Chapter V: V V (2)

The sub-umbrella invariably shows a velum as an inwardly projecting
ridge or rim at its margin, within the circle of tentacles; hence the
medusae of this sub-class are termed craspedote. The manubrium is
absent altogether in the fresh-water medusa _Limnocnida_, in which the
diameter of the mouth exceeds half that of the umbrella; on the other
hand, the manubrium may attain a great length, owing to the centre of
the sub-umbrella with the stomach being drawn into it, as it were, to
form a long proboscis, as in _Geryonia_. The mouth may be a simple,
circular pore at the extremity of the manubrium, or by folding of the
edges it may become square or shaped like a Maltese cross, with four
corners and four lips. The corners of the mouth may then be drawn out
into lobes or lappets, which may have a branched or fringed outline
(fig. 27), and in _Margelidae_ the subdivisions of the fringe simulate
tentacles (fig. 24).

FIG. 25.--_Aeginura grimaldii._]

The internal anatomy of the Hydromedusae shows numerous variations.
The stomach may be altogether lodged in the manubrium, from which the
radial canals then take origin directly as in _Geryonia_
(Trachomedusae); it may be with or without gastric pouches. The radial
canals may be simple or branched, primarily four, rarely six in
number. The ring-canal is drawn out in Narcomedusae into festoons
corresponding with the lobes of the margin, and may be obliterated
altogether (_Solmaris_). In this order the radial canals are
represented only by wide gastric pouches, and in the family Solmaridae
are suppressed altogether, so that the tentacles and the festoons of
the ring-canal arise directly from the stomach. In _Geryonia_,
centripetal canals, ending blindly, arise from the ring-canal and run
in a radial direction towards the centre of the umbrella (fig. 26).

_Histology of the Hydromedusa._--The histology described above for the
polyp may be taken as the primitive type, from which that of the
medusa differs only in greater elaboration and differentiation of the
cell-elements, which are also more concentrated to form distinct
tissues.

a, Nerve ring,
a´, Radial nerve.
b, Tentaculocyst.
c, Circular canal.
e, Radiating canal,
g´´. Ovary.
h, Peronia or cartilaginous process ascending from the cartilaginous
margin of the disk centripetally in the outer surface of the
jelly-like disk; six of these are perradial, six interradial,
corresponding to the twelve solid larval tentacles, resembling those
of _Cunina_.
k, Dilatation (stomach) of the manubrium.
l, Jelly of the disk.
p, Manubrium.
t, Tentacle (hollow and tertiary, i.e. preceded by six perradial and
six interradial solid larval tentacles).
u, Cartilaginous margin of the disk covered by thread-cells.
v. Velum.]

The ectoderm furnishes the general epithelial covering of the body,
and the muscular tissue, nervous system and sense-organs. The external
epithelium is flat on the ex-umbral surface, more columnar on the
sub-umbral surface, where it forms the muscular tissue of the
sub-umbrella and the velum. The nematocysts of the ectoderm may be
grouped to form batteries on the tentacles, umbrellar margin and oral
lappets. In places the nematocysts may be crowded so thickly as to
form a tough, supporting, "chondral" tissue, resembling cartilage,
chiefly developed at the margin of the umbrella and forming streaks or
bars supporting the tentacles ("Tentakelspangen," _peronia_) or the
tentaculocysts ("Gehörspangen," _otoporpae_).

FIG. 27.--_Stomotoca divisa_, one of the _Tiaridae_ (Anthomedusae).]

The muscular tissue of the Hydromedusae is entirely ectodermal. The
muscle-fibres arise as processes from the bases of the epithelial
cells; such cells may individually become sub-epithelial in position,
as in the polyp; or, in places where muscular tissue is greatly
developed, as in the velum or sub-umbrella, the entire muscular
epithelium may be thrown into folds in order to increase its surface,
so that a deeper sub-epithelial muscular layer becomes separated
completely from a more superficial body-epithelium.

In its arrangement the muscular tissue forms two systems: the one
composed of striated fibres arranged circularly, that is to say,
concentrically round the central axis of the umbrella; the other of
non-striated fibres running longitudinally, that is to say, in a
radial direction from, or (in the manubrium) parallel to, the same
ideal axis. The circular system is developed continuously over the
entire sub-umbral surface, and the velum represents a special local
development of this system, at a region where it is able to act at the
greatest mechanical advantage in producing the contractions of the
umbrella by which the animal progresses. The longitudinal system is
discontinuous, and is subdivided into proximal, medial and distal
portions. The proximal portion forms the retractor muscles of the
manubrium, or proboscis, well developed, for example, in _Geryonia_.
The medial portion forms radiating tracts of fibres, the so-called
"bell-muscles" running underneath, and parallel to, the radial canals;
when greatly developed, as in _Tiaridae_, they form ridges, so-called
mesenteries, projecting into the sub-umbral cavity. The distal
portions form the muscles of the tentacles. In contrast with the
polyp, the longitudinal muscle-system is entirely ectodermal, there
being no endodermal muscles in craspedote medusae.

The nervous system of the medusa consists of sub-epithelial
ganglion-cells, which form, in the first place, a diffuse plexus of
nervous tissue, as in the polyp, but developed chiefly on the
sub-umbral surface; and which are concentrated, in the second place,
to form a definite central nervous system, never found in the polyp.
In Hydromedusae the central nervous system forms two concentric
nerve-rings at the margin of the umbrella, near the base of the velum.
One, the "upper" or ex-umbral nerve-ring, is derived from the ectoderm
on the ex-umbral side of the velum; it is the larger of the two rings,
containing more numerous but smaller ganglion-cells, and innervates
the tentacles. The other, the "lower" or sub-umbral nerve-ring, is
derived from the ectoderm on the sub-umbral side of the velum; it
contains fewer but larger ganglion-cells and innervates the muscles of
the velum (see diagram in article MEDUSAE). The two nerve-rings are
connected by fibres passing from one to the other.

FIG. 29.--_Tiaropsis rosea_ (Ag. and Mayer) showing the eight adradial
Statocysts, each close to an Ocellus. Cf. fig. 30.]

The sensory cells are slender epithelial cells, often with a cilium or
stiff protoplasmic process, and should perhaps be regarded as the only
ectoderm-cells which retain the primitive ciliation of the larval
ectoderm, otherwise lost in all Hydrozoa. The sense-cells form, in the
first place, a diffuse system of scattered sensory cells, as in the
polyp, developed chiefly on the manubrium, the tentacles and the
margin of the umbrella, where they form a sensory ciliated epithelium
covering the nerve-centres; in the second place, the sense-cells are
concentrated to form definite sense-organs, situated always at the
margin of the umbrella, hence often termed "marginal bodies." The
possession of definite sense-organs at once distinguishes the medusa
from the polyp, in which they are never found.

