c1U + [xi] = F cos [theta], c2V = - F sin [theta], (17)
dx F cos² [theta] F sin² [theta] [xi]
-- = U cos [theta] - V sin [theta] = -------------- + -------------- - ---- cos [theta], (18)
dt c1 c2 c1
d[mu] / F F \ [xi]
----- = U sin [theta] + V cos [theta] = ( -- - -- ) sin [theta] cos [theta] - ---- sin [theta], (19)
dt \ c1 c2 / c1
d²[theta] / F² F² \ F[xi] d[mu]
C3--------- = ( -- - -- ) sin [theta] cos [theta] - ----- sin [theta] = F-----, (20)
dt² \ c1 c2 / c1 dt
_ _
d[theta] / | F² cos² [theta] F² sin² [theta] F[xi] |
C3-------- = Fy = / | - --------------- - --------------- + 2----- cos [theta] + H |; (21)
dt \/ |_ c1 c2 c1 _|
so that cos [theta] and y is an elliptic function of the time.
When [xi] is absent, dx/dt is always positive, and the centre of the
body cannot describe loops; but with [xi], the influence may be great
enough to make dx/dt change sign, and so loops occur, as shown in A.
B. Basset's _Hydrodynamics_, i. 192, resembling the trochoidal curves,
which can be looped, investigated in § 29 for the motion of a cylinder
under gravity, when surrounded by a vortex.
The branch of hydrodynamics which discusses wave motion in a liquid or
gas is given now in the articles SOUND and WAVE; while the influence
of viscosity is considered under HYDRAULICS.
REFERENCES.--For the history and references to the original memoirs
see _Report to the British Association_, by G. G. Stokes (1846), and
W. M. Hicks (1882). See also the _Fortschritte der Mathematik_, and A.
E. H. Love, "Hydrodynamik" in the _Encyklöpadie der mathematischen
Wissenschaften_ (1901). (A. G. G.)
HYDROMEDUSAE, a group of marine animals, recognized as belonging to the
Hydrozoa (q.v.) by the following characters. (1) The polyp (hydropolyp)
is of simple structure, typically much longer than broad, without
ectodermal oesophagus or mesenteries, such as are seen in the anthopolyp
(see article ANTHOZOA); the mouth is usually raised above the peristome
on a short conical elevation or hypostome; the ectoderm is without cilia.
(2) With very few exceptions, the polyp is not the only type of
individual that occurs, but alternates in the life-cycle of a given
species, with a distinct type, the medusa (q.v.), while in other cases
the polyp-stage may be absent altogether, so that only medusa-individuals
occur in the life-cycle.
The Hydromedusae represent, therefore, a sub-class of the Hydrozoa. The
only other sub-class is the Scyphomedusae (q.v.). The Hydromedusae
contrast with the Scyphomedusae in the following points. (1) The polyp,
when present, is without the strongly developed longitudinal retractor
muscles, forming ridges (_taeniolae_) projecting into the digestive
cavity, seen in the scyphistoma or scyphopolyp. (2) The medusa, when
present, has a velum and is hence said to be _craspedote_; the nervous
system forms two continuous rings running above and below the velum; the
margin of the umbrella is not lobed (except in Narcomedusae) but entire;
there are characteristic differences in the sense-organs (see below, and
SCYPHOMEDUSAE); and gastral filaments (phacellae), subgenital pits, &c.,
are absent. (3) The gonads, whether formed in the polyp or the medusa,
are developed in the ectoderm.
The Hydromedusae form a widespread, dominant and highly differentiated
group of animals, typically marine, and found in all seas and in all
zones of marine life. Fresh-water forms, however, are also known, very
few as regards species or genera, but often extremely abundant as
individuals. In the British fresh-water fauna only two genera, _Hydra_
and _Cordylophora_, are found; in America occurs an additional genus,
_Microhydra_. The paucity of fresh-water forms contrasts sharply, with
the great abundance of marine genera common in all seas and on every
shore. The species of _Hydra_, however, are extremely common and
familiar inhabitants of ponds and ditches.
In fresh-water Hydromedusae the life-cycle is usually secondarily
simplified, but in marine forms the life-cycle may be extremely
complicated, and a given species often passes in the course of its
history through widely different forms adapted to different habitats and
modes of life. Apart from larval or embryonic forms there are found
typically two types of person, as already stated, the polyp and the
medusa, each of which may vary independently of the other, since their
environment and life-conditions are usually quite different. Hence both
polyp and medusa present characters for classification, and a given
species, genus or other taxonomic category may be defined by
polyp-characters or medusa-characters or by both combined. If our
knowledge of the life-histories of these organisms were perfect, their
polymorphism would present no difficulties to classification; but
unfortunately this is far from being the case. In the majority of cases
we do not know the polyp corresponding to a given medusa, or the medusa
that arises from a given polyp.[1] Even when a medusa is seen to be
budded, from a polyp under observation in an aquarium, the difficulty is
not always solved, since the freshly-liberated, immature medusa may
differ greatly from the full-grown, sexually-mature medusa after several
months of life on the high seas (see figs. 11, B, C, and 59, a, b, c).
