In external form and appearance the Hydrozoa exhibit such striking differences that there would seem at first sight to be little in common between the more divergent members of the group. Nevertheless there is no other class in the animal kingdom with better marked characteristics, or with more uniform morphological peculiarities underlying the utmost diversity of superficial characters.
HYDROMEDUSAE, a group of marine animals, recognized as belonging to the Hydrozoa by the following characters.
Sub-class Hydromedusae (Hydrozoa Craspedota).
Cylindrical or tapering from The numbers in square brackets  refer to the bibliography at the end of this article; but when the number is preceded by the word Hydrozoa, it refers to the bibliography at the end of the articl Hydrozoa.
The budding of this medusa has been worked out in detail by Chun (Hydrozoa, ), to whom the reader must be referred for the interesting laws of budding regulating the sequence and order of formation of the buds.
For further information see C. Chun (Hydrozoa ,Pl.
See C. Viguier  and Delage and Herouard (Hydrozoa ).
Schaudinn  and Delage and Herouard (Hydrozoa ).
Delage and Herouard (Hydrozoa ) were the first to make an heroic attempt to unite the two classifications into one, to which Hickson (Hydrozoa ) has made some additions and slight modifications.
See C. Chun (Hydrozoa).
- This order has been constituted for a peculiar group of palaeozoic fossils, which have been interpreted as the remains of the skeletons of Hydrozoa of an extinct type.
Since no graptolites are known living, or, indeed, since palaeozoic times, the interpretation of their structure and affinities must of necessity be extremely conjectural, and it is by no means certain that they are Hydrozoa at all.
It can only be said that their organization, so far as the state of their preservation permits it to be ascertained, offers closer analogies with the Hydrozoa, especially the Calyptoblastea, than with any other existing group of the animal kingdom.
See the treatise of Delage and Herouard (Hydrozoa, ), and the article Graptolites.
- p elagic floating Hydrozoa with great differentiation of parts, each performing a special function; generally regarded as colonies showing differentiation of individuals in correspondence with a physiological division of labour.
Herouard (Hydrozoa ) and more recently by R.
Chun (Hydrozoa ) maintains the older views of Leuckart and Claus, according to which the cormus is to be compared to a floating hydroid colony.
- In addition to the works cited below, see the general works cited in the article Hydrozoa, in some of which very full bibliographies will be found.
In 18J5, however, Owen included under Lamarck's term Radiaria the Echinodermata, Anthozoa, Acalepha and Hydrozoa, while Agassiz also clung to the term Radiata as including Echinodermata, Acalepha and Polypi, regarding their separation into Coelenterata and Echinodermata as "an exaggeration of their anatomical differences" (Essay on Classification, London, 1859).
The Heteropoda belong to the " pelagic fauna " occurring near the surface in the Mediterranean and great oceans in company with the Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and other specially-modified transparent swimming representatives of various groups of the animal kingdom.
Classes: Hydrozoa, Actinozoa.
HYDROZOA, one of the most widely spread and prolific groups of aquatic animals.
The Hydrozoa comprise the hydroids, so abundant on all shores, most of which resemble vegetable organisms to the unassisted eye; the hydrocorallines, which, as their name implies, have a massive stony skeleton and resemble corals; the jelly-fishes so called; and the Siphonophora, of which the species best known by repute is the so-called "Portuguese man-of-war" (Physalia), dreaded by sailors on account of its terrible stinging powers.
All Hydrozoa, in the first place, exhibit the three structural features distinctive of the Coelentera.
To these three characters the Hydrozoa add a fourth which is distinctive of the subdivision of the Coelenterata termed the Cnidaria; that is to say, they always possess peculiar stinging organs known as nettle-cells, or nematocysts (Cnidae), each produced in a cell forming an integral part of the animal's tissues.
The Hydrozoa are thus shown to belong to the group of Coelenterata Cnidaria, and it remains to consider more fully their distinctive features, and in particular those which mark them off from the other main division of the Cnidaria, the Anthozoa, comprising the corals and sea-anemones.
The great diversity, to which reference has already been made, in the form and structure of the Hydrozoa is due to two principal causes.
