[Illustration: FIG. 4.--Diagram showing the change of the Actinula into a Medusa. A, Vertical section of the actinula; _a-b_ and _c-d_ as in fig. 3, B, transitional stage, showing preponderating growth in the horizontal plane. C, C', D, D', two types of medusa organization; C and D are composite sections, showing a radius (R) on one side, an interradius (IR) on the other; C' and D' are plans; the mouth and manubrium are indicated at the centre, leading into the gastral cavity subdivided by the four areas of concrescence in each interradius (IR). t, tentacle; g.p, gastric pouch; r.c, radial canal not present in C and C'; c.c, circular or ring-canal; e.l, endoderm-lamella formed by concrescence. For a more detailed diagram of medusa-structure see article MEDUSA.] Returning now to the actinula, this form may, as already stated, develop into a medusa, a type of individual found only in the Hydrozoa, as here understood. To become a medusa, the actinula grows scarcely at all in the direction of the principal axis, but greatly along a plane at right angles to it. Thus the body becomes umbrella-shaped, the concave side representing the peristome, and the convex side the column, of the polyp. Hence the tentacles are found at the edge of the umbrella, and the hypostome forms usually a projecting tube, with the mouth at the extremity, forming the _manubrium_ or handle of the umbrella. The medusa has a pronounced radial symmetry, and the positions of the primary tentacles, usually four in number, mark out the so-called _radii_, alternating with which are four _interradii_. The ectoderm retains its ciliation only in the sensory organs. The mesogloea becomes enormously increased in quantity (hence the popular name "jelly-fish"), and in correlation with this the endoderm-layer lining the coelenteron becomes pressed together in the interradial areas and undergoes concrescence, forming a more or less complicated gastrovascular system (see MEDUSA). It is sufficient to state here that the medusa is usually a free-swimming animal, floating mouth downwards on the open seas, but in some cases it may be attached by its aboral pole, like a polyp, to some firm basis, either temporarily or permanently. Thus the development of the two types of individual seen in the Hydrozoa may be summarized as follows:-- / Egg | | Free | Blastula | | "Planula" < Parenchymula | | Stage | Gastrula | | \ Actinula / \ / \ Polyp Medusa