To establish the exact relationship it is necessary not only to breed
but to rear the medusa, which cannot always be done in confinement. The
alternative is to fish all stages of the medusa in its growth in the
open sea, a slow and laborious method in which the chance of error is
very great, unless the series of stages is very complete.
At present, therefore, classifications of the Hydromedusae have a more
or less tentative character, and are liable to revision with increased
knowledge of the life-histories of these organisms. Many groups bear at
present two names, the one representing the group as defined by
polyp-characters, the other as defined by medusa-characters. It is not
even possible in all cases to be certain that the polyp-group
corresponds exactly to the medusa-group, especially in minor systematic
categories, such as families.
The following is the main outline of the classification that is Adopted
in the present article. Groups founded on polyp-characters are printed
in ordinary type, those founded on medusa-characters in italics. For
definitions of the groups see below.
Sub-class Hydromedusae (_Hydrozoa Craspedota_).
Order I. Eleutheroblastea.
" II. Hydroidea (_Leptolinae_).
Sub-order 1. Gymnoblastea (_Anthomedusae_).
" 2. Calyptoblastea (_Leptomedusae_).
Order III. Hydrocorallinae.
" IV. Graptolitoidea.
" V. Trachylinae.
Sub-order 1. _Trachomedusae_.
" 2. _Narcomedusae_.
Order VI. Siphonophora.
Sub-order 1. Chondrophorida.
" 2. Calycophorida.
" 3. Physophorida.
" 4. Cystophorida.
_Organization and Morphology of the Hydromedusae._
As already stated, there occur in the Hydromedusae two distinct types of
person, the polyp and the medusa; and either of them is capable of
non-sexual reproduction by budding, a process which may lead to the
formation of colonies, composed of more or fewer individuals combined
and connected together. The morphology of the group thus falls naturally
into four sections--(1) the hydropolyp, (2) the polyp-colony, (3) the
hydromedusa, (4) the medusa-colonies. Since, however, medusa-colonies
occur only in one group, the Siphonophora, and divergent views are held
with regard to the morphological interpretation of the members of a
siphonophore, only the first three of the above subdivisions of
hydromedusa morphology will be dealt with here in a general way, and the
morphology of the Siphonophora will be considered under the heading of
the group itself.
[Illustration: FIG. 1.--Diagram of a typical Hydropolyp.
a, Hydranth;
b, Hydrocaulus;
c, Hydrorhiza;
t, Tentacle;
ps, Perisarc, forming in the region of the hydranth a cup or
hydrotheca (h, t),--which, however, is only found in polyps of the
order Calyptoblastea.]
1. _The Hydropolyp_ (fig. 1)--The general characters of this organism
are described above and in the articles HYDROZOA and POLYP. It is
rarely free, but usually fixed and incapable of locomotion. The foot
by which it is attached often sends out root-like processes--the
_hydrorhiza_ (c). The column (b) is generally long, slender and
stalk-like (_hydrocaulus_). Just below the crown of tentacles,
however, the body widens out to form a "head," termed, the _hydranth_
(a), containing a stomach-like dilatation of the digestive cavity. On
the upper face of the hydranth the crown of tentacles (t) surrounds
the peristome, from which rises the conical hypostome, bearing the
mouth at its extremity. The general ectoderm covering the surface of
the body has entirely lost the cilia present in the earlier larval
stages (planula), and may be naked, or clothed in a cuticle or
exoskeleton, the perisarc (ps), which in its simplest condition is a
chitinous membrane secreted by the ectoderm. The perisarc when present
invests the hydrorhiza and hydrocaulus; it may stop short below the
hydranth, or it may extend farther. In general there are two types of
exoskeleton, characteristic of the two principal divisions of the
Hydroidea. In the Gymnoblastea the perisarc either stops below the
hydranth, or, if continued on to it, forms a closely-fitting
investment extending as a thin cuticle as far as the bases of the
tentacles (e.g. _Bimeria_, see G. J. Allman [1],[2] pl. xii. figs, 1
and 3). In the Calyptoblastea the perisarc is always continued above
the hydrocaulus, and forms a cup, the hydrangium or hydrotheca (h, t),
standing off from the body, into which the hydranth can be retracted
for shelter and protection.
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 2.--_Stauridium productum_, portion of the colony magnified; p,
polyp; rh, hydrorhiza.]
[Illustration: FIG. 3.--Diagram of _Corymorpha_. A, A hydriform person
giving rise to medusiform persons by budding from the margin of the
disk; B, free swimming medusa (_Steenstrupia_ of Forbes) detached from
the same, with manubrial genitalia, (_Anthomedusae_) and only one
tentacle. (After Allman).]
