ORDER I. Eleutheroblastea.--Simple polyps which become sexually mature
and which also reproduce non-sexually, but without any medusoid stage in
the life-cycle.
The sub-order includes the family _Hydridae_, containing the common
fresh-water polyps of the genus _Hydra_. Certain other forms of doubtful
affinities have also been referred provisionally to this section.
_Hydra._--This genus comprises fresh-water polyps of simple structure.
The body bears tentacles, but shows no division into hydrorhiza,
hydrocaulus or hydranth; it is temporarily fixed and has no perisarc.
The polyp is usually hermaphrodite, developing both ovaries and testes
in the same individual. There is no free-swimming planula larva, but
the stage corresponding to it is passed over in an enveloping cyst,
which is secreted round the embryo by its own ectodermal layer,
shortly after the germ-layer formation is complete, i.e. in the
parenchymula-stage. The envelope is double, consisting of an external
chitinous stratified shell, and an internal thin elastic membrane.
Protected by the double envelope, the embryo is set free as a
so-called "egg," and in Europe it passes the winter in this condition.
In the spring the embryo bursts its shell and is set free as a minute
actinula which becomes a _Hydra_.
Many species are known, of which three are common in European waters.
It has been shown by C. F. Jickeli (28) that the species are
distinguishable by the characters of their nematocysts. They also show
characteristic differences in the egg (Brauer [2]). In _Hydra viridis_
the polyp is of a green colour and produces a spherical egg with a
smooth shell which is dropped into the mud. _H. grisea_ is greyish in
tint and produces a spherical egg with a spiky shell, which also is
dropped into the mud. _H. fusca_ (= _H. vulgaris_) is brown in colour,
and produces a bun-shaped egg, spiky on the convex surface, and
attached to a water-weed or some object by its flattened side. Brauer
found a fourth species, similar in appearance to _H. fusca_, but
differing from the three other species in being of separate sexes, and
in producing a spherical egg with a knobby shell, which is attached
like that of _H. fusca_.
The fact already noted that the species of _Hydra_ can be
distinguished by the characters of their nematocysts is a point of
great interest. In each species, two or three kinds of nematocysts
occur, some large, some small, and for specific identification the
nematocysts must be studied collectively in each species. It is very
remarkable that this method of characterizing and diagnozing species
has never been extended to the marine hydroids. It is quite possible
that the characters of the nematocysts might afford data as useful to
the systematist in this group as do the spicules of sponges, for
instance. It would be particularly interesting to ascertain how the
nematocysts of a polyp are related to those possessed by the medusa
budded from it, and it is possible that in this manner obscure
questions of relationship might be cleared up.
_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 [1]. Pl. I.).
[Illustration: FIG. 49.--Diagram showing possible modifications of
persons of a gymnoblastic _Hydromedusa_. (After Allman.)
a, Hydrocaulus (stem).
b, Hydrorhiza (root).
c, Enteric cavity.
d, Endoderm.
e, Ectoderm.
f, Perisarc, (horny case).
g, Hydranth (hydriform person) expanded.
g', Hydranth (hydriform person) contracted.
h, Hypostome, bearing mouth at its extremity.
k, Sporosac springing from the hydrocaulus.
k', Sporosac springing from m, a modified hydriform person
(blastostyle): the genitalia are seen surrounding the spadix or
manubrium.
l, Medusiform person or medusa.
m, Blastostyle.]
_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 [54].
_Tetraplatia volitans_ Viguier is a remarkable floating marine form.
See C. Viguier [56] and Delage and Hérouard (Hydrozoa [2]).
_Haleremita_ Schaudinn. See F. Schaudinn [50] and Delage and Hérouard
(HYDROZOA [2]).
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 [22] 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.
[Illustration: FIG. 50.--_Sarsia (Dipurena) gemnifera._ b, The long
manubrium, bearing medusiform buds; a, mouth.]
[Illustration: FIG. 51.--_Sarsia prolifera._ Ocelli are seen at the base
of the tentacles, and also (as an exception) groups of medusiform buds.]
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_.