passes for digestion. The single opening into it serves at once as
mouth and anus; the sac itself is the alimentary tract. Frequently
lateral diverticula or branched canals are given off from the central
sac which distribute the nourishment to the peripheral parts of the
"body, and thus functionally replace the vascular system of higher
Since this gastrovascular system is primarily for nourishment,
it is an error to call it a body cavity and to say that the coelenterates
are stomachless. On the other hand, the term ' coelenteron, ' that
is a cavity which is at once gastric and ccelomic (p. 158), is perfectly
defensible, since in many higher animals which possess a true body
cavity this is seen in development to arise as diverticula from
the primitive stomach (enteron). Since such diverticula occur in
coelenterates without becoming independent, one can say that the
gastrovascular system consists not only of intestinal portions but,
in potentia, of the coelom as well.
To even a superficial observation the Coelenterata are more
clearly animals than are the sponges. The single animals, though
often united in colonies, and fixed to some support, are capable of
quick and energetic motion. These movements are most striking
in the tentacles long tactile threads, in the neighborhood of the
mouth, which have the functions of feeling for food, grasping it,
and conveying it to the mouth. The means of killing the prey are
the cnidae, nematocysts, or nettle cells (fig. 171), which with rare
FIG. 171. Nettle cells of Coelenterata (After Hertwig, Lendenfeld, and Hamann.)
exceptions in Protozoa, Turbellaria, and molluscs occur in no
other group. These structures, of great systematic importance,
are oval vesicles with fluid .contents and firm membrane. Each is
drawn out at one end into a long tube, so delicate as to appear as
a thread (hence an additional name, thread cells). This thread is
sometimes armed throughout its length with retrorse hooks, or it
may have only a few stronger hooks on its basal portion, which is
thicker than the rest. In the resting stage the thread is spirally
coiled inside the cell. On stimulation the thread is quickly
extended (< explosion of cell ') and produces a wound into which
passes the irritating fluid contents. Some coelenterates (e.g+
Physalia) can produce in this way very painful nettling even in man..
The nettle capsule arises as a plasma product inside a cell.
When fully developed the nettle cell extends to the surface and
ends with a tactile process (cnidocill) which upon contact stimu-
lates the protoplasm and causes the explosion. The cell itself is.
frequently enclosed by a muscular sheath or a network of muscle
Among the ccelenterates both sexual and asexual reproduction.
may occur, the latter usually by budding, more rarely by division.
Sexual and asexual types of reproduction can be combined in the
same species, producing an alternation of generations.
In comparison with the sponges the Co3lenterata may be called
epithelial organisms. A mesoderm (' mesogloaa') may be entirely
lacking or may have but a subordinate development. The ectoderm
and entoderm, on the other hand, are the important tissues pro-
ducing muscles, nerves, sense organs, sexual products and cnidae.
Hence the group is often called Diploblastica two-layered animals.
Class I. Hydrozoa (Hydromedusae).
According to varying standpoints the Hydrozoa can be placed
either higher or lower than the Anthozoa in the system, since in the
former group two forms are frequently introduced into the life
Mstory, one agreeing well in structure with the Anthozoa, the
other standing on a higher grade. The first is the sessile and
usually colonial polyp, the second the free-swimming medusa, well
provided with sense organs. These are usually related to each
other by an alternation of generations. The polyp is asexual and
by budding produces medusae; the medusa, on the other hand, is
the sexual stage, and from its eggs polyps arise.
The polyp of the Hydrozoa is the hydropolyp, forming in the
branch of coelenterates an important archetype from which all
other conditions medusae, scyphopolyp, and even the coral polyp
may be derived. Our best example of this is the fresh-water
Hydra, so common in pools and streams. The body (fig. 172) is a
sac, the hinder closed end of which, the pedal disc, is used for
attachment. The other end bears the mouth which leads to the
internal gastrovascular (digestive) cavity. Around the mouth is a
circle of tentacles used in capturing food (mostly small Crustacea).
These are outgrowths of the body wall; the circle dividing the
body into a peristome inside the circle and a column constituting
the rest of the outer wall.
Hydra has but two body layers (fig. 173), an entoderm of
flagellate cells lining the gastrovascular space, and the ectoderm
covering the outer surface. Between the two is the supporting
layer (mesogloea), a structureless membrane without cells and hence
not a body layer. Both layers consist of epithelial muscular cells
(cf. p. 92), the basal ends of which are produced into smooth
muscle fibres, those of the ectoderm running lengthwise, those of
the entoderm around the body. The ectoderm further contains
ganglion, nettle and sex cells. The nettle cells on the tentacles
are crowded into small ridges or batteries. The sex cells (at cer-
tain times) produce swellings on the column; a circle of male
swellings close beneath the tentacles, the female cells farther down
the column (fig. 172). Individuals reproducing by budding are
more common than the sexually mature (fig. 90). Small eleva-
s ek c
FIG. 172. Hydra viridis,* testes above; ovarian enlargement below.