The architecture of the hydropolyp, simple though it be, furnishes a
long series of variations affecting each part of the body. The
greatest variation, however, is seen in the tentacles. As regards
number, we find in the aberrant forms _Protohydra_ and _Microhydra_
tentacles entirely absent. In the curious hydroid _Monobrachium_ a
single tentacle is present, and the same is the case in _Clathrozoon_;
in _Amphibrachium_ and in _Lar_ (fig. 11, A) the polyp bears two
tentacles only. The reduction of the tentacles in all these forms may
be correlated with their mode of life, and especially with living in a
constant current of water, which brings food-particles always from one
direction and renders a complete whorl or circle of tentacles
unnecessary. Thus _Microhydra_ lives amongst Bryozoa, and appears to
utilize the currents produced by these animals. _Protohydra_ occurs in
oyster-banks and _Monobrachium_ also grows on the shells of bivalves,
and both these hydroids probably fish in the currents produced by the
lamellibranchs. _Amphibrachium_ grows in the tissues of a sponge,
_Euplectella_, and protrudes its hydranth into the canal-system of the
sponge; and _Lar_ grows on the tubes of the worm _Sabella_. With the
exception of these forms, reduced for the most part in correlation
with a semi-parasitic mode of life, the tentacles are usually
numerous. It is rare to find in the polyp a regular, symmetrical
disposition of the tentacles as in the medusa. The primitive number of
four in a whorl is seen, however, in _Stauridium_ (fig. 2) and
_Cladonema_ (Allman [1], pl. xvii.), and in _Clavatella_ each whorl
consists regularly of eight (Allman, _loc. cit._ pl. xviii.). As a
rule, however, the number in a whorl is irregular. The tentacles may
form a single whorl, or more than one; thus in _Corymorpha_ (fig. 3)
and _Tubularia_ (fig. 4) there are two circlets; in _Stauridium_ (fig.
2) several; in _Coryne_ and _Cordylophora_ the tentacles are scattered
irregularly over the elongated hydranth.
As regards form, the tentacles show a number of types, of which the
most important are (1) filiform, _i.e._ cylindrical or tapering from
base to extremity, as in _Clava_ (fig. 5); (2) capitate, i.e. knobbed
at the extremity, as in _Coryne_ (see Allman, loc. cit. pl. iv.); (3)
branched, a rare form in the polyp, but seen in _Cladocoryne_ (see
Allman, loc. cit. p. 380, fig. 82). Sometimes more than one type of
form is found in the same polyp; in _Pennaria_ and _Stauridium_ (fig.
2) the upper whorls are capitate, the lower filiform. Finally, as
regards structure, the tentacles may retain their primitive hollow
nature, or become solid by obliteration of the axial cavity.
The hypostome of the hydropolyp may be small, or, on the other hand,
as in _Eudendrium_ (Allman, loc. cit. pls. xiii., xiv.), large and
trumpet-shaped. In the curious polyp _Myriothela_ the body of the
polyp is differentiated into nutritive and reproductive portions.
[Illustration: FIG. 4.--Diagram of _Tubularia indivisa_. A single
hydriform person a bearing a stalk carrying numerous degenerate
medusiform persons or sporosacs b. (After Allman.)]
_Histology._--The ectoderm of the hydropolyp is chiefly sensory,
contractile and protective in function. It may also be glandular in
places. It consists of two regions, an external epithelial layer and a
more internal sub-epithelial layer.
The epithelial layer consists of (1) so-called "indifferent" cells
secreting the perisarc or cuticle and modified to form glandular cells
in places; for example, the adhesive cells in the foot. (2) Sensory
cells, which may be fairly numerous in places, especially on the
tentacles, but which occur always scattered and isolated, never
aggregated to form sense-organs as in the medusa. (3) Contractile or
myo-epithelial cells, with the cell prolonged at the base into a
contractile muscle-fibre (fig. 6, B). In the hydropolyp the ectodermal
muscle-fibres are always directed longitudinally. Belonging primarily
to the epithelial layer, the muscular cells may become secondarily
sub-epithelial.
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 5.--Colonies of _Clava_. A, _Clava squamata_, magnified. B, _C.
multicornis_, natural size; p, _polyp_; _gon_, gonophores; rh,
hydrorhiza.]
The sub-epithelial layer consists primarily of the so-called
interstitial cells, lodged between the narrowed basal portions of the
epithelial cells. From them are developed two distinct types of
histological elements; the genital cells and the cnidoblasts or
mother-cells of the nematocysts. The sub-epithelial layer thus
primarily constituted may be recruited by immigration from without of
other elements, more especially by nervous (ganglion) cells and
muscle-cells derived from the epithelial layer. In its fullest
development, therefore, the sub-epithelial layer consists of four
classes of cell-elements.