In the second place, the power of non-sexual reproduction by budding is practically of universal occurrence among the Hydrozoa, and by the buds failing to separate from the parent stock, colonies are produced, more or less complicated in structure and often of great size.
Hence we have a primary subdivision of the colonies of Hydrozoa into those produced by budding of polyps and those produced by budding of medusae.
The morphology of the Hydrozoa reduces itself, therefore, to a consideration of the morphology of the polyp, of the medusa and of the colony.
In the development of the Hydrozoa, and indeed of the Cnidaria generally, the egg usually gives rise to an oval larva which swims about by means of a coating of cilia on the surface of the body.
After the gastrula stage, which is found as a developmental stage in all Enterozoa, the embryo of the Hydrozoa proceeds to develop characters which are peculiar to the Coelen- a terata only.
It is convenient to distinguish two types of polyp by the names hydro polyp and anthopolyp, characteristic of the Hydrozoa and From Gegenbaur's FIG.
Returning now to the actinula, this form may, as already stated, develop into a medusa, a type of individual found only in the Hydrozoa, as here understood.
Thus the development of the two types of individual seen in the Hydrozoa may be summarized as follows: - Egg Free Blastula "Planula" Parenchymula Stage I Gastrula Actinula 1 Polyp Medusa This development, though probably representing the primitive sequence of events, is never actually found in its full extent, but is always abbreviated by omission or elimination of one or more of the stages.
The Hydrozoa may be defined, therefore, as Cnidaria in which two types of individual, the polyp and the medusa, may be present, each type developed along divergent lines from the primitive actinula form.
GENERAL WORKS ON HYDROZOA.-C. Chun, "Coelenterata (Hohlthiere)," Bronn's Klassen and Ordnungen des Thier-Reichs ii.
The medusa occurs as one type of individual in the class Hydrozoa, the other type being the polyp. In a typical medusa we can distinguish the following parts.
(For other figures of medusae see Hydrozoa.) of the mouth.
As in all Hydrozoa (q.v.) the body wall is composed of two cell-layers, the ectoderm and endoderm.
(For figures see Hydrozoa.) The actinula is polyp-like, with a sack-like or rounded body; a crown of tentacles surrounds a wide peristome, in the centre of which is the mouth, usually raised on a conical process termed the hypostome.
Neomenia and its allies are marine animals living at depths of 15 to Boo fathoms on soft muddy ground; they are found crawling on corals and hydrozoa, on which they feed.
In the above-given classification, the Scyphomedusae, formerly included with the Hydromedusae as Hydrozoa, are placed nearer the Anthozoa.
SCYPHOMEDUSAE or Acalephae, one of the two subdivisions of the Hydrozoa, the other being the Hydromedusae.
The umbrella has a lobed, indented margin, a character only seen amongst Hydromedusae in the order Narcomedusae, and it is without the characteristic velum of the Hydromedusae; hence the Scyphomedusae are sometimes termed Hydrozoa Acraspeda.
By some authorities the Scyphomedusae have been removed from the Hydrozoa and united with the Anthozoa in a common group termed Scyphozoa.
The fact that the Scyphomedusae have a number of well-marked peculiarities of form and structure is not incompatible with placing them in the Hydrozoa as a distinct sub-class, contrasting sharply in many ways with the Hydromedusae.
See also the general works cited in the article HYDROZOA and the bibliographies given in them.
In the subdivision Anthozoa, comprising the sea-anemones and corals, the individual is always a polyp; in the Hydrozoa, however, the individual may be either a polyp or a medusa.
As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic Af one of the two classes, Hydrozoa and Anthozoa.
At one time they were referred by some to the Polyzoa (Bryozoa), and later, by almost general consent, to the Hydroida (Calyptoblastea) among the Hydrozoa (Hydromedusae).
Such forms he distinguished as Coelentera, and showed that they had no special affinity with echinoderms, polyzoa, &c. He divided the Coelentera into a group Hydrozoa, in which the sexually produced embryos were usually set free from the surface of the body, and a group Actinozoa, in which the embryos are detached from the interior of the body and escape generally by the oral aperture.