The sense-organs of medusae are of two kinds--first, organs sensitive
to light, usually termed _ocelli_ (fig. 29); secondly, organs commonly
termed _otocysts_, on account of their resemblance to the auditory
vesicles of higher animals, but serving for the sense of balance and
orientation, and therefore given the special name of _statocysts_
(fig. 30). The sense-organs may be _tentaculocysts_, i.e.
modifications of a tentacle, as in Trachylinae, or developed from the
margin of the umbrella, in no connexion with a tentacle (or, if so
connected, not producing any modification in the tentacle), as in
Leptolinae. In Hydromedusae the sense-organs are always exposed at the
umbrellar margin (hence _Gymnophthalmata_), while in Scyphomedusae
they are covered over by flaps of the umbrellar margin (hence
_Steganophthalmata_).

FIG. 30.--Section of a Statocyst and Ocellus of _Tiaropsis diademata_;
cf. fig. 29.

ex, Ex-umbral ectoderm.
sub, Sub-umbral ectoderm.
c.c, Circular canal.
v, Velum.
st.e, Cavity of statocyst.
con, Concrement-cell with otolith.]

The _statocysts_ present in general the structure of either a knob or
a closed vesicle, composed of (1) indifferent supporting epithelium:
(2) sensory, so-called auditory epithelium of slender cells, each
bearing at its free upper end a stiff bristle and running out at its
base into a nerve-fibre; (3) concrement-cells, which produce
intercellular concretions, so-called otoliths. By means of vibrations
or shocks transmitted through the water, or by displacements in the
balance or position of the animal, the otoliths are caused to impinge
against the bristles of the sensory cells, now on one side, now on the
other, causing shocks or stimuli which are transmitted by the basal
nerve-fibre to the central nervous system. Two stages in the
development of the otocyst can be recognized, the first that of an
open pit on a freely-projecting knob, in which the otoliths are
exposed, the second that of a closed vesicle, in which the otoliths
are covered over. Further, two distinct types of otocyst can be
recognized in the Hydromedusae: that of the Leptolinae, in which the
entire organ is ectodermal, concrement-cells and all, and the organ is
not a tentaculocyst; and that of the Trachylinae, in which the organ
is a tentaculocyst, and the concrement-cells are endodermal, derived
from the endoderm of the modified tentacle, while the rest of the
organ is ectodermal.

FIG. 31.--Section of a Statocyst of _Mitrocoma annae_.

sub, Sub-umbral ectoderm.
c.c, Circular canal.
v, Velum.
st.c,. Cavity of statocyst.
con, Concrement-cell with otolith.]

FIG. 32.--Section of a Statocyst of _Phialidium_.

ex, Ex-umbral ectoderm.
sub, Sub-umbral ectoderm.
v, Velum.
st.c, Cavity of statocyst.
con, Concrement-cell with otolith.]

FIG. 33.--Optical Section of a Statocyst of _Octorchis_.

con, Concrement-cell with otolith.
st.c, Cavity of statocyst.]

In the Leptolinae the otocysts are seen in their first stage in
_Mitrocoma annae_ (fig. 31) and _Tiaropsis_ (figs. 29, 30) as an open
pit at the base of the velum, on its sub-umbral side. The pit has its
opening turned towards the sub-umbral cavity, while its base or fundus
forms a bulge, more or less pronounced, on the ex-umbral side of the
velum. At the _fundus_ are placed the concrement-cells with their
conspicuous otoliths (_con_) and the inconspicuous auditory cells,
which are connected with. the sub-umbral nerve-ring. From the open
condition arises the closed condition very simply by closing up of the
aperture of the pit. We then find the typical otocyst of the
Leptomedusae, a vesicle bulging on the ex-umbral side of the velum
(figs. 32, 33). The otocysts are placed on the outer wall of the
vesicle (the fundus of the original pit) or on its sides; their
arrangement and number vary greatly and furnish useful characters for
distinguishing genera. The sense-cells are innervated, as before, from
the sub-umbral nerve-ring. The inner wall of the vesicle (region of
closure) is frequently thickened to form a so-called "sense-cushion,"
apparently a ganglionic offshoot from the sub-umbral nerve-ring. In
many Leptomedusae the otocysts are very small, inconspicuous and
embedded completely in the tissues; hence they may be easily
overlooked in badly-preserved material, and perhaps are present in
many cases where they have been said to have been wanting.

FIG. 34.--Tentaculocyst (statorhabd) of _Cunina solmaris_. n.c,
Nerve-cushion; end, endodermal concrement-cells; con, otolith.]

FIG. 35.--Tentaculocyst of _Cunina lativentris_.

ect, Ectoderm.
n.c, Nerve-cushion.
end, Endodermal concrement-cells.
con, Otolith.]

In the Trachylinae the simplest condition of the otocyst is a freely
projecting club, a so-called _statorhabd_ (figs. 34, 35), representing
a tentacle greatly reduced in size, covered with sensory ectodermal
epithelium (_ect._), and containing an endodermal core (_end._), which
is at first continuous with the endoderm of the ring-canal, but later
becomes separated from it. In the endoderm large concretions are
formed (_con._). Other sensory cells with long cilia cover a sort of
cushion (_n.c._) at the base of the club; the club may be long and the
cushion small, or the cushion large and the club small. The whole
structure is innervated, like the tentacles, from the ex-umbral
nerve-ring. An advance towards the second stage is seen in such a form
as _Rhopalonema_ (fig. 36), where the ectoderm of the cushion rises up
in a double fold to enclose the club in a protective covering forming
a cup or vesicle, at first open distally; finally the opening closes
and the closed vesicle may sink inwards and be found far removed from
the surface, as in _Geryonia_ (fig. 37).

The _ocelli_ are seen in their simplest form as a pigmented patch of
ectoderm, which consists of two kinds of cells--(1) pigment-cells,
which are ordinary indifferent cells of the epithelium containing
pigment-granules, and (2) visual cells, slender sensory epithelial
cells of the usual type, which may develop visual cones or rods at
their free extremity. The ocelli occur usually either on the inner or
outer sides of the tentacles; if on the inner side, the tentacle is
turned upwards and carried over the ex-umbrella, so as to expose the
ocellus to the light; if the ocellus be on the outer side of a
tentacle, two nerves run round the base of the tentacle to it. In
other cases ocelli may occur between tentacles, as in _Tiaropsis_
(fig. 29).