[Illustration: FIG. 6 A.--Portion of the body-wall of _Hydra_, showing
ectoderm cells above, separated by "structureless lamella" from three
flagellate endoderm cells below. The latter are vacuolated, and
contain each a nucleus and several dark granules. In the middle
ectoderm cell are seen a nucleus and three nematocysts, with trigger
hairs projecting beyond the cuticle. A large nematocyst, with everted
thread, is seen in the right-hand ectodermal cell. (After F. E.
Schulze.)]
The genital cells are simple wandering cells (archaeocytes), at first
minute and without any specially distinctive features, until they
begin to develop into germ-cells. According to Wulfert [60] the
primitive germ-cells of _Gonothyraea_ can be distinguished soon after
the fixation of the planula, appearing amongst the interstitial cells
of the ectoderm. The germ-cells are capable of extensive migrations,
not only in the body of the same polyp, but also from parent to bud
through many non-sexual generations of polyps in a colony (A. Weismann
[58]).
[Illustration: FIG. 6 B.--Epidermo-muscular cells of _Hydra_. m,
muscular-fibre processes. (After Kleinenberg, from Gegenbaur.)]
The cnidoblasts are the mother-cells of the nematocysts, each cell
producing one nematocyst in its interior. The complete nematocyst
(fig. 7) is a spherical or oval capsule containing a hollow thread,
usually barbed, coiled in its interior. The capsule has a double wall,
an outer one (o.c.), tough and rigid in nature, and an inner one
(i.c.) of more flexible consistence. The outer wall of the capsule is
incomplete at one pole, leaving an aperture through which the thread
is discharged. The inner membrane is continuous with the wall of the
hollow thread at a spot immediately below the aperture in the outer
wall, so that the thread itself (f) is simply a hollow prolongation of
the wall of the inner capsule inverted and pushed into its cavity. The
entire nematocyst is enclosed in the cnidoblast which formed it. When
the nematocyst is completely developed, the cnidoblast passes outwards
so as to occupy a superficial position in the ectoderm, and a delicate
protoplasmic process of sensory nature, termed the _cnidocil_ (cn)
projects from the cnidoblast like a fine hair or cilium. Many points
in the development and mechanism of the nematocyst are disputed, but
it is tolerably certain (1) that the cnidocil is of sensory nature,
and that stimulation, by contact with prey or in other ways, causes a
reflex discharge of the nematocyst; (2) that the discharge is an
explosive change whereby the in-turned thread is suddenly everted and
turned inside out, being thus shot through the opening in the outer
wall of the capsule, and forced violently into the tissues of the
prey, or, it may be, of an enemy; (3) that the thread inflicts not
merely a mechanical wound, but instils an irritant poison, numbing and
paralysing in its action. The points most in dispute are, first, how
the explosive discharge is brought about, whether by pressure exerted
external to the capsule (i.e. by contraction of the cnidoblast) or by
internal pressure. N. Iwanzov [27] has brought forward strong grounds
for the latter view, pointing out that the cnidoblast has no
contractile mechanism and that measurements show discharged capsules
to be on the average slightly larger than undischarged ones. He
believes that the capsule contains a substance which swells very
rapidly when brought into contact with water, and that in the
undischarged condition the capsule has its opening closed by a plug of
protoplasm (x, fig. 7) which prevents access of water to the contents;
when the cnidocil is stimulated it sets in action a mechanism or
perhaps a series of chemical changes by which the plug is dissolved or
removed; as a result water penetrates into the capsule and causes its
contents to swell, with the result that the thread is everted
violently. A second point of dispute concerns the spot at which the
poison is lodged. Iwanzov believes it to be contained within the
thread itself before discharge, and to be introduced into the tissues
of the prey by the eversion of the thread. A third point of dispute is
whether the nematocysts are formed _in situ_, or whether the
cnidoblasts migrate with them to the region where they are most
needed; the fact that in _Hydra_, for example, there are no
interstitial cells in the tentacles, where nematocysts are very
abundant, is certainly in favour of the view that the cnidoblasts
migrate on to the tentacles from the body, and that like the genital
cells the cnidoblasts are wandering cells.
[Illustration: FIG. 7.--Diagrams to show the structure of Nematocysts
and their mode of working. (After Iwanzov.)
a, Undischarged nematocyst.
b, Commencing discharge.
c, Discharge complete.
cn, Cnidocil.
N, Nucleus of cnidoblast.
o.c, Outer capsule.
x, Plug closing the opening of the outer capsule.
i.c., Inner capsule, continuous with the wall of the filament, f.
b, Barbs.]