FIG. 173. Body layers of Hydra. (After Schulze, from Hatschek.) c, cuticula; en,
nettle cells; eto, ectoderm; en, entoderm; s, supporting layer.
tions appear on the column, enlarge, form tentacles, and at last a
mouth, after which they may separate from the parent.
In the sea are numerous hydroid polyps which, while agreeing in
the main with Hydra, are distinguished from it in two important
respects: (1) they do not directly produce sexual organs; (2) they
reproduce asexually, and by incomplete budding form persistent
colonies. In this formation of colonies a series of parts have
arisen which require special designations (fig. 174). The separate
animals are the hydranths, and are connected together by a system
of tubes, the ccenosarc, which, like the hydranths, consist of ecto-
derm, entoderm, and mesogloea, and since the gastro vascular space
continues in them, these effect a distribution of food throughout
the colony. The coenosarcal tubes may creep over some support
(stone, alga, snail-shell, etc. ) and form a network, the hydrorhiza,
or it may stand erect and tree-like, forming a liydrocaulus. Usually
both hydrorhiza and hydrocaulus occur in the same colony.
FiQ.llLCampanulnrfajohnstoni. (After Allman.) o, hydranth with hydrotheca;
6, retracted; rf, hydrocaulus; /, gonotheca, with blastostyle and medusa buds;
g, free medusa.
FIG. 175. Section of Eudendrium ramnsum. ek, ectoderm; en, entoderm; p, perisarc;
,s, supporting layer.
Usually the colony is strengthened and protected by the perisarc,
a cuticular tubular secretion of the ectoderm. In some (fig. 175)
the perisarc stops at the base of the hydranth; in others (fig. 176)
it expands distally into a wide-mouthed bell, the hydrotheca, into
which the hydranth may retract at times of danger. In rare cases
FIG. 176. Campanularia geniculata. eft, ectoderm; en, entoderm; p, perisarc, ex-
panded around hydranth to a hydrotheca; s, supporting layer.
this perisarc may be greatly increased and calcified, forming large
coral-like masses with openings from which the hydranths may
protrude (fig. 177).
FIG. 177. A bit of Millepora alcicornis, enlarged. (After Agassiz.)
The lack of sexual organs, which distinguishes the marine
species from the fresh-water Hydra, is explained by the fact that
sexual individuals of special form are produced from the colony
by budding. These, the medusae, may separate early from the
colony and swim freely. A medusa (figs. 178, 179) has the form
FIG. 178. Rhopalonema velatum. c, ring canal; e, exumbrella; g, gonads; h, otocysts:
m, stomach; n, nerve ring; o, mouth; s, subumbrella; t', t'\ tentacles of first
and second order ; v, velum.
of a dome-like or disc-like bell and consists chiefly of an ex-
traordinarily watery jelly. The bell or umbrella of the medusa is
covered on both its surfaces the concave or subuinbrella, the con-
vex or exumbrella with ectodermal epithelium. At the margin
of the bell the sub- and exumbrellar ectoderm is produced into a
two-layered sheet with a central opening, the velum or craspedon
(fig. 178, B, v) of systematic importance, since these medusae are
often spoken of as Craspedota. Tentacles (usually 4, 8, or
multiples in number) also arise from the edge of the bell just
outside the velum.
FIG. 179. Tiara pileata. (After Haeckel, from Hatschek.)
Comparable to the tongue of the bell or the handle of the
umbrella is the manufrrium, hanging from the highest point of the
subumbrella and bearing the mouth at its tip. It contains the
chief digestive space from which radial canals run on the sub-
umbrellar surface to a ring canal in the margin of the umbrella.
The radial canals are usually four in number, but in some species
the number is increased during growth even to a hundred or more.
Manubrium and canals are lined by entoderm, which also extends
into the tentacles and forms their axes.