The simple form of ocellus described in the foregoing paragraph may
become folded into a pit or cup, the interior of which becomes filled
with a clear gelatinous secretion forming a sort of vitreous body. The
distal portion of the vitreous body may project from the cavity of the
cup, forming a non-cellular lens as in _Lizzia_ (fig. 28). Beyond this
simple condition the visual organs of the Hydromedusae do not advance,
and are far from reaching the wonderful development of the eyes of
Scyphomedusae (_Charybdaea_).

FIG. 37.--Section of statocyst of _Geryonia_ (_Carmarina hastata_).

st.c, Statocyst containing the minute tentaculocyst.
nr1, Ex-umbral nerve-ring.
nr2, Sub-umbral nerve-ring.
ex, Ex-umbral ectoderm.
sub, Sub-umbral ectoderm.
c.c, Circular canal.
v, Velum.]

Besides the ordinary type of ocellus just described, there is found in
one genus (_Tiaropsis_) a type of ocellus in which the visual elements
are inverted, and have their cones turned away from the light, as in
the human retina (fig. 30). In this case the pigment-cells are
endodermal, forming a cup of pigment in which the visual cones are
embedded. A similar ocellus is formed in _Aurelia_ among the
Scyphomedusae (q.v.).

Other sense organs of Hydromedusae are the so-called _sense-clubs_ or
_cordyli_ found in a few Leptomedusae, especially in those genera in
which otocysts are inconspicuous or absent (fig. 39). Each cordylus is
a tentacle-like structure with an endodermal axis containing an axial
cavity which may be continuous with the ring-canal, or may be
partially occluded. Externally the cordylus is covered, by very
flattened ectoderm, and bears no otoliths or sense-cells, but the base
of the club rests upon the ex-umbral nerve-ring. Brooks regards these
organs as sensory, serving for the sense of balance, and representing
a primitive stage of the tentaculocysts of Trachylinae; Linko, on the
other hand, finding no nerve-elements connected with them, regards
them as digestive (?) in function.

The sense-organs of the two fresh-water medusae _Limnocodium_ and
_Limnocnida_ are peculiar and of rather doubtful nature (see E. T.
Browne [10]).

FIG. 38.--Ocellus of _Lizzia koellikeri_. oc, Pigmented ectodermal
cells; l, lens. (After Hertwig.)

The endoderm of the medusa shows the same general types of structure
as in the polyp, described above. We can distinguish (1) digestive
endoderm, in the stomach, often with special glandular elements; (2)
circulatory endoderm, in the radial and ring-canals; (3) supporting
endoderm in the axes of the tentacles and in the endoderm-lamella; the
latter is primitively a double layer of cells, produced by
concrescence of the ex-umbral and sub-umbral layers of the
coelenteron, but it is usually found as a single layer of flattened
cells (fig. 40); in _Geryonia_, however, it remains double, and the
centripetal canals arise by parting of the two layers; (4) excretory
endoderm, lining pores at the margin of the umbrella, occurring in
certain Leptomedusae as so-called "marginal tubercles," opening, on
the one hand, into the ring-canal and, on the other hand, to the
exterior by "marginal funnels," which debouch into the sub-umbral
cavity above the velum. As has been described above, the endoderm may
also contribute to the sense-organs, but such contributions are always
of an accessory nature, for instance, concrement-cells in the
otocysts, pigment in the ocelli, and never of sensory nature,
sense-cells being in all cases ectodermal.

The reproductive cells may be regarded as belonging primarily to
neither ectoderm nor endoderm, though lodged in the ectoderm in all
Hydromedusae. As described for the polyp, they are wandering cells
capable of extensive migrations before reaching the particular spot at
which they ripen. In the Hydromedusae they usually, if not invariably,
ripen in the ectoderm, but in the neighbourhood of the main sources of
nutriment, that is to say, not far from the stomach. Hence the gonads
are found on the manubrium in Anthomedusae generally; on the base of
the manubrium, or under the gastral pouches, or in both these
situations (_Octorchidae_), or under the radial canals, in
Trachomedusae; under the gastral pouches or radial canals, in
Narcomedusae. When ripe, the germ-cells are dehisced directly to the
exterior.

FIG. 39.--Section of a Cordylus of _Laodice_.

c.c, Circular canal.
v, Velum.
t, Tentacle.
c, Cordylus, composed of flattened ectoderm ec covering a large-celled
endodermal axis en.]

Hydromedusae are of separate sexes, the only known exception being
_Amphogona apsteini_, one of the Trachomedusae (Browne [9]). 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. (In _Hydra_, on the other hand, the individual is usually
hermaphrodite.) The medusa always reproduces itself sexually, and in
some cases non-sexually also. The non-sexual reproduction takes the
form of fission, budding or sporogony, the details of which are
described below. Buds may be produced from the manubrium, radial
canals, ring-canal, or tentacle-bases, or from an aboral stolon
(Narcomedusae). In all cases only medusa-buds are produced, never
polyp-buds.

The mesogloea of the medusa is largely developed and of great
thickness in the umbrella. The sub-epithelial tissues, i.e. the
nervous and muscular cells, are lodged in the mesogloea, but in
Hydromedusae it never contains tissue-cells or mesogloeal corpuscles.

(b) _The Medusae as a Subordinate Individuality._--It has been shown
above that polyps are budded only from polyps and that the medusae may
be budded either from polyps or from medusae. In any case the
daughter-individuals produced from the buds may be imagined as
remaining attached to the parent and forming a colony of individuals
in organic connexion with one another, and thus three possible cases
arise. The first case gives a colony entirely composed of polyps, as
in many Hydroidea. The second case gives a colony partly composed of
polyp-individuals, partly of medusa-individuals, a possibility also
realized in many colonies of Hydroidea. The third case gives a colony
entirely composed of medusa-individuals, a possibility perhaps
realized in the Siphonophora, which will be discussed in dealing with
this group.

el, Endoderm lamella.
m, Muscular processes of the ectoderm-cells in cross section.
d, Ectoderm.
en, Endoderm lining the enteric cavity.
e, Wandering endoderm cells of the gelatinous substance.]