The muscular tissue consists primarily of processes from the bases of
the epithelial cells, processes which are contractile in nature and
may be distinctly striated. A further stage in evolution is that the
muscle-cells lose their connexion with the epithelium and come to lie
entirely beneath it, forming a sub-epithelial contractile layer,
developed chiefly in the tentacles of the polyp. The evolution of the
ganglion-cells, is probably similar; an epithelial cell develops
processes of nervous nature from the base, which come into connexion
with the bases of the sensory cells, with the muscular cells, and with
the similar processes of other nerve-cells; next the nerve-cell loses
its connexion with the outer epithelium and becomes a sub-epithelial
ganglion-cell which is closely connected with the muscular layer,
conveying stimuli from the sensory cells to the contractile elements.
The ganglion-cells of Hydromedusae are generally very small. In the
polyp the nervous tissue is always in the form of a scattered plexus,
never concentrated to form a definite nervous system as in the medusa.
[Illustration: From Gegenbaur's _Elements of Comparative Anatomy_.
FIG. 8.--Vacuolated Endoderm Cells of cartilaginous consistence from
the axis of the tentacle of a Medusa (_Cunina_).]
The endoderm of the polyp is typically a flagellated epithelium of
large cells (fig. 6), from the bases of which arise contractile
muscular processes lying in the plane of the transverse section of the
body. In different parts of the coelenteron the endoderm may be of
three principal types--(1) digestive endoderm, the primitive type,
with cells of large size and considerably vacuolated, found in the
hydranth; some of these cells may become special glandular cells,
without flagella or contractile processes; (2) circulatory endoderm,
without vacuoles and without basal contractile processes, found in the
hydrorhiza and hydrocaulus; (3) supporting endoderm (fig. 8), seen in
solid tentacles as a row of cubical vacuolated cells, occupying the
axis of the tentacle, greatly resembling notochordal tissue,
particularly that of _Amphioxus_ at a certain stage of development; as
a fourth variety of endodermal cells excretory cells should perhaps be
reckoned, as seen in the pores in the foot of _Hydra_ and elsewhere
(cf. C. Chun, HYDROZOA [1], pp. 314, 315).
The mesogloea in the hydropolyp is a thin elastic layer, in which may
be lodged the muscular fibres and ganglion cells mentioned above, but
which never contains any connective tissue or skeletogenous cells or
any other kind of special mesogloeal corpuscles.
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 9.--Colony of _Hydractinia echinata_, growing on the Shell of a
Whelk. Natural size.]
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 10.--Polyps from a Colony of _Hydractinia_, magnified. dz,
dactylozoid; gz, gastrozoid: b, blastostyle; gon, gonophores; rh,
hydrorhiza.]
2. _The Polyp-colony._--All known hydropolyps possess the power of
reproduction by budding, and the buds produced may become either
polyps or medusae. The buds may all become detached after a time and
give rise to separate and independent individuals, as in the common
_Hydra_, in which only polyp-individuals are produced and sexual
elements are developed upon the polyps themselves; or, on the other
hand, the polyp-individuals produced by budding may remain permanently
in connexion with the parent polyp, in which case sexual elements are
never developed on polyp-individuals but only on medusa-individuals,
and a true colony is formed. 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. The polyps are all non-sexual
individuals whose function is purely nutritive. After a time the
polyps, or certain of them, produce by budding medusa-individuals,
which sooner or later develop sexual elements; in some cases, however,
the founder-polyp remains solitary, that is to say, does not produce
polyp-buds, but only medusa-buds, from the first (_Corymorpha_, fig.
3, _Myriothela_, &c.). In primitive forms the medusa-individuals are
set free before reaching sexual maturity and do not contribute
anything to the colony. 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 medusa-individuals, which never exercise a nutritive function
while attached to the colony. As a general rule polyp-buds are
produced from the hydrorhiza and hydrocaulus, while medusa-buds are
formed on the hydranth. In some cases, however, medusa-buds are formed
on the hydrorhiza, as in Hydrocorallines.
In such a colony of connected individuals, the exact limits of the
separate "persons" are not always clearly marked out. Hence it is
necessary to distinguish between, first, the "zooids," indicated in
the case of the polyps by the hydranths, each with mouth and
tentacles; and, secondly, the "coenosarc," or common flesh, which
cannot be assigned more to one individual than another, but consists
of a more or less complicated network of tubes, corresponding to the
hydrocaulus and hydrorhiza of the primitive independent
polyp-individual. The coenosarc constitutes a system by which the
digestive cavity of any one polyp is put into communication with that
of any other individual either of the trophosome or gonosome. In this
manner the food absorbed by one individual contributes to the welfare
of the whole colony, and the coenosarc has the function of circulating
and distributing nutriment through the colony.
The hydroid colony shows many variations in form and architecture
which depend simply upon differences in the methods in which polyps
are budded.