All other important organs arise from the ectoderm. Gonads
arise in many species (fig. 179) from the ectoderm of the manu-
brium; in others from the same layer covering the subumbrellar
surface of the radial canals (fig. 178), forming in either case con-
spicuous, often orange or red, thickenings. Longitudinal ectoder-
mal muscles move the tentacles in a snaky fashion, whence the
name medusa. Circular striped muscles run on the subumbrellar
side of bell and velum, causing the characteristic motion. By this
contraction the bell becomes more arched and narrowed, while the
FlG. 180. Otocysts of Medusae. A, Cunina ; B, Rhopalonema ; C, Carmarinn (Trachy-
medusae) ; D. Octorchis (Leptomedusan). a, epithelium ; /i, auditory cells ; hf y
origin of hairs ; hh, auditory hairs ; hp, auditory cushion ; o, otoliths ; ?i, audi-
tory nerve ; nr, nerve ring.
velum (which hangs down when at rest fig. 178, A) contracts
like a diaphragm across the mouth of the bell (fig. 178, B). Since
water is thus forced out through the opening the medusa is forced
forward by the reaction.
The circular muscles of umbrella and velum are separated by
the nerve ring, with which are connected the sensory organs
eyes of the simplest type; red pigment spots with or without a
lens (fig. 81); and open or closed auditory vesicles (otocysts).
Tactile hairs are abundant on the tentacles.
The auditory organs are of two types, both beginning as free organs
and receiving their highest development as closed vesicles (otocysts). One
type, the tentacular organs, occur in the Trachymedusae, the other, or
velar organ, in the Leptomedusae. The tentacular organs are modified
tentacles, the entodermal axis of which forms the otoliths and the
ectodermal covering the sense cells. In the ^Eginidae (Fig. 180, A) the
club-like tentacles, seated on an auditory cushion, project freely into the
water; in the Trachynemidse (Fig. 180, B) they are partially transformed
into vesicles by the upgrowth of epithelium, and in the Geryonidae (Fig.
180, C) they are completely enclosed arid are sunk in the jelly of the bell.
The velar organs of the Leptomedusa3 are placed on the subumbrellar sur-
face of the velum. They may be either simple pits (Fig. 180, J?), or the
mouths of the pits may close (Fig. 180, Z>). In these both sense cells and
otoliths are ectodermal. Eyes and otocysts occur in different forms, a
fact which formerly lead to a division of medusae into ocellate and vesicu-
While polyps and medusae apparently differ so greatly from each
other, their morphology shows that the medusae are only highly
modified polyps adapted to a swimming life. The long axis of
the polyp has been greatly shortened (fig. 181) and the cylindrical
FIG. 181. Diagram of sections of (A) a polyp and (B) a medusa, ek, ectoderm; ck', of
exumbrella; efc 2 , of subumbrella; ek\ of manubrium; e7, endoderm (cathamnal)
layer arising from obliteration of digestive space ; en, entoderm ; r, ring canal ;
s, subumbrella ; t, tentacles; V, velum ; x, supporting layer (gelatinous in B).
body developed into a disc; the mesoglcea of column and disc thick-
ened to a conspicuous layer of jelly; while manubrial cavity,
radial and ring canals are to be interpreted as remnants of the
large gastrovascular space of the polyp, obliterated in part by the
pressure of the mesogloea. To the parts thus formed only the
yelum and sense organs are added.
This comparison of medusa with polyp is of importance in
understanding the development, which usually is complicated by
an alternation of generations. From the eggs of the medusae a
small ciliated embryo (planula) escapes, which becomes attached,
develops mouth and tentacles, and, by budding, produces a.
hydroid colony. This hydroid colony lacks sexual organs. It
produces by budding the sexual individuals, the medusae, which
separate and swim freely. Since polyp and medusas are morpho-
logically comparable, there is a time before the escape of the
medusae when the colony is polymorphic, consisting of asexual
individuals (hydranths) which reproduce only asexually and of
others which have taken over the sexual reproduction (medusas).
Hence we conclude that the alternation of generations here has
arisen from a division of labor or polymorphism of individuals
originally of equivalent value, in which some individuals (the
sexual) have separated and acquired a peculiar structure.
While alternation of generation has arisen from polymorphism,
it can again produce it. This occurs when the medusae, instead
of separating, remain permanently attached to the colony. They
then degenerate into ' sporosacs/ which always lack mouth, tentacles,
and velum (fig. 182), often also radial and ring canals, so that at last
FIG. 182. Comparison of a medusa and a sporosac (orig.). A, fully developed medusa;
B, medusa with the manubrium closed, still attached to the blastostyle ; C,
', last stage, eggs being pro-
medusa reduced to a simple manubrium (sporosac) ; D
duced in the body wall (Hydra).
there remains only the manubrium ('spadix') and the sexual
organs, the latter enveloped by the rudiments of the umbrella.
Since medusae and sporosac replace each other in closely allied
species, a common name, gonopliore, has been applied to both.