The first step towards the formation of a mixed hydroid colony is
undoubtedly a hastening of the sexual maturity of the
medusa-individual. Normally the medusae are liberated in quite an
immature state; they swim away, feed, grow and become adult mature
individuals. From the bionomical point of view, the medusa is to be
considered as a means of spreading the species, supplementing the
deficiencies of the sessile polyp. It may be, however, that increased
reproductiveness becomes of greater importance to the species than
wide diffusion; such a condition will be brought about if the medusae
mature quickly and are either set free in a mature condition or remain
in the shelter of the polyp-colony, protected from risks of a free
life in the open sea. In this way the medusa sinks from an independent
personality to an organ of the polyp-colony, becoming a so-called
_medusoid gonophore_, or bearer of the reproductive organs, and losing
gradually all organs necessary for an independent existence, namely
those of sense, locomotion and nutrition.

In some cases both free medusae and gonophores may be produced from
the same hydroid colony. This is the case in _Syncoryne mirabilis_
(Allman [1], p. 278) and in _Campanularia volubilis_; in the latter,
free medusae are produced in summer, gonophores in winter (Duplessis
[14]). Again in _Pennaria_, the male medusae are set free in a state
of maturity, and have ocelli; the female medusae remain attached and
have no sense organs.

FIG. 41.--Diagrams of the Structure of the Gonophores of various
Hydromedusae, based on the figures of G. J. Allman and A. Weismann.

A, "Meconidium" of _Gonothyraea_.
B, Type of _Tubularia_.
C, Type of _Garveia_, &c.
D, Type of _Plumularia_, _Agalma_, &c.
E, Type of _Coryne_, _Forskalia_, &c.
F, G, H, Sporosacs.
F, With simple spadix.
G, With spadix prolonged (_Eudendrium_).
H, With spadix branched (_Cordylophora_).
s.c, Sub-umbral cavity.
t, Tentacles.
c.c, Circular canal,
g, Gonads.
sp, Spadix.
e.l, Endoderm-lamella.
ex, Ex-umbral ectoderm.
ect, Ectotheca.]

The gonophores of different hydroids differ greatly in structure from
one another, and form a series showing degeneration of the
medusa-individual, which is gradually stripped, as it were, of its
characteristic features of medusan organization and finally reduced to
the simplest structure. A very early stage in the degeneration is well
exemplified by the so-called "meconidium" of _Gonothyraea_ (fig. 41,
A). Here the medusoid, attached by the centre of its ex-umbral
surface, has lost its velum and sub-umbral muscles, its sense organs
and mouth, though still retaining rudimentary tentacles. The gonads
(g) are produced on the manubrium, which has a hollow endodermal axis,
termed the spadix (sp.), in open communication with the coenosarc of
the polyp-colony and serving for the nutrition of the generative
cells. A very similar condition is seen in _Tubularia_ (fig. 41, B),
where, however, the tentacles have quite disappeared, and the circular
rim formed by the margin of the umbrella has nearly closed over the
manubrium leaving only a small aperture through which the embryos
emerge. The next step is illustrated by the female gonophores of
_Cladocoryne_, where the radial and ring-canals have become
obliterated by coalescence of their walls, so that the entire endoderm
of the umbrella is in the condition of the endoderm-lamella. Next the
opening of the umbrella closes up completely and disappears, so that
the sub-umbral cavity forms a closed space surrounding the manubrium,
on which the gonads are developed; such a condition is seen in the
male gonophore of _Cladocoryne_ and in _Garveia_ (fig. 41, C), where,
however, there is a further complication in the form of an
adventitious envelope or ectotheca (ect.) split off from the gonophore
as a protective covering, and not present in _Cladocoryne_. The
sub-umbral cavity (s.c.) functions as a brood-space for the developing
embryos, which are set free by rupture of the wall. It is evident that
the outer envelope of the gonophore represents the ex-umbral ectoderm
(ex.), and that the inner ectoderm lining the cavity represents the
sub-umbral ectoderm of the free medusa. The next step is the gradual
obliteration of the sub-umbral cavity (s.c.) by disappearance of which
the sub-umbral ectoderm comes into contact with the ectoderm of the
manubrium. Such a type is found in _Plumularia_ and also in _Agalma_
(fig. 41, D); centrally is seen the spadix (sp.), bearing the
generative cells (g), and external to these (1) a layer of ectoderm
representing the epithelium of the manubrium; (2) the layer of
sub-umbral ectoderm; (3) the endoderm-lamella (e.l.); (4) the
ex-umbral ectoderm (ex.); and (5) there may or may not be present also
an ectotheca. Thus the gonads are covered over by at least four layers
of epithelium, and since these are unnecessary, presenting merely
obstacles to the dehiscence of the gonads, they gradually undergo
reduction. The sub-umbral ectoderm and that covering the manubrium
undergo concrescence to form a single layer (fig. 41, E), which
finally disappears altogether, and the endoderm-lamella disappears.
The gonophore is now reduced to its simplest condition, known as the
_sporosac_ (fig. 41, F, G, H), and consists of the spadix bearing the
gonads covered by a single layer of ectoderm (ex.), with or without
the addition of an ectotheca. It cannot be too strongly emphasized,
however, that the sporosac should not be compared simply with the
manubrium of the medusa, as is sometimes done. The endodermal spadix
(sp) of the sporosac represents the endoderm of the manubrium; the
ectodermal lining of the sporosac (ex.) represents the ex-umbral
ectoderm of the medusa; and the intervening layers, together with the
sub-umbral cavity, have disappeared. The spadix, as the organ of
nutrition for the gonads, may be developed in various ways, being
simple (fig. 41, F) or branched (fig. 41, H); in _Eudendrium_ (fig.
41, G) it curls round the single large ovum.

FIG. 42.--Gonophores of _Dicoryne conferta_.

A, A male gonophore still enclosed in its ectotheca.
B and C, Two views of a female gonophore after liberation.
t, Tentacles.
ov, Ova, two carried on each female gonophore.
sp, Testis.]

The hydroid _Dicoryne_ is remarkable for the possession of gonophores,
which are ciliate and become detached and swim away by means of their
cilia. Each such sporosac has two long tentacle-like processes thickly
ciliated.

It has been maintained that the gonads of _Hydra_ represent sporosacs
or gonophores greatly reduced, with the last traces of medusoid
structure completely obliterated. There is, however, no evidence
whatever for this, the gonads of _Hydra_ being purely ectodermal
structures, while all medusoid gonophores have an endodermal portion.
_Hydra_ is, moreover, bisexual, in contrast with what is known of
hydroid colonies.