[Illustration: After Hincks, Forbes, and Browne. A and B modified from
Hincks; C modified from Forbes's _Brit. Naked-eyed Medusae_.
FIG. 11.--_Lar sabellarum_ and two stages of its Medusa, _Willia
stellata_. A, colony of _Lar_; B and C, young and adult medusae.]
In the first place, buds may be produced only from the hydrorhiza,
which grows out and branches to form a basal _stolon_, typically
net-like, spreading over the substratum to which the founder-polyp
attached itself. From the stolon the daughter-polyps grow up
vertically. The result is a spreading or creeping colony, with the
coenosarc in the form of a root-like horizontal network (fig. 5, B;
11, A). Such a colony may undergo two principal modifications. The
meshes of the basal network may become very small or virtually
obliterated, so that the coenosarc becomes a crust of tubes tending to
fuse together, and covered over by a common perisarc. Encrusting
colonies of this kind are seen in _Clava squamata_ (fig. 5, A) and
_Hydractinia_ (figs. 9, 10), the latter having the perisarc calcified.
A further very important modification is seen when the tubes of the
basal perisarc do not remain spread out in one plane, but grow in all
planes forming a felt-work; the result is a massive colony, such as is
seen in the so-called Hydrocorallines (fig. 60), where the interspaces
between the coenosarcal tubes are filled up with calcareous matter, or
_coenosteum_, replacing the chitinous perisarc. The result is a stony,
solid mass, which contributes to the building up of coral reefs. In
massive colonies of this kind no sharp distinction can be drawn
between hydrorhiza and hydrocaulus in the coenosarc; it is practically
all hydrorhiza. Massive colonies may assume various forms and are
often branching or tree-like. A further peculiarity of this type of
colony is that the entire coenosarcal complex is covered externally by
a common layer of ectoderm; it is not clear how this covering layer is
developed.
[Illustration: FIG. 12.--Colony of _Bougainvillea fruticosa_, natural
size, attached to the underside of a piece of floating timber. (After
Allman.)]
In the second place, the buds may be produced from the hydrocaulus,
growing out laterally from it; the result is an arborescent, tree-like
colony (figs. 12, 13). Budding from the hydrocaulus may be combined
with budding from the hydrorhiza, so that numerous branching colonies
arise from a common basal stolon. In the formation of arborescent
colonies, two sharply distinct types of budding are found, which are
best described in botanical terminology as the monopodial or racemose,
and the sympodial or cymose types respectively; each is characteristic
of one of the two sub-orders of the Hydroidea, the Gymnoblastea and
Calyptoblastea.
In the monopodial method (figs. 12, 14) the founder-polyp is,
theoretically, of unlimited growth in a vertical direction, and as it
grows up it throws out buds right and left alternately, so that the
first bud produced by it is the lowest down, the second bud is above
the first, the third above this again, and so on. Each bud produced by
the founder proceeds to grow and to bud in the same way as the founder
did, producing a side branch of the main stem. Hence, in a colony of
gymnoblastic hydroids, the oldest polyp of each system, that is to
say, of the main stem or of a branch, is the topmost polyp; the
youngest polyp of the system is the one nearest to the topmost polyp;
and the axis of the system is a true axis.
[Illustration: FIG. 13.--Portion of colony of _Bougainvillea
fruticosa_ (_Anthomedusae-Gymnoblastea_) more magnified. (From
Lubbock, after Allman.)]
[Illustration: FIG. 14.--Diagrams of the monopodial method of budding,
shown in five stages (1-5). F, the founder-polyp; 1, 2, 3, 4, the
succession of polyps budded from the founder-polyp; a´, b´, c´, the
succession of polyps budded from 1; a², b², polyps budded from 2; a³,
polyp budded from 3.]
In the sympodial method of budding, on the other hand, the
founder-polyp is of limited growth, and forms a bud from its side,
which is also of limited growth, and forms a bud in its turn, and so
on (figs. 15, 16). Hence, in a colony of calyptoblastic hydroids, the
oldest polyp of a system is the lowest; the youngest polyp is the
topmost one; and the axis of the system is a false axis composed of
portions of each of the consecutive polyps. In this method of budding
there are two types. In one, the biserial type (fig. 15), the polyps
produce buds right and left alternately, so that the hydranths are
arranged in a zigzag fashion, forming a "scorpioid cyme," as in
_Obelia_ and _Sertularia_. In the other, the uniserial type (fig. 16),
the buds are formed always on the same side, forming a "helicoid
cyme," as in _Hydrallmania_, according to H. Driesch, in which,
however, the primitively uniserial arrangement becomes masked later by
secondary torsions of the hydranths.
[Illustration: FIG. 15.--Diagram of sympodial budding, biserial type,
shown in five stages (1-5). F, founder-polyp; 1, 2, 3, 4, 5, 6,
succession of polyps budded from the founder; a, b, c, second series
of polyps budded from the founder; a³, b³, series budded from 3.]