This developmental history may be modified in two ways:
either the polypoid or the medusan generation may be suppressed:
In the first case we have polyps which reproduce both sexually and
asexually, in the other medusae whose eggs develop directly into
other medusae. (A few medusae may produce new medusae by
budding.) Thus we can have four conditions: (1) Polyps which
produce sometimes asexually, sometimes sexually, but always
polpys; (2) Medusae which always produce medusae; (3) Polyps
and medusae in alternating generations; (4) Polyps and sessile
medusae (sporosacs) united in a polymorphic colony.
The Hydrozoa are almost exclusively marine. The colonial forms occur
mostly on rocky coasts down to a depth of 100 fathoms, but have been
found in water 4000 fathoms deep. The medusae belong to the pelagic
fauna. For a long time the only fresh-water species known belonged to
the cosmopolitan genus Hydra, but more recently both hydroid (Proto-
hydra ryderi,* America ; Polypodium hydriforme, Russia) and medusan
forms (Limnocodium soivei'byi, Brazil ; Limnocnida tanganyicce, Africa ;
Halomises lacustris, Trinidad) have been found. Cordylophora lucustris *
occurs in the brackish waters of Europe and America.
The Hydrozoa may be classified according to characters, derived either
from the hydroid or the medusan stage. The former basis gives us four
(1) Hydraria. Polyps with asexual and sexual reproduction ; no per-
sistent colonies, no perisarc, no gonophores (fig. 172).
(2) Tubulariae. Mostly colonial, with perisarc but without hydrothecae.
Reproduction by gonophores (medusae or sporosacs, figs. 91, 175).
(3) Campanulariae. Colonial, with perisarc and hydrotheca. Repro-
duction by gonophores arising in special perisarcal envelopes, the gonotheca
(figs. 174, 176).
(4) Hydrocorallina. Colonial, with massive, calcified perisarc, resem-
bling coral. Reproduction by sporosacs or short-lived medusae.
FIG. 183. American Trachy and Narcomedusae. A, Liriope scutigera. (After Fewkes.)
.B, Cunocantha octonaria. (After Brooks.)
The characters derived from the medusae also give five groups :
(1) Anthomedusae (Ocellatae). Gonads on the manubrium ; no audi-
tory organs ; eyes usually present ; polyp generation present.
(2) Leptomedusae. Gonads on radial canals ; usually velar auditory
organs ; polyp generation present.
(3) Trachymedusae. Gonads on the radial canals ; tentacular auditory
organs ; develop directly to medusae (fig. 183, A.)
(4) Narcomedusae. Gonads on the manubrium or gastral pouches ;
tentacular auditory organs ; no polypoid stage (fig. 183, B.)
(5) Siphonophora. Polymorphic, free-swimming colonies of Anthome-
<dusae ; no polyp generation.
From this it is seen that there are medusae without polyp stages and
polyps without medusas, so that a true system must take into account both
these features. When this is done and life histories are traced it is seen
that the Anthomedusse and the Tubulariae are connected by an alternation
of generations, and the same holds good for Leptomedusse and Campanu-
lariae. There are three groups Trachymedusae, Narcomedusae, and
;Siphonophora without a hydroid stage, and two in which the polyp
plays the chief role, the medusa being rudimentary in the Hydrocorallinae,
lacking in the Hydraria. The hydroid polyps are usually but a few
FIG. 184. American hydrqzoan medusae. (Mostly after Fewkes.) A, Eutima gracilis;
B, Hydrichthys mirabilis; C, Obelia; D, Euchilota ventricularis; E, Lizzia grata; F,
Turritopsis nutricola; (?, Dipurena strangulata.
millimetres or fractions of a millimetre in size, but the huge Monocaulis
imperator, of the deep seas, a yard- in length, forms an exception. The
colonies are usually only a few inches in extent. The medusae have bells
varying between a millimetre and a few inches in diameter, reaching in
forskalea a diameter of sixteen inches.
Order I. Hydraria.
Until recently only the cosmopolitan species of Hydra were known.
During most of the year they reproduce by budding (fig. 90), only occa-
sionally developing gonads (fig. 172). The eggs remain in connexion
with the mother during segmentation, and later form an embryonal shell,
protecting them from drought or cold. In this * encysted stage ' they
can be distributed by wind or water birds. These animals formed the
basis of the celebrated researches of Trembley on regeneration. He
.showed that small portions when they contained both body layers could
/. HTDROZOA: EJDROCOEALLIN^J.
regenerate the whole animal. His experiments upon turning the animals
inside out have not been fully confirmed ; for in such cases the layers
resume their normal positions. Hydra grisea * (fusca). large brown
species ; H. viridis* green, from the presence of symbiotic algae. Pro-
tohydra ryderi* without tentacles. Polypodium hydriforme, parasitic
on sturgeon eggs in Russia, needs more study. The marine Haleremita
cumulans may belong here.