In some Leptomedusae the gonads are formed on the radial canals and
form protruding masses resembling sporosacs superficially, but not in
structure. Allman, however, regarded this type of gonad as equivalent
to a sporosac, and considered the medusa bearing them as a non-sexual
organism, a "blastocheme" as he termed it, producing by budding
medusoid gonophores. As medusae are known to bud medusae from the
radial canals there is nothing impossible in Allman's theory, but it
cannot be said to have received satisfactory proof.

_Reproduction and Ontogeny of the Hydromedusae._

Nearly every possible method of reproduction occurs amongst the Hydromedusae. In classifying methods of generation it is usual to make use of the sexual or non-sexual nature of the reproduction as a primary difference, but a more scientific classification is afforded by the distinction between tissue-cells (histocytes) and germinal cells, actual or potential (archaeocytes), amongst the constituent cells of the animal body. In this way we may distinguish, first, _vegetative_ reproduction, the result of discontinuous growth of the tissues and cell-layers of the body as a whole, leading to (1) _fission_, (2) _autotomy_, or (3) _vegetative budding_; secondly, _germinal_ reproduction, the result of the reproductive activity of the archaeocytes or germinal tissue. In germinal reproduction the proliferating cells may be _undifferentiated_, so-called primitive germ-cells, or they may be _differentiated_ as sexual cells, male or female, i.e. spermatozoa and ova. If the germ-cells are _undifferentiated_, the offspring may arise from many cells or from a single cell; the first type is (4) _germinal budding_, the second is (5) _sporogony_. If the germ-cells are _differentiated_, the offspring arises by _syngamy_ or sexual union of the ordinary type between an ovum and spermatozoon, so-called fertilization, of the ovum, or by _parthenogenesis_, i.e. development of an ovum without fertilization. The only one of these possible modes of reproduction not known to occur in Hydromedusae is parthenogenesis.

(1) True _fission_ or longitudinal division of an individual into two equal and similar daughter-individuals is not common but occurs in _Gastroblasta_, where it has been described in detail by Arnold Lang [30].

(2) _Autotomy_, sometimes termed transverse fission, is the name given to a process of unequal fission in which a portion of the body separates off with subsequent regeneration. In _Tubularia_ by a process of decapitation the hydranths may separate off and give rise to a separate individual, while the remainder of the body grows a new hydranth. Similarly in _Schizocladium_ portions of the hydrocaulus are cut off to form so-called "spores," which grow into new individuals (see Allman [1]).

FIG. 43.--Direct Budding of _Cunina_.

A, B, C, E, F, In vertical section.
D, Sketch of external view.
st, Stomach.
m, Manubrium.
t. Tentacle.
s.o, Sense organ.
v, Velum.
s.c, Sub-umbral cavity.
n.s, Nervous system.]

(3) _Vegetative budding_ is almost universal in the Hydromedusae. By budding is understood the formation of a new individual from a fresh growth of undifferentiated material. It is convenient to distinguish buds that give rise to polyps from those that form medusae.

(a) _The Polyp._--The buds that form polyps are very simple in mode of
formation. Four stages may be distinguished; the first is a simple
outgrowth of both layers, ectoderm and endoderm, containing a
prolongation of the coelenteric cavity; in the second stage the
tentacles grow out as secondary diverticula from the side of the first
outgrowth; in the third stage the mouth is formed as a perforation of
the two layers; and, lastly, if the bud is to be separated, it becomes
nipped off from the parent polyp and begins a free existence.

(b) _The Medusae._--Two types of budding must be distinguished--the
direct, so-called, palingenetic type, and the _indirect_, so-called
coenogenetic type.

The direct type of budding is rare, but is seen in _Cunina_ and
_Millepora_. In _Cunina_ there arises, first, a simple outgrowth of
both layers, as in a polyp-bud (fig. 43, A); in this the mouth is
formed distally as a perforation (B); next the sides of the tube so
formed bulge out laterally near the attachment to form the umbrella,
while the distal undilated portion of the tube represents the
manubrium (C); the umbrella now grows out into a number of lobes or
lappets, and the tentacles and tentaculocysts grow out, the former in
a notch between two lappets, the latter on the apex of each lappet (D,
E); finally, the velum arises as a growth of the ectoderm alone, the
whole bud shapes itself, so to speak, and the little medusa is
separated off by rupture of the thin stalk connecting it with the
parent (F). The direct method of medusa-budding only differs from the
polyp-bud by its greater complexity of parts and organs.

A, B, C, D, F, Successive stages in vertical section.
E, Transverse section of a stage similar to D.
Gc, Entocodon.
s.c, Cavity of entocodon, forming the future sub-umbral cavity.
st, Stomach.
r.c, Radial canal.
c.c, Circular canal.
e.l, Endoderm lamella.
m, Manubrium.
v, Velum.
t, Tentacle.]

The indirect mode of budding (figs. 44, 45) is the commonest method by
which medusa-buds are formed. It is marked by the formation in the bud
of a characteristic structure termed the _entocodon_ (_Knospenkern_,
_Glockenkern_).

The first stage is a simple hollow outgrowth of both body-layers (fig.
44, A); at the tip of this is formed a thickening of the ectoderm,
arising primitively as a hollow ingrowth (fig. 44, B), but more
usually as a solid mass of ectoderm-cells (fig. 45, A). The ectodermal
ingrowth is the entocodon (Gc.); it bulges into, and pushes down, the
endoderm at the apex of the bud, and if solid it soon acquires a
cavity (fig. 44, C, s.c.). The cavity of the entocodon increases
continually in size, while the endoderm pushes up at the sides of it
to form a cup with hollow walls, enclosing but not quite surrounding
the entocodon, which remains in contact at its outer side with the
ectoderm covering the bud (fig. 44, D, v). The next changes that take
place are chiefly in the endoderm-cup (fig. 44, D, E); the cavity
between the two walls of the cup becomes reduced by concrescence to
form the radial canals (r.c.), ring-canal (c.c.), and endoderm-lamella
(e.l., fig. 44, E), and at the same time the base of the cup is thrust
upwards to form the manubrium (m), converting the cavity of the
entocodon into a space which is crescentic or horse-shoe-like in
section. Next tentacles (t, fig. 44, F) grow out from the ring-canal,
and the double plate of ectoderm on the distal side of the entocodon
becomes perforated, leaving a circular rim composed of two layers of
ectoderm, the velum (v) of the medusa. Finally, a mouth is formed by
breaking through at the apex of the manubrium, and the now
fully-formed medusa becomes separated by rupture of the stalk of the
bud and swims away.

