In the curious hydroid Monobrachium a single tentacle is present, and the same is the case in Clathrozoon; in Amphibrachium and in Lar (fig.
Thus the typical hydroid colony starts from a " founder " polyp, which in the vast majority of cases is fixed, but which may be floating, as in Nemopsis, Pelagohydra, &c. The founder-polyp usually produces by budding polyp-individuals, and these in their turn produce other buds.
In other cases, however, the medusa-individuals become sexually mature while still attached to the parent polyp, and are then not set free at all, but become appanages of the hydroid colony and undergo degenerative changes leading to reduction and even to complete obliteration of their original medusan structure.
In this way the hydroid colony becomes composed of two portions of different function, the nutritive " trophosome," composed of non-sexual polyps, and the reproductive " gonosome," composed of sexual medusaindividuals, which never exercise a nutritive function while attached to the colony.
The hydroid colony shows many variations in form and architec- ture which depend simply upon differences in the methods in which polyps are budded.
Whereas primitively any polyp in a colony may produce medusa-buds, in many hydroid colonies medusae are budded only by certain polyps termed blastostyles (fig.
Besides the three types of individual above mentioned, there are other appendages of hydroid colonies, of which the individuality is doubtful.
Such are the " guard-polyps " (machopolyps) of Plumularidae, which are often regarded as individuals of the nature of dactylozoids, but from a study of the mode of budding in this hydroid family Driesch concluded that the guard-polyps were not true polyp-individuals, although each is enclosed in a small protecting cup of the perisarc, known as a nematophore.
- In the Hydromedusae the medusa-individual occurs, as already stated, in one of two conditions, either as an independent organism leading a true life c2 a2 in the open seas, or as a subordinate individuality in the hydroid c colony, from which it is never set free; it then becomes a mere reproductive appendage or gono- phore, losing suc FIG.
Moreover, all the medusae budded from a given hydroid colony are either male or female, so that even the non-sexual polyp must be considered to have a latent sex.
The first step towards the formation of a mixed hydroid colony is undoubtedly a hastening of the sexual maturity of the medusaindividual.
In some cases both free medusae and gonophores may be produced from the same hydroid colony.
The hydroid Dicoryne 'is re- ' markable for the possession of gonophores, which are ciliate and become detached and swim away by means of their cilia.
Hydra is, moreover, bisexual, in contrast with what is known of hydroid colonies.
Common genera are the hydroid Bougainvillea (figs.
The typical genus is the well-known hydroid Podocoryne, budding the medusa known as Dysmorphosa; Thamnostylus, Cytaeis, &c., are other medusae with unknown hydroids.
9, io) is a familiar hydroid genus, bearing gonophores.
4), a well-known British hydroid, bears gonophores.
Here belong the deep-sea genera Monocaulus and Branchiocerianthus, including the largest hydroid polyps known, both genera producing sessile gonophores.
Dendroclava, a hydroid, produces the medusa known as Turritopsis.
5), a common British hydroid, produces gonophores; so also does Cordylophora, a form inhabiting fresh or brackish water.
Cladocoryne is another hydroid genus; Codonium and Dipurena (fig.
The remarkable hydroid Lar (fig.
- The genus Monobrachium is a colonyforming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle.
The British genus Gemmaria, however, is budded from a hydroid referable to the family Corynidae.
The body, representing the hydranth of an ordinary hydroid, has the aboral portion modified into a float, from which hangs down a proboscis bearing the mouth.
T his common British hydroid belongs by its characters to the family Bougainvillidae; it produces, however, a medusa of the genusTiara (fig.
As in the Gymnoblastea, the difficulty of uniting the hydroid and medusan systems into one scheme of classification is very great in the present state of our knowledge.
In a great many Leptomedusae the hydroid stage is as yet unknown, and it is by no means certain even that they possess one.
The hydroid genus Lafoea is remarkable for producing gonothecae on the hydrorhiza, each containing a blastostyle which bears a single gonophore; this portion of the colony was formerly regarded as an independent parasitic hydroid, and was named Coppinia.
(After coralline may be regarded as a form of Moseley.) hydroid colony in which the coenosarc forms a felt-work ramifying in all planes, and in which the chitinous perisarc is replaced by a massive calcareous skeleton.
(From Moseley.) hydroid such as Hydractinia to the hydrocoralline Millepora is not great.
Each such projection is regarded as representing a cup or hydrotheca, similar to those borne by a calyptoblastic hydroid, such as Sertularia.
Without a hydroid phase; the medusa develops directly from the actinula larva, which may, however, multiply by budding.
In the same tanks a small hydroid, very similar to Microhydra, has been found, which bears medusa-buds and is probably the stock from which the medusa is budded.
The hydroid phase, if any, is not known.
Gonostyles, appendages which produce by budding medusae or gonophores, like the blastostyles of a hydroid colony.
The gonostyles have been compared to the blastostyles of a hydroid colony, or to the manubrium of a medusa which produces free or sessile medusa-buds.
They show every transition between free medusae and sporosacs, as already described for hydroid colonies.
Huxley, therefore, considered a hydroid colony, for example, as a single individual, and each separate polyp or medusa budded from it as having the value of an organ and not of an individual.
Chun (Hydrozoa ) maintains the older views of Leuckart and Claus, according to which the cormus is to be compared to a floating hydroid colony.
Dendy, " On a Free-swimming Hydroid, Pelagohydra mirabilis," n.
Hincks, A History of British Hydroid Zoophytes (2 vols., London, 1868); 27.
I, End of hydroid of the thalloid Liverwort Blyttia, showing the thick lignified wall penetrated by simple pits.
J, End of hydroid of the Moss Mnium, showing particularly thin oblique end-wall.
N, One side of the end of hydroid (tracheid) of a Pteridophyte (fern), with scalariform pits.
P, Part of spiral hydroid (tracheid) of Phanerogam (Flowering Plant).
The hydroid of a Pteridophyte or of a Phanerogam is characteristically a dead, usually elongated, cell containing air and water, and either thin-walled with lignified (woody) spiral (fig.
Their structure is eminently that of degenerate forms. Many frequent growths of coralline Algae and hydroid polyps, upon the juices of which they feed, and in some cases a species of gall is produced in hydroids by the penetration of the larval Pantopod into the tissues of the polyp.
He showed (1830) that the organisms like Flustra are not hydroid Polyps, but of a more complex structure resembling Molluscs, and he gave them the name Polyzoa.