Protohydra is a marine genus characterized by the absence of tentacles, by a great similarity to Hydra in histological structure, and by reproduction by transverse fission. It was found originally in an oyster-farm at Ostend. The sexual reproduction is unknown. For further information see C. Chun (Hydrozoa . Pl. I.).
Polypodium hydriforme Ussow is a fresh-water form parasitic on the eggs of the sterlet. A “stolon” of unknown origin produces thirty-two buds, which become as many Polypodia; each has twenty-four tentacles and divides by fission repeated twice into four individuals, each with six tentacles. The daughter-individuals grow, form the full number of twenty-four tentacles and divide again. The polyps are free and walk on their tentacles. See Ussow .
Tetraplatia volitans Viguier is a remarkable floating marine form. See C. Viguier  and Delage and Hérouard (Hydrozoa ).
Haleremita Schaudinn. See F. Schaudinn  and Delage and Hérouard (Hydrozoa ).
In all the above-mentioned genera, with the exception of Hydra, the life-cycle is so imperfectly known that their true position cannot be determined in the present state of our knowledge. They may prove eventually to belong to other orders. Hence only the genus Hydra can be considered as truly representing the order Eleutheroblastea. The phylogenetic position of this genus has been discussed above.
Order II. Hydroidea seu Leptolinae.—Hydromedusae with alternation of generations (metagenesis) in which a non-sexual polyp-generation (trophosome) produces by budding a sexual medusa-generation (gonosome). The polyp may be solitary, but more usually produces polyps by budding and forms a polyp-colony. The polyp usually has the body distinctly divisible into hydranth, hydrocaulus and hydrorhiza, and is usually clothed in a perisarc. The medusae may be set free or may remain attached to the polyp-colony and degenerate into a gonophore. When fully developed the medusa is characterized by the sense organs being composed entirely of ectoderm, developed independently of the tentacles, and innervated from the sub-umbral nerve-ring.
The two kinds of persons present in the typical Hydroidea make the classification of the group extremely difficult, for reasons explained above. Hence the systematic arrangement that follows must be considered purely provisional. A natural classification of the Hydroidea has yet to be put forward. Many genera and families are separated by purely artificial characters, mere shelf-and-bottle groupings devised, for the convenience of the museum curator and the collector. Thus many subdivisions are diagnosed by setting free medusae in one case, or producing gonophores in another, although it is very obvious, as pointed out above, that a genus producing medusae may be far more closely allied to one producing gonophores than to another producing medusae, or vice versa, and that in some cases the production of medusae or gonophores varies with the season or the sex. Moreover, P. Hallez  has recently shown that hydroids hitherto regarded as distinct species are only forms of the same species grown under different conditions.
Sub-Order 1. Hydroidea Gymnoblastea (Anthomedusae).—Trophosome without hydrothecae or gonothecae, with monopodial type of budding. Gonosome with free medusae or gonophores; medusae usually with ocelli, never with otocysts. The gymnoblastic polyp usually has a distinct perisarc investing the hydrorhiza and the hydrocaulus, sometimes also the hydranth as far as the bases of the tentacles (Bimeria); but in such cases the perisarc forms a closely-fitting investment or cuticule on the hydranth, never a hydrotheca standing off from it, as in the next sub-order. The polyps may be solitary, or form colonies, which may be of the spreading or encrusting type, or arborescent, and then always of monopodial growth and budding. In some cases, any polyp of the colony may bud medusae; in other cases, only certain polyps, the blastostyles, have this power. When blastostyles are present, however, they are never enclosed in special gonothecae as in the next sub-order. In this sub-order the characters of the hydranth are very variable, probably owing to the fact that it is exposed and not protected by a hydrotheca, as in Calyptoblastea.
Speaking generally, three principal types of hydranth can be distinguished, each with subordinate varieties of form.
1. Club-shaped hydranths with numerous tentacles, generally scattered irregularly, sometimes with a spiral arrangement, or in whorls (“verticillate”).
- (a) Tentacles filiform; type of Clava (fig. 5), Cordylophora, &c.
- (b) Tentacles capitate, simple; type of Coryne and Syncoryne; Myriothela is an aberrant form with some of the tentacles modified as “claspers” to hold the ova.
- (c) Tentacles capitate, branched, wholly or in part; type of Cladocoryne.
- (d) Tentacles filiform or capitate, tending to be arranged in definite whorls; type of Stauridium (fig. 2), Cladonema and Pennaria.
2. Hydranth more shortened, daisy-like in form, with two whorls of tentacles, oral and aboral.
- (a) Tentacles filiform, simple, radially arranged or scattered irregularly; type of Tubularia (fig. 4), Corymorpha (fig. 3), Nemopsis, Pelagohydra, &c.
- (b) Tentacles with a bilateral arrangement, branched tentacles in addition to simple filiform ones; type of Branchiocerianthus.
3. Hydranth with a single circlet of tentacles.
- (a) With filiform tentacles; the commonest type, seen in Bougainvillea (fig. 13), Eudendrium, &c.
- (b) With capitate tentacles; type of Clavatella.
4. Hydranth with tentacles reduced below four; type of Lar (fig. 11), Monobrachium, &c.