I, Ideally primitive method of budding, in which the mouth is formed
first (Ia), next the tentacles (Ib), and lastly the umbrella.
II, Method. of _Cunina_; (a) the mouth arises, next the umbrella
(b), and lastly the tentacles (c).
III, Hypothetical transition from II to the indirect method with an
entocodon; the formation of the manubrium is retarded, that of the
umbrella hastened (IIIa, b).
IV, a, b, c, budding with an entocodon (cf. fig. 44).
V, Budding with a solid entocodon (cf. fig. 45).]

If the bud, however, is destined to give rise not to a free medusa,
but to a gonophore, the development is similar but becomes arrested at
various points, according to the degree to which the gonophore is
degenerate. The entocodon is usually formed, proving the medusoid
nature of the bud, but in sporosacs the entocodon may be rudimentary
or absent altogether. The process of budding as above described may be
varied or complicated in various ways; thus a secondary, amnion-like,
ectodermal covering or ectotheca (fig. 45, C, ect.) may be formed over
all, as in _Garveia_, &c.; or the entocodon may remain solid and
without cavity until after the formation of the manubrium, or may
never acquire a cavity at all, as described above for the gonophores.

_Phylogenetic Significance of the Entocodon._--It is seen from the
foregoing account of medusa-budding that the entocodon is a very
important constituent of the bud, furnishing some of the most
essential portions of the medusa; its cavity becomes the sub-umbral
cavity, and its lining furnishes the ectodermal epithelium of the
manubrium and of the sub-umbral cavity as far as the edge of the
velum. Hence the entocodon represents a precocious formation of the
sub-umbral surface, equivalent to the peristome of the polyp,
differentiated in the bud prior to other portions of the organism
which must be regarded as antecedent to it in phylogeny.

If the three principal organ-systems of the medusa, namely mouth,
tentacles and umbrella, be considered in the light of phylogeny, it is
evident that the manubrium bearing the mouth must be the oldest, as
representing a common property of all the Coelentera, even of the
gastrula embryo of all Enterozoa. Next in order come the tentacles,
common to all Cnidaria. The special property of the medusa is the
umbrella, distinguishing the medusa at once from other morphological
types among the Coelentera. If, therefore, the formation of these
three systems of organs took place according to a strictly
phylogenetic sequence, we should expect them to appear in the order
set forth above (fig. 46, Ia, b, c). The nearest approach to the
phylogenetic sequence is seen in the budding of _Cunina_, where the
manubrium and mouth appear first, but the umbrella is formed before
the tentacles (fig. 46, IIa, b, c). In the indirect or coenogenetic
method of budding, the first two members of the sequence exhibited by
Cunina change places, and the umbrella is formed first, the manubrium
next, and then the tentacles; the actual mouth-perforation being
delayed to the very last (fig. 46, IVa, b, c). Hence the budding of
medusae exemplifies very clearly a common phenomenon in development, a
phylogenetic series of events completely dislocated in the ontogenetic
time-sequence.

The entocodon is to be regarded, therefore, not as primarily an
ingrowth of ectoderm, but rather as an upgrowth of both body-layers,
in the form of a circular rim (IVa), representing the umbrellar
margin; it is comparable to the bulging that forms the umbrella in the
direct method of budding, but takes place before a manubrium is
formed, and is greatly reduced in size, so as to become a little pit.
By a simple modification, the open pit becomes a solid ectodermal
ingrowth, just as in Teleostean fishes the hollow medullary tube, or
the auditory pit of other vertebrate embryos, is formed at first as a
solid cord of cells, which acquires a cavity secondarily. Moreover,
the entocodon, however developed, gives rise at first to a closed
cavity, representing a closing over of the umbrella, temporary in the
bud destined to be a free medusa, but usually permanent in the sessile
gonophore. As has been shown above, the closing up of the sub-umbral
cavity is one of the earliest degenerative changes in the evolution of
the gonophore, and we may regard it as the umbrellar fold taking on a
protective function, either temporarily for the bud or permanently for
the gonophore.

To sum up, the entocodon is a precocious formation of the umbrella,
closing over to protect the organs in the umbrellar cavity. The
possession of an entocodon proves the medusa-nature of the bud, and
can only be explained on the theory that gonophores are degenerate
medusae, and is inexplicable on the opposed view that medusae are
derived from gonophores secondarily set free. In the sporosac,
however, the medusa-individual has become so degenerate that even the
documentary proof, so to speak, of its medusoid nature may have been
destroyed, and only circumstantial evidence of its nature can be
produced.

4. _Germinal Budding._--This method of budding is commonly described as budding from a single body-layer, instead of from both layers. The layer that produces the bud is invariably the ectoderm, i.e. the layer in which, in Hydromedusae, the generative cells are lodged; and in some cases the buds are produced in the exact spot in which later the gonads appear. From these facts, and from those of the sporogony, to be described below, we may regard budding to this type as taking place from the germinal epithelium rather than from ordinary ectoderm.

(a) _The Polyp._--Budding from the ectoderm alone has been described
by A. Lang [29] in _Hydra_ and other polyps. The tissues of the bud
become differentiated into ectoderm and endoderm, and the endoderm of
the bud becomes secondarily continuous with that of the parent, but no
part of the parental endoderm contributes to the building up of the
daughter-polyp. Lang regarded this method of budding as universal in
polyps, a notion disproved by O. Seeliger [52] who went to the
opposite extreme and regarded the type of budding described by Lang as
non-existent. In view, however, both of the statements and figures of
Lang and of the facts to be described presently for medusae
(_Margellium_), it is at least theoretically possible that both
germinal and vegetative budding may occur in polyps as well as in
medusae.

(b) _The Medusa._--The clearest instance of germinal budding is
furnished by _Margellium (Rathkea) octopunctatum_, one of the
_Margelidae_. The budding of this medusa has been worked out in detail
by Chun (HYDROZOA, [1]), to whom the reader must be referred for the
interesting laws of budding regulating the sequence and order of
formation of the buds.

