d. Testis. The testes are always placed near the base of the tentacular circle, and are, as here, generally more than one in number (8-9). They are formed by the local multiplication of interstitial ectoderm cells, which are developed into spermatozoa and raise the covering-cells into a pointed capsule. This capsule dehisces at its apex, setting free its contents into the water.

e. Ovum. The ovum is as a rule solitary. It is produced by the growth of one out of a mass of cells produced by the division of interstitial ectoderm cells. The outer cells undergo regressive changes, and the products formed serve as food to the growing ovum. The ovum is the only ectoderm cell which forms chlorophyl corpuscles, and it may be noted that these bodies have colourless predecessors. The covering-cells are raised by the ovum and burst The ovum is then fertilised, and undergoes fission: the superficial cells are completely used up in this species but not in H. fusca to form (1) a chitinous coat, (2) a vitelline membrane, and (3) a mucous coat. Thus protected, the ovum drops away from the parent and remains quiescent.

f. Adhesive or pedal disc. The ectoderm cells of this disc are glandular and secrete a clear tenacious fluid, and they emit pseudopodia, and hence move the animal slowly. Hydra swims by creeping up to the surface of the water and exposing its disc; and it creeps by bending the body and bringing a tentacle into contact with the surface upon which it rests, then detaching the disc, and bringing it up to the tentacle and reattaching it.

Hydra rarely occurs hermaphrodite as figured here, but one and the same individual may produce both testes and ovaria at different times. These structures are organs in Hydra, not zooids. This fact is proved (1) by their development out of interstitial ectoderm cells, as in the organs of many sexual Medusae, and (2) by the absence of any evagination of the endoderm as in the reduced sexual zooids known as sporosacs or gonophores. It may be added that if a Hydra which is budding actively be starved, the buds dwindle away and sexual organs are evolved.

The ectoderm consists of (1) covering cells, the surface of which is exposed, the body and base irregular, the latter reaching the supporting lamina; (2) epithelial muscle cells, the surface of which is exposed, the base forming a muscle filament, disposed in a longitudinal direction and applied to the supporting lamina; (3) interstitial cells, small, irregular, in masses between the bases of (1); (4) young cnidoblasts placed deeply; (5) fully formed cnidoblasts which are superficial; (6) gland cells, restricted to the pedal disc; (7) ganglion cells with numerous outrunners which are certainly continuous, as in Eudendrium, with cnidoblasts and perhaps inter se on the tentacles, as also in Eudendrium. Sense-cells appear to be wanting. A 'supporting lamina' everywhere separates ecto- from endo-derm save at the edge of the mouth. It is delicate, and contains fibrils emitted from both the ecto- and endo-dermal muscle cells (Jickeli). The endoderm cells are ciliated. They are of three kinds - (1) endodermal cells so called, which contain chlorophyl corpuscles (see on Symbiosis, p. 243), are vacuolated to a certain extent and throw out pseudo-podia during digestion (Parker), and also develope in the walls of the gastric cavity muscle-filaments, which appear to run both circularly and longitudinally; (2) small granular gland cells in the hypostome; (3) vacuolated gland cells at the base of the gastric cavity.

According to Jickeli cnidoblasts are present in the endoderm both of Hydra and Eudendrium. The chlorophyl corpuscles are present chiefly in the marginal part of the endoderm cells, only when very plentiful in their bases. They are spherical, and consist of an outer envelope containing chlorophyl, usually entire but sometimes in plates, and central protoplasmic contents. They therefore closely resemble the chlorophyl bodies of plants.

It was formerly asserted that a Hydra could be turned inside out, and continue to live in this condition (Trembley). More modern researches have disproved the statement. But it appears that within certain limits the animal can be propagated by artificial division.

Hydra. Kleinenberg, Leipzig, 1872; Jickeli, M. J. viii. 1882; Figures, Atlas of Practical Elementary Biology, Howes, 1885.

Tentacle. Development, Jung, M. J. viii. 1882. Pedal disc. Hamann, J. Z. xv. 1882, p. 552.

Muscle cells. Korotneff, A. Z. Expt. v. 1876.

Endodermal pseudopodia. T. J. Parker, P. R. S. xxx. 1880. Chlorophyl corpuscles. Ray Lankester, Q. J. M. xxii. 1882; Id. Nature, xxvii. 1882-83; Hamann Z. W. Z. xxxvii. 1882; Id. Z. A. vi. 1883; Brandt, Z. A. vi. 1883.

Sexual organs, Marshall, Studies Biol. Lab. Owen's College, i. 1886.

Embryology. Korotneff, Z. W. Z. xxxviii. 1883. (Transl. A. N. H. (5) xi. 1883.) Artificial reversal of animal. Engelmann, Z. A. i. 1878 and Marshall, 'Vital phenomena, etc, and new species of Hydra viridis,' Z. W. Z. xxxvii. 1882.

FIG. 7. Portion of tentacle of Eucopdla campanularia; from Von Lendenfeld, Z. W. Z. xxxviii. 1883, P1. xxviii. Fig. 4.

THE tentacle, as in all Hydrozoa and Anthozoa, consists of ectoderm, supporting lamina and endoderm. The ectoderm consists of three layers, shown in A and B.

A. The superficial stratum of covering cells, fully formed cnidoblasts and sense cells. The covering cells have polygonal outlines, a superficial delicate cuticle, a nucleus centrally placed, and protoplasm much vacuolated, and disposed in a mass round the nucleus with irregular peripheral strands, as seen in three of the more centrally placed cells. These details are omitted in the remaining cells for the sake of clearness, as also is the delicate cilium which each cell carries. Three projections, two on the right and one on the left, are cnidocils (see Fig. 8), and the nematocysts corresponding to them are seen as oval bodies at their base. Both cnidocils and nematocysts are disposed at an angle of 450 to the axis of the tentacle. At its tip, however, they are placed at right angles. Sense-cells are not figured (see Fig. 8, s'), but they occur principally towards the apex of the tentacle, scattered among the covering or supporting cells.