[Illustration: FIG. 16.--Diagram of sympodial budding, uniserial type,
shown in four stages (1-4). F, founder-polyp; 1, 2, 3, succession of
polyps budded from the founder.]
In a colony formed by sympodial budding, a polyp always produces first
a bud, which contributes to the system to which it belongs, i.e.
continues the stem or branch of which its parent forms a part. The
polyp may then form a second bud, which becomes the starting point of
a new system, the beginning, that is, of a new branch; and even a
third bud, starting yet another system, may be produced from the same
polyp. Hence the colonies of Calyptoblastea may be complexly branched,
and the budding may be biserial throughout, uniserial throughout, or
partly one, partly the other. Thus in _Plumularidae_ (figs. 17, 18)
there is formed a main stem by biserial budding; each polyp on the
main stem forms a second bud, which usually forms a side branch or
_pinnule_ by uniserial budding. In this way are formed the familiar
feathery colonies of _Plumularia_, in which the pinnules are all in
one plane, while in the allied _Antennularia_ the pinnules are
arranged in whorls round the main biserial stem. The pinnules never
branch again, since in the uniserial mode of budding a polyp never
forms a second polyp-bud. On the other hand, a polyp on the main stem
may form a second bud which, instead of forming a pinnule by uniserial
budding, produces by biserial budding a branch, from which pinnules
arise as from the main stem (fig. 18--3, 6). Or a polyp on the main
stem, after having budded a second time to form a pinnule, may give
rise to a third bud, which starts a new biserial system, from which
uniserial pinnules arise as from the main stem--type of _Aglaophenia_
(fig. 19). The laws of budding in hydroids have been worked out in an
interesting manner by H. Driesch [13], to whose memoirs the reader
must be referred for further details.
[Illustration: FIG. 17.--Diagram of sympodial budding, simple
unbranched _Plumularia_-type. F, founder; 1-8, main axis formed by
biserial budding from founder; a-e, pinnule formed by uniserial
budding from founder; a¹-d¹, branch formed by similar budding from 1;
a²-d² from 2, and so forth.]
_Individualization of Polyp-Colonies._--As in other cases where animal
colonies are formed by organic union of separate individuals, there is
ever a tendency for the polyp-colony as a whole to act as a single
individual, and for the members to become subordinated to the needs of
the colony and to undergo specialization for particular functions,
with the result that they simulate organs and their individuality
becomes masked to a greater or less degree. Perhaps the earliest of
such specializations is connected with the reproductive function.
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. 10, b). At first not differing in any way from
other polyps (fig. 5), the blastostyles gradually lose their nutritive
function and the organs connected with it; the mouth and tentacles
disappear, and the blastostyle obtains the nutriment necessary for its
activity by way of the coenosarc. In the Calyptoblastea, where the
polyps are protected by special capsules of the perisarc, the
_gonothecae_ enclosing the blastostyles differ from the hydrothecae
protecting the hydranths (fig. 54).
[Illustration: FIG. 18.--Diagram showing method of branching in the
_Plumularia_-type; compare with fig. 17. Polyps 3 and 6, instead of
producing uniserial pinnules, have produced biserial branches (3¹, 3²,
3³, 3^4; 6¹-6³), which give off uniserial branches in their turn.]
In other colonies the two functions of the nutritive polyp, namely,
capture and digestion of food, may be shared between different polyps
(fig. 10). One class of polyps, the _dactylozoids_ (dz), lose their
mouth and stomach, and become elongated and tentacle-like, showing
great activity of movement. Another class, the _gastrozoids_ (gz),
have the tentacles reduced or absent, but have the mouth and stomach
enlarged. The dactylozoids capture food, and pass it on to the
gastrozoids, which swallow and digest it.
[Illustration: FIG. 19.--Diagram showing method of branching in the
_Aglaophenia_-type. Polyp 7 has produced as its first bud, 8; as its
second bud, a^7, which starts a uniserial pinnule; and as a third bud
I^7, which starts a biserial branch (II^7-VI^7) that repeats the
structure of the main stem and gives off pinnules. The main stem is
indicated by-·-·-·, the new stem by ······.]
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. Again, the spines arising from the basal crust
of _Podocoryne_ have been interpreted by some authors as reduced
polyps.
3. _The Medusa._--In the Hydromedusae the medusa-individual occurs, as
already stated, in one of two conditions, either as an independent
organism leading a true life in the open seas, or as a subordinate
individuality in the hydroid colony, from which it is never set free;
it then becomes a mere reproductive appendage or _gonophore_, losing
successively its organs of sense, locomotion and nutrition, until its
medusoid nature and organization become scarcely recognizable. Hence
it is convenient to consider the morphology of the medusa from these
two aspects.
