[Illustration: Fig. 107. Young Nanomia; magnified.]
[Illustration: Fig, 108. Young Nanomia with rudimentary Medusæ.]
[Illustration: Fig. 109. Young Nanomia, older than Fig. 108.]
[Illustration: Fig. 110. Heart-shaped swimming bell of Nanomia; magnified.]
Besides these locomotive members, the community contains three kinds of Hydræ arising as buds from the primitive Hydra below the swimming bells, the latter remaining always nearest the oil bubble at the top, while the first Hydra, the founder of the community, in proportion as the new individuals are added, is gradually pushed downward, and remains always at the end of the string, the stem of which is formed by the elongated neck of the primitive Hydra. All the three sets of Hydræ have certain features in common, while they have other distinguishing characteristics marking them as distinct individuals. They are all accompanied by triangular shields (Fig. 111), arising with them at the same point on the parent stem, and all are furnished with tentacles hanging down from the summit of the Hydra at the side opposite the shield. These facts are important to remember, since we shall presently perceive, upon analyzing their parts, that these Hydræ have a close homology to the Hybocodon. The tentacles differ in structure as well as in number for each kind of Hydra. Having shown in what characters they agree, let us now take each set individually, and see what differences they present.
[Illustration: Fig. 111. Cluster of Medusæ with tentacles having pendent knobs.]
In the first set which we will examine the Hydra is open-mouthed. Like the original Hydra, it is only a digestive tube, similar in all respects to the proboscis of a Medusa-disk. Its only function is that of feeding, and it shows a laudable fidelity to its calling, being very constantly and earnestly engaged in the work. Let us add, however, that in this instance the occupation is not a wholly selfish one, since the cavity of every Hydra communicates with that of the stem, and the food taken in at these over-gaping mouths, is at once circulated through all parts of the community, with the exception of the oil bubble, from which it is excluded by the transverse partition dividing it from all the lower members of the stock. The shields share in this general nourishment of the compound body by means of chymiferous tubes extending toward the outer surface, and opening into the cavity of the stem. The mouth of this Hydra is very flexible (Fig. 111), expanding and contracting at the will of the animal, and sometimes acting as a sucker, fastening itself, leech-like, on the object from which it seeks to draw its sustenance. (See Fig. 111.) The tentacles attached to this set of Hydræ are exceedingly long and delicate. They arise in a cluster at the upper and inner edge of the Hydra, just at its point of juncture with the stem, and being extremely flexible and contractile, their long tendril-like sprays are thrown out in an endless variety of attitudes. (See Fig. 115.) Along the whole length of this kind of tentacle are attached little pendent knobs at even distances; Fig. 112 represents such a knob greatly magnified, and absolutely paved with lasso-cells, the inner and smaller ones being surrounded by a row of larger ones.
