Hydroids are particularly interesting as exemplifying the close resemblance that may exist in outward appearance between animal and vegetable life and as illustrations of communal life and of the alternation of generation. A few examples of different types are given below. ORDER =LEPTOLINIÆ= The members of this order agree in all essential particulars being branched colonies having two principal forms of zoöids, the nutritive and the reproductive. Some genera attain the length of several inches, or even feet; others are very small tufts growing on shells and seaweeds. The cup may completely inclose the zoöid and be close to the stem (sessile), as in sertularians; it may be on the end of a short stalk, as in campanularians; or it may not reach above the base of the zoöid, as in tubularians. The genera are based upon these differences in the perisarc. The hydroids, like all other classes, exist in such great variety that it would be impracticable to describe here the many named species; but to recognize the genera is simple. A long tubular pedicel without a cup is characteristic of the tubularians; the campanularians have an arborescent form and bell-shaped cups on stalks; the sertularians have sessile cups; and the plumularians have a feather-like form, with zoöid-cups on one side only of the branches. The beautiful and varied structure of these "animal plants" is most interesting, and to be fully appreciated they should be seen [pg122] in life and examined with a glass. Some species are confined to deep water, but many are littoral and to be found in tide-pools, in the chinks and crannies of rocks, under stones, and under the hanging /Fucus/. The horny skeletons of large varieties are frequently washed ashore, and in their tangled masses smaller living species often may be found. THE TUBULARIANS This division is characterized by zoöids borne on long, slender stems which are sometimes simple and small, sometimes branching and eight to ten inches long. The zoöid has two rows of tentacles, the central one being sometimes on a kind of proboscis. The reproductive zoöids are in bunches, sometimes below the outer row of tentacles, sometimes between the two rows. The perisarc does not cover the zoöid. In color they are commonly red or yellow. GENUS ^Clava^