(a) _The Medusa as an Independent Organism._--The general structure
and characteristics of the medusa are described elsewhere (see
articles HYDROZOA and MEDUSA), and it is only necessary here to deal
with the peculiarities of the Hydromedusa.
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 20.--_Cladonema radiatum_, the medusa walking on the basal
branches of its tentacles (t), which are turned up over the body.]
[Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of
the Council of the Ray Society.
FIG. 21.--_Clavatella prolifera_, ambulatory medusa. t, tentacles; oc,
ocelli.]
As regards habit of life the vast majority of Hydromedusae are pelagic
organisms, floating on the surface of the open sea, propelling
themselves feebly by the pumping movements of the umbrella produced by
contraction of the sub-umbral musculature, and capturing their prey
with their tentacles. The genera _Cladonema_ (fig. 20) and
_Clavatella_ (fig. 21), however, are ambulatory, creeping forms,
living in rock-pools and walking, as it were, on the tips of the
proximal branches of each of the tentacles, while the remaining
branches serve for capture of food. _Cladonema_ still has the typical
medusan structure, and is able to swim about, but in _Clavatella_ the
umbrella is so much reduced, that swimming is no longer possible. The
remarkable medusa _Mnestra parasites_ is ecto-parasitic throughout
life on the pelagic mollusc _Phyllirrhoe_, attached to it by the
sub-umbral surface, and its tentacles have become rudimentary or
absent. It is interesting to note that _Mnestra_ has been shown by J.
W. Fewkes [15] and R. T. Günther [19] to belong to the same family
(_Cladonemidae_) as _Cladonema_ and _Clavatella_, and it is reasonable
to suppose that the non-parasitic ancestor of _Mnestra_ was, like the
other two genera, an ambulatory medusa which acquired louse-like
habits. In some species of the genus _Cunina_ (Narcomedusae) the
youngest individuals (actinulae) are parasitic on other medusae (see
below), but in later life the parasitic habit is abandoned. No other
instances are known of sessile habit in Hydromedusae.
[Illustration: After E. T. Browne, from _Proc. Zool. Soc. of London_.
FIG. 22.--_Corymorpha nutans_, adult female Medusa. Magnified 10
diameters.]
The external form of the Hydromedusae varies from that of a deep bell
or thimble, characteristic of the Anthomedusae, to the shallow
saucer-like form characteristic of the Leptomedusae. It is usual for
the umbrella to have an even, circular, uninterrupted margin; but in
the order Narcomedusae secondary down-growths between the tentacles
produce a lobed, indented margin to the umbrella. The marginal
tentacles are rarely absent in non-parasitic forms, and are typically
four in number, corresponding to the four perradii marked by the
radial canals. Interradial tentacles may be also developed, so that
the total number present may be increased to eight or to an
indefinitely large number. In _Willia_, _Geryonia_, &c., however, the
tentacles and radial canals are on the plan of six instead of four
(figs. 11 and 26). On the other hand, in some cases the tentacles are
less in number than the perradii; in _Corymorpha_ (figs. 3 and 22)
there is but a single tentacle, while two are found in _Amphinema_ and
_Gemmaria_ (Anthomedusae), and in _Solmundella bitentaculata_ (fig.
67) and _Aeginopsis hensenii_ (fig. 23) (Narcomedusae). The tentacles
also vary considerably in other ways than in number: first, in form,
being usually simple, with a basal bulb, but in _Cladonemidae_ they
are branched, often in complicated fashion; secondly, in grouping,
being usually given off singly, and at regular intervals from the
margin of the umbrella, but in _Margelidae_ and in some Trachomedusae
they are given off in tufts or bunches (fig. 24); thirdly, in position
and origin, being usually implanted on the extreme edge of the
umbrella, but in Narcomedusae they become secondarily shifted and are
given off high up on the ex-umbrella (figs. 23 and 25); and, fourthly,
in structure, being hollow or solid, as in the polyp. In some medusae,
for instance, the remarkable deep-sea family _Pectyllidae_, the
tentacles may bear suckers, by which the animal may attach itself
temporarily. It should be mentioned finally that the tentacles are
very contractile and extensible, and may therefore present themselves,
in one and the same individual, as long, drawn-out threads, or in the
form of short corkscrew-like ringlets; they may stream downwards from
the sub-umbrella, or be held out horizontally, or be directed upwards
over the ex-umbrella (fig. 23). Each species of medusa usually has a
characteristic method of carrying its tentacles.
[Illustration: After O. Maas, _Die craspedoten Medusen der Plankton
Expedition_, by permission of Lipsius and Tischer.
FIG. 23.--_Aeginopsis hensenii_, slightly magnified, showing the
manner in which the tentacles are carried in life.]
[Illustration: After O. Maas, _Craspedoten Medusen der
Siboga-Expedition_, by permission of E. S. Brill & Co.