as forming, with the Cœlenterates, the old division of Radiata. The term Echinoderm signifies ‘hedgehog skin,’ and is applied to the group on account of the fact that the majority of its species possess a skin that is either distinctly spiny, or exhibits numerous more or less defined prominences. This skin is also supported and hardened by the deposit of little plates or spicules of carbonate of lime, all joined together so as to form a kind of scaffolding or ‘test’ for the protection of the animal; and this secretion of carbonate of lime is not always confined to the outer skin, for, in some cases, it occurs in the walls of the internal organs as well.
Most of the animals of this sub-kingdom display a regular radiate symmetry; that is, the parts of their bodies are arranged regularly round a common axis, and the arrangement is usually a five-fold one, as may be observed in the case of the common Five-fingered Starfish of our coasts (see [Plate IV.]), and it is worthy of note that this radiate disposition of parts is not merely external, but that, as in the case of anemones and jelly-fishes, it also obtains within, and determines the arrangement of the internal organs. Further, although this radiate symmetry characterises the adult animals of the group we are considering, yet some show a tendency towards bilateral symmetry (parts arranged equally on two opposite sides of a common axis), while this is the rule, rather than the exception, with the early stages or larvæ of these creatures. Observe, for instance, the larva of the common Brittle Starfish, the adult of which species exhibits an almost perfect radiate symmetry, and we see something more than a mere trace of a two-sided disposition.
We have not to look far into the structure of any typical echinoderm to see that it is a distinct advance on the anemones in the matter of organisation. To begin with its digestive system—this consists of a tube having no communication with the general body-cavity, but remaining quite distinct throughout its length, with both ends communicating directly with the exterior. Its nervous system also is more highly developed, for it has a well-formed ring of nerve matter round the mouth, from which pass two or three systems of nerve fibres, each system having its own special function to perform. The sense organs, however, do not appear to be well developed, though there exist certain ‘pigment spots,’ in which nerve fibres terminate, and which are supposed to serve the purpose of eyes.
Fig. 104.—Larva of the Brittle Starfish