The buds of _Margellium_ are produced on the manubrium in each of the
four interradii, and they arise from the ectoderm, that is to say, the
germinal epithelium, which later gives rise to the gonads. The buds do
not appear simultaneously but successively on each of the four sides
of the manubrium, thus:

1
3 4
2

and secondary buds may be produced on the medusa-buds before the
latter are set free as medusae. Each bud arises as a thickening of the
epithelium, which first forms two or three layers (fig. 47, A), and
becomes separated into a superficial layer, future ectoderm,
surrounding a central mass, future endoderm (fig. 47, B). The
ectodermal epithelium on the distal side of the bud becomes thickened,
grows inwards, and forms a typical entocodon (fig. 37, D, E, F). The
remaining development of the bud is just as described above for the
indirect method of medusa-budding (fig. 47, G, H). When the bud is
nearly complete, the body-wall of the parent immediately below it
becomes perforated, placing the coelenteric cavity of the parent in
secondary communication with that of the bud (H), doubtless for the
better nutrition of the latter.

Especially noteworthy in the germinal budding of _Margellium_ is the
formation of the entocodon, as in the vegetative budding of the
indirect type.

5. _Sporogony._--This method of reproduction has been described by E. Metchnikoff in _Cunina_ and allied genera. In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm. They divide each into two sister-cells, one of which--the spore--becomes enveloped by the other. The spore-cell multiplies by division, while the enveloping cell is nutrient and protective. The spore cell gives rise to a "spore-larva," which is set free in the coelenteron and grows into a medusa. Whether sporogony occurs also in the polyp or not remains to be proved.

6. _Sexual Reproduction and Embryology._--The ovum of Hydromedusae is usually one of a large number of oögonia, and grows at the expense of its sister-cells. No regular follicle is formed, but the oöcyte absorbs nutriment from the remaining oögonia. In _Hydra_ the oöcyte is a large amoeboid cell, which sends out pseudopodia amongst the oögonia and absorbs nutriment from them. When the oöcyte is full grown, the residual oögonia die off and disintegrate.

A, The epithelium becomes two-layered.
B, The lower layer forms a solid mass of cells, which (C) becomes a
vesicle, the future endoderm, containing the coelenteric cavity
(coel), while the outer layer furnishes the future ectoderm.
D, E, F, a thickening of the ectoderm on the distal side of the bud
forms an entocodon (Gc).
G,H, Formation of the medusae.
s.c, Sub-umbral cavity.
r.c, Radial canal.
st, Stomach, which in H acquires a secondary communication with the
digestive cavity of the mother.
cc, Circular canal.
v, Velum.
t, Tentacle.]

The spermatogenesis and maturation and fertilization of the germ-cells present nothing out of the common and need not be described here. These processes have been studied in detail by A. Brauer [2] for _Hydra_.

The general course of the development is described in the article
HYDROZOA. We may distinguish the following series of stages: (1) ovum;
(2) cleavage, leading to formation of a blastula; (3) formation of an
inner mass or parenchyma, the future endoderm, by immigration or
delamination, leading to the so-called parenchymula-stage; (4)
formation of an archenteric cavity, the future coelenteron, by a
splitting of the internal parenchyma, and of a blastopore, the future
mouth, by perforation at one pole, leading to the gastrula-stage; (5)
the outgrowth of tentacles round the mouth (blastopore), leading to
the actinula-stage; and (6) the actinula becomes the polyp or medusa
in the manner described elsewhere (see articles HYDROZOA, POLYP and
MEDUSA). This is the full, ideal development, which is always
contracted or shortened to a greater or less extent. If the embryo is
set free as a free-swimming, so-called planula-larva, in the blastula,
parenchymula, or gastrula stage, then a free actinula stage is not
found; if, on the other hand, a free actinula occurs, then there is no
free planula stage.

The cleavage of the ovum follows two types, both seen in _Tubularia_
(Brauer [3]). In the first, a cleavage follows each nuclear division;
in the second, the nuclei multiply by division a number of times, and
then the ovum divides into as many blastomeres as there are nuclei
present. The result of cleavage in all cases is a typical blastula,
which when set free becomes oval and develops a flagellum to each
cell, but when not set free, it remains spherical in form and has no
flagella.

The germ-layer formation is always by immigration or delamination,
never by invagination. When the blastula is oval and free-swimming the
inner mass is formed by unipolar immigration from the hinder pole.
When the blastula is spherical and not set free, the germ-layer
formation is always multipolar, either by immigration or by
delamination, i.e. by tangential division of the cells of the
blastoderm, as in _Geryonia_, or by a mixture of immigration and
delamination, as in _Hydra_, _Tubularia_, &c. The blastopore is formed
as a secondary perforation at one spot, in free-swimming forms at the
hinder pole. Formation of archenteron and blastopore may, however, be
deferred till a later stage (actinula or after).

The actinula stage is usually suppressed or not set free, but it is
seen in _Tubularia_ (fig. 48), where it is ambulatory, in _Gonionemus_
(Trachomedusae), and in _Cunina_ (Narcomedusae), where it is
parasitic.

FIG. 48.--Free Actinula of _Tubularia_.]

In Leptolinae the embryonic development culminates in a polyp, which
is usually formed by fixation of a planula (parenchymula), rarely by
fixation of an actinula. The planula may fix itself (1) by one end,
and then becomes the hydrocaulus and hydranth, while the hydrorhiza
grows out from the base; or (2) partly by one side and then gives rise
to the hydrorhiza as well as to the other parts of the polyp; or (3)
entirely by its side, and then forms a recumbent hydrorhiza from which
a polyp appears to be budded as an upgrowth.

In Trachylinae the development produces always a medusa, and there is
no polyp-stage. The medusa arises direct from the actinula-stage and
there is no entocodon formed, as in the budding described above.

