^Hydranth^ ("water flower"): A nutritive zoöid. ^Hydrorhiza^ ("water-root"): The part of the colony which fixes it to other bodies, like a root. ^Hydrosoma^ ("water-body"): The entire hydroid colony. ^Hydrotheca^ ("water-receptacle"): The cup-like, horny receptacle which protects the hydranth. ^Mesoderm^ ("middle skin"): A layer which lies between the ectoderm and the endoderm. ^Nematophore^ ("thread-bearing"): The name of peculiar bodies developed in certain genera from definite points; characteristic of plumularians. ^Perisarc^ ("around flesh"): The transparent, chitinous shell, or unorganized outer membrane of horny consistency, which covers to a greater or less extent the soft parts of the colony. ^Zoöid^ ("animal form"): One of the animals which form the colony. [pg119] CLASS =HYDROZOA= SEAWEED-LIKE ZOÖPHYTES AND SMALL JELLYFISHES The hydroids have been called the nurses of jellyfishes. From casual observation these two forms would not be associated together, for the shrub-like organisms, which so much resemble plants that they are often collected and preserved as seaweeds, suggest only vegetable life. Examined with a glass, however, they disclose their animal nature. Along the stems, arranged in various ways, are small cups, from which protrude the numerous moving tentacles of the little polyps living within them. /Hydroids are colonies of associated animals living a communal life./ The multitude of individuals composing the colony are invested with a horny covering, the /perisarc/, which in some genera assumes a tree-like form. Through these stems and branches runs a fleshy tube, a thread of animal substance, which connects in one living whole the zoöids, or individuals of the community. There is division of labor, as in other communities: some of the zoöids obtain the food for the colony, and have tentacles around their open mouths; others have no mouths, but reproduce the species, and at certain stages of development liberate swimming-bells, or small jellyfishes (medusæ). The typical hydroid colony is attached by a kind of creeping stem from which arises a vertical axis, which gives off short lateral, alternate branches bearing zoöids at their ends. There is often more complex branching. The zoöids in certain genera (tubularians) are uncovered; in others (sertularians) they are incased in a glassy, cup-like, horny sheath.