Fig. 350.—Medusæ, Jelly-fish.
The free-swimming jelly-fish ([Fig. 350], and Plate XVII., c and d) belong to the order Scyphomedusæ. These are characterised by their delicate colouring, and from the arrangement of their nervous system, which can only be made out by staining. Some new and curious forms were dredged from a depth of more than 6,000 feet off the coast of New Zealand, varying in size from an inch to twenty inches; many having from four to eight or ten eyes arranged along the margin.
Anthozoa.—From the free-swimming we turn to a group of permanently fixed polyp forms, the sea-anemones and corals. The development of Monoxenia commences with the egg, repeatedly dividing into many parts ([Fig. 351], C, D, and E), by a process common to the animal kingdom, termed egg-segmentation, in this particular instance proceeding from an apparently hollow sphere, A, enclosing a single layer of cells, G. Each cell sends out a long cilia, or whip-like process, F, by means of which the larva turns about and swims in the body fluid of the parent polyp. One half of the sphere now becomes enfolded into the other half, H, and forms what is termed a gastrula, I, K. The gastrula stage of Monoxenia is of the simplest kind, the larva forming a sac, with walls consisting of two layers, an outer, or ectoderm, and an inner, or endoderm. The transition from the flat dish shape, H, to the sac with a narrow mouth is at once clear, and the knowledge that all the Cœlenterates proceed from similar larvæ, and that all the complications of their various systems are developed from a simple gastrula, throws much light on their anatomy. During these transitions the endoderm, whose cells multiply, continues as an uninterrupted lining to the stomach and its appendages, while the ectoderm yields the cuticular elements.
Fig. 351.—Stages in development of Monoxenia Darwinii, × 600.—(Warne.)
A third intermediate gelatinous layer, the mesoglæa, arises between the two layers in which muscles and connective interstitial tissue appear. In the mesoglæa of one species of coral calcification takes place; this internal calcification has but a small share in the work of the great rock-making corals, their most important calcification being external. In Monoxenia, although the transition from the gastrula larva to the adult animal has not been seen, there can be no doubt as to how this is carried out, the transformations having been watched throughout in other species. The larva attaches itself with the end opposite the mouth, the cilia disappear, and after the mouth-tube has been formed by the folding in of the anterior end along the longitudinal axis of the body, and has thus become marked off from the stomach, eight hollow tentacles rise round the mouth as outgrowths of the body cavity, or as direct continuations of the stomach.
Like all other corals, Monoxenia periodically multiply by means of eggs, which are formed either in the walls of the radiating partitions or septa, or along the free edges. These are ejected through the oral opening. As a rule, the polyps are either male or female; but in stock-forming species individuals of the two sexes are often mixed. Monoxenia may be taken as the simplest type of the regularly radiate polyps; in all the different organs being repeated in regular rings round a central axis; the mouth also is circular. From this interesting account, drawn by Haeckel, of a simple polyp, it will be at once seen what kind of radiate animal it is that builds up the coral reefs. “No garden on earth can match the gardens of the sea that circle the northern part of Australia. As the tide ebbs in azure sunset, coral-reefs peer out symmetrically arranged in beds and intersected by emerald pathways coursing through corals of all hues and tints fathoms deep in the channels.”
In a growing polyp-stock the individuals usually remain in organic connection; that is to say, each first provides for itself and then shares its superfluity with others, sometimes by means of a continuous reticulated system of canals perforating the calcareous substance which often separates the members of one stock from another. The whole colony may thus be physiologically one creature with many mouths. There are others that remain single, as the inverted pyramidal-looking bodies, Fungidæ, commonly called “Sea-mushrooms,” found in great variety. The colour of the polypidom is white, of a flattened round shape, made up of thin plates or scales, imbedded in a translucent jelly-like substance, and within is concealed a polyp; the footstalk, by means of which the animal is attached to the rock, is of a calcareous nature ([Fig. 352], No. 1).