_Life-cycles of the Hydromedusae._--The life-cycle of the Leptolinae
consists of an alternation of generations in which non-sexual
individuals, polyps, produce by budding sexual individuals, medusae,
which give rise by the sexual process to the non-sexual polyps again,
so completing the cycle. Hence the alternation is of the type termed
metagenesis. The Leptolinae are chiefly forms belonging to the inshore
fauna. The Trachylinae, on the other hand, are above all oceanic
forms, and have no polyp-stage, and hence there is typically no
alternation in their life-cycle. It is commonly assumed that the
Trachylinae are forms which have lost the alternation of generations
possessed by them ancestrally, through secondary simplification of the
life-cycle. Hence the Trachylinae are termed "hypogenetic" medusae to
contrast them with the metagenetic Leptolinae. The whole question has,
however, been argued at length by W. K. Brooks [4], who adduces strong
evidence for a contrary view, that is to say, for regarding the direct
type of development seen in Trachylinae as more primitive, and the
metagenesis seen in Leptolinae as a secondary complication introduced
into the life-cycle by the acquisition of _larval budding_. The polyp
is regarded, on this view, as a form phylogenetically older than the
medusa, in short, as nothing more than a sessile actinula. In
Trachylinae the polyp-stage is passed over, and is represented only by
the actinula as a transitory embryonic stage. In Leptolinae the
actinula becomes the sessile polyp which has acquired the power of
budding and producing individuals either of its own or of a higher
rank; it represents a persistent larval stage and remains in a
sexually immature condition as a neutral individual, sex being an
attribute only of the final stage in the development, namely the
medusa. The polyp of the Leptolinae has reached the limit of its
individual development and is incapable of becoming itself a medusa,
but only produces medusa-buds; hence a true alternation of generations
is produced. In Trachylinae also the beginnings of a similar
metagenesis can be found. Thus in _Cunina octonaria_, the ovum
develops into an actinula which buds daughter-actinulae; all of them,
both parent and offspring, develop into medusae, so that there is no
alternation of generations, but only larval multiplication. In _Cunina
parasitica_, however, the ovum develops into an actinula, which buds
actinulae as before, but only the daughter-actinulae develop into
medusae, while the original, parent-actinula dies off; here,
therefore, larval budding has led to a true alternation of
generations. In _Gonionemus_ the actinula becomes fixed and
polyp-like, and reproduces by budding, so that here also an
alternation of generations may occur. In the Leptolinae we must first
substitute polyp for actinula, and then a condition is found which can
be compared to the case of _Cunina parasitica_ or Gonionemus, if we
suppose that neither the parent-actinula (i.e. founder-polyp) nor its
offspring by budding (polyps of the colony) have the power of becoming
medusae, but only of producing medusae by budding. For further
arguments and illustrations the reader must be referred to Brooks's
most interesting memoir. The whole theory is one most intimately
connected with the question of the relation between polyp and medusa,
to be discussed presently. It will be seen elsewhere, however, that
whatever view may be held as to the origin of metagenesis in
Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is
possible than that the alternation of generations is the direct result
of larval proliferation.

To complete our survey of life-cycles in the Hydromedusae it is
necessary to add a few words about the position of _Hydra_ and its
allies. If we accept the view that _Hydra_ is a true sexual polyp, and
that its gonads are not gonophores (i.e. medusa-buds) in the extreme
of degeneration, then it follows from Brooks's theory that _Hydra_
must be descended from an archaic form in which the medusan type of
organization had not yet been evolved. _Hydra_ must, in short, be a
living representative of the ancestor of which the actinula-stage is a
transient reminiscence in the development of higher forms. It may be
pointed out in this connexion that the fixation of _Hydra_ is only
temporary, and that the animal is able at all times to detach itself,
to move to a new situation, and to fix itself again. There is no
difficulty whatever in regarding _Hydra_ as bearing the same relation
to the actinula-stage of other Hydromedusae that a Rotifer bears to a
trochophore-larva or a fish to a tadpole.

_The Relation of Polyp and Medusa._--Many views have been put forward as to the morphological relationship between the two types of person in the Hydromedusae. For the most part, polyp and medusa have been regarded as modifications of a common type, a view supported by the existence, among Scyphomedusae (q.v.), of sessile polyp-like medusae (_Lucernaria_, &c.). R. Leuckart in 1848 compared medusae in general terms to flattened polyps. G. J. Allman [1] put forward a more detailed view, which was as follows. In some polyps the tentacles are webbed at the base, and it was supposed that a medusa was a polyp of this kind set free, the umbrella being a greatly developed web or membrane extending between the tentacles. A very different theory was enunciated by E. Metchnikoff. In some hydroids the founder-polyp, developed from a planula after fixation, throws out numerous outgrowths from the base to form the hydrorhiza; these outgrowths may be radially arranged so as to form by contact or coalescence a flat plate. Mechnikov considered the plate thus formed at the base of the polyp as equivalent to the umbrella, and the body of the polyp as equivalent to the manubrium, of the medusa; on this view the marginal tentacles almost invariably present in medusae are new formations, and the tentacles of the polyp are represented in the medusa by the oral arms which may occur round the mouth, and which sometimes, e.g. in _Margelidae_, have the appearance and structure of tentacles. Apart from the weighty arguments which the development furnishes against the theories of Allman and Mechnikov, it may be pointed out that neither hypothesis gives a satisfactory explanation of a structure universally present in medusae of whatever class, namely the endoderm-lamella, discovered by the brothers O. and R. Hertwig. It would be necessary to regard this structure as a secondary extension of the endoderm in the tentacle-web, on Allman's theory, or between the outgrowths of the hydrorhiza, on Mechnikov's hypothesis. The development, on the contrary, shows unequivocally that the endoderm-lamella arises as a local coalescence of the endodermal linings of a primitively extensive gastral space.

The question is one intimately connected with the view taken as to the nature and individuality of polyp, medusa and gonophore respectively. On this point the following theories have been put forward.

1. The theory that the medusa is simply an organ, which has become
detached and has acquired a certain degree of independence, like the
well-known instance of the hectocotyle of the cuttle-fish. On this
view, put forward by E. van Beneden and T. H. Huxley, the sporosac is
the starting-point of an evolution leading up through the various
types of gonophores to the free medusa as the culminating point of a
phyletic series. The evidence against this view may be classed under
two heads: first, comparative evidence; hydroids very different in
their structural characters and widely separate in the systematic
classification of these organisms may produce medusae very similar, at
least so far as the essential features of medusan organization are
concerned; on the other hydroids closely allied, perhaps almost
indistinguishable, may produce gonophores in the one case, medusae in
the other; for example, _Hydractinia_ (gonophores) and _Podocoryne_
(medusae), _Tubularia_ (gonophores) and _Ectopleura_ (medusae),
_Coryne_ (gonophores) and _Syncoryne_ (medusae), and so on. If it is
assumed that all these genera bore gonophores ancestrally, then medusa
of similar type must have been evolved quite independently in a great
number of cases. Secondly, there is the evidence from the development,
namely, the presence of the entocodon in the medusa-bud, a structure
which, as explained above, can only be accounted for satisfactorily by
derivation from a medusan type of organization. Hence it may be
concluded that the gonophores are degenerate medusae, and not that the
medusae are highly elaborated gonophores, as the organ-theory
requires.