Three kinds of zoöids, polyps, or hydranths—as they are indiscriminately called—are attached to the stem. Those having an [pg120] open end and a crown of tentacles are the nutritive individuals. Small, club-like dilations are immature zoöids. The /blastostyles/, or reproductive zoöids, are long, cylindrical, mouthless, and covered. At maturity the cover is ruptured, and the medusæ have the appearance of a pile of thin saucers attached by the middle of the convex side. When at length these saucers are set free as little medusæ, or jellyfishes, the convex side of each saucer, or swimming-bell, is called the /ex-umbrella/; the concave, under side, the /subumbrella/. From the center of the subumbrella projects the /manubrium/, or stomach of the animal. At the free end of the manubrium is a four-cornered mouth. From the attached end of the /manubrium/ four tubes or canals diverge, and, extending through the animal, open into a circular canal which runs around the margin of the umbrella. When the medusa is as above described, it has reached the highest point in its development. When the medusa has matured, it lays eggs, known as /planulæ/. These are spherical bodies covered with cilia (hairs), by means of which they swim about for a time; but they finally attach themselves to some object, there to grow and develop into hydroid colonies. The cycle of life is thus completed. This process is known as /alternation of generation/, or /metagenesis/, one life-history containing two quite different forms of being. The term of life of an individual is one year, the zoöphyte stage beginning in the autumn and the medusa stage in the spring. Some medusæ, besides reproducing by means of eggs, multiply by budding, small medusæ growing on the manubrium or on the margin of the umbrella. /Sarsia/ and /Lizzia/ sometimes increase by budding. The /Hydrozoa/ are not all of the above type. In the sertularians the zoöids perish on the stem and have no medusa life, their reproductive element giving rise to the hydroid form without metamorphosis. The /Trachylinæ/ have no hydroid life, being always free-swimming medusæ; others, the /Siphonophora/, live a hydroid life which is unattached, the colony floating on the ocean; the millepores secrete calcareous skeletons and always remain fixed, reproducing by budding. [pg121] Hydroids are very abundant, but are comprised in the few groups mentioned: namely, those which live only in the fixed colonial state; those which have alternation of generation, being first hydroids and then swimming-bells, or medusæ; those which live always in the medusa state, the eggs of the jellyfish developing at once into other medusæ; and the /Siphonophora/, or those which have a floating colonial state, the hydroid never being attached, but floating at large and capable of locomotion, some of the colony having the function of propulsion.