Order II. Hydrocorallinae.
Exclusively marine, forming colonies of thousands of individuals whose
calcareous skeletons so closely resemble true corals that they were asso-
ciated with them until the animals were studied. Millepora alcicornis*
stag-horn coral, in Florida. The rosy Stylasters occur in tropical seas.
Order III. Tubulariae = Anthomedusae (Gymnoblastea).
As a rule these colonial forms with perisarc but without hydrotheca
produce anthozoan medusae, but there are forms like Clava * (pink, on
FIG. 185. American Tubularian hydroids. A, Myrioihelia phryqiana (after Danielssen
andKoren); B. 6'arsi'a?-osari'a(afterFewkes); C, Monocaulispendula(a,fter Agassiz);
D, Clava leptostyla; E, Parypha crocea; F, Podocoryne mirabilis (after Agassiz).
rockweed) and Hydractinia * (on shells inhabited by hermit crabs)
which have sporosacs. Indeed the genera Corymorpha * and Mono-
caulis * are only differentiated by the existence of medusae in the former
and of sporosacs in the latter. In the forms with alternation of genera-
tions different names are applied to the hydroid and medusan stages as
Bougainvillea. Hippocrene, Margelis.
Other common genera in American waters are, of hydroids, besides
those mentioned, Eudendrium, Tubularia, and Thamnocnida; of medusae,
Tiaris, Turritopsis, Dipurena, Lizzia, Nemopsis, and Hydrichthis.
Order IV. Campanulariae = Leptomedusae (Calyphoblastea).
These forms are readily distinguished from the last by the fact that
they are always colonial and possess hydrotheeae, the medusas always being
LeptomedussB (p. 239). A peculiarity of the group is the existence of
gonothecae, closed perisarcal envelopes, inside which the gonophores arise
from the blastostyle, a specialized polyp, without mouth or tentacles
(fig. 174, /). The typical Campanulariae produce medusae, while some
forms, like Thaumantia * and JEquoria * have no hydroid stage, and
on the other hand Sertularia * and Plumularia * have no medusa stage.
FIG. 186. American Campanularians. (After Verrill.) A, Clytia noliformis ; B r
Calycellasyringa; C, Obelia dichotoma ,' Z>, Opercularella pumila.
Other common genera, Clytia,* Dipliasia* and Aglaophetiia* among
hydroids; Obelia,* Tima* RUegmatodes* among medusae. Possibly the
fossil group of GRAPTOLITES belongs near here. Only the perisarc is
known, and this is composed of hydrothecae, in which it is supposed the
Order V. Trachymedusae.
These medusae, mostly from warmer seas, have no hydroid stage. The
characters are given on p. 239, Trachynema, Liriope*(ftg. 183), and Cam-
panella in our own waters, Geryonia, etc., in Europe.
Order VI. Narcomedusae.
In addition to the characters on p. 239 may be added the fact that the
tentacles arise from the outside above the rim of the bell. Cunocantha *
(fig. 183), and Cunina * in our warmer waters, jEgina in Europe.
7. HTDROZOA : SIPHONOPHORA.
Order VII. Siphonophora.
The Siphonophora are among the most beautiful of pelagic
animals, some transparent, some brightly colored. Each (fig,.
187) consists of a colony of individ-
uals springing from a common C03-
nosarcal tube which is strongly mus-
cular and contains a central canal
lined with entoderm by which the
members of the colony receive their
nourishment. At one end the tube
is usually closed by a float filled with
air, the pneumataphore, which acts
as a hydrostatic apparatus, and keeps
the colony vertical in the water.
The individuals, springing from
the coenosarcal axis, perform differ-
ent functions and hence have differ-
ent structures. Close behind the
float commonly come several swim-
ming bells (nectocalyces) which re-
tain of medusal structures only those
(bell, velum) necessary for swimming
and those (ring and radial canals)
for the distribution of nourishment
received from the common tube.
Then come, scattered through the
colony, the covering scales, for pro-
tection, firm gelatinous plates which
have lost the ring canal, the muscles,
and the bell shape of the medusae.
Food is taken by wide-mouthed feed-
ing tubes (In/} which may be com-
pared to polyps (fig. 57) or the m
nubrium of a medusa. They digest
the food by means of large masses of
glands (' liver bands ') and convey it calyx); st ' stalk '