FRESHWATER POLYPS
(HYDRIDA).
INTRODUCTION TO PART II.
I.
THE PHYLUM COELENTERATA AND THE CLASS HYDROZOA.
The second of the great groups or phyla into which the metazoa are
divided is the Coelenterata, in which are included most of the animals
commonly known as zoophytes, and also the corals, sea-anemones and
jelly-fish. These animals are distinguished from the sponges on the one
hand and from the worms, molluscs, arthropods, vertebrates, etc., on the
other by possessing a central cavity (the coelenteron or "hollow
inside") the walls of which are the walls of the body and consist of
_two_ layers of cells separated by a structureless, or apparently
structureless, jelly. This cavity has as a main function that of a
digestive cavity.
An ideally simple coelenterate would not differ much in general
appearance from an olynthus (p. 27), but it would have no pores in the
body-wall and its upper orifice would probably be surrounded by
prolongations of the body-wall in the form of tentacles. There would be
no collar-cells, and the cells of the body generally would have a much
more fixed and definite position and more regular functions than those
of any sponge. The most characteristic of them would be the so-called
cnidoblasts. Each of these cells contains a capsule[AK] from which a
long thread-like body can be suddenly uncoiled and shot out.
[Footnote AK: Similar capsules are found in the tissues of
certain worms and molluscs, but there is the strongest
evidence that these animals, which habitually devour
coelenterates, are able to swallow the capsules uninjured
and to use them as weapons of defence (see Martin, Q. J.
Micro. Sci. London, lii, p. 261, 1908, and Grosvenor, Proc.
Roy. Soc. London, lxxii, p. 462, 1903). The "trichocysts" of
certain protozoa bear a certain resemblance to the
nettle-cells of coelenterates and probably have similar
functions.]
The simplest in structure of the coelenterates are those that constitute
the class Hydrozoa. In this class the primitive central cavity is not
divided up by muscular partitions and there is no folding in of the
anterior part of the body to form an oesophagus or stomatodæum such as
is found in the sea-anemones and coral polyps. In many species and
genera the life-history is complex, illustrating what is called the
alternation of generations. That is to say, only alternate generations
attain sexual maturity, those that do so being produced as buds from a
sexless generation, which itself arises from the fertilized eggs of a
previous sexual generation. The sexual forms as a rule differ
considerably in structure from the sexless ones; many medusæ are the
sexual individuals in a life-cycle in which those of the sexless
generation are sedentary.
An excellent general account of the coelenterates will be found in the
Cambridge Natural History, vol. i (by Prof. Hickson).
STRUCTURE OF HYDRA.
_Hydra_, the freshwater polyp, is one of the simplest of the Hydrozoa
both as regards structure and as regards life-history. Indeed, it
differs little as regards structure from the ideally simple coelenterate
sketched in a former paragraph, while its descent is direct from one
polyp to another, every generation laying its own eggs[AL]. The animal
may be described as consisting of the following parts:--(1) an upright
(or potentially upright) column or body, (2) a circle of contractile
tentacles at the upper extremity of the column, (3) an oral disk or
peristome surrounding the mouth and surrounded by the tentacles, and (4)
a basal or aboral disk at the opposite extremity. The whole animal is
soft and naked. The column, when the animal is at rest, is almost
cylindrical in some forms but in others has the basal part distinctly
narrower than the upper part. It is highly contractile and when
contracted sometimes assumes an annulate appearance; but as a rule the
external surface is smooth.
[Footnote AL: The statement is not strictly accurate as
regards the Calcutta phase of _H. vulgaris_, for the summer
brood apparently does not lay eggs but reproduces its
species by means of buds only. This state of affairs,
however, is probably an abnormality directly due to
environment.]
The tentacles vary in number, but are never very numerous. They are
disposed in a single circle round the oral disk and are hollow, each
containing a prolongation of the central cavity of the column. Like the
column but to an even greater degree they are contractile, and in some
forms they are capable of great elongation. They cannot seize any object
between them, but are able to move in all directions.
The disk that surrounds the mouth, which is a circular aperture, is
narrow and can to some extent assume the form of a conical proboscis,
although this feature is never so marked as it is in some hydroids. The
basal disk is even narrower and is not splayed out round the edges.
[Illustration: Fig. 27.--Nettle-cells of _Hydra_.
A=capsules from nettle-cells of a single specimen of the summer phase of
_H. vulgaris_ from Calcutta, × 480: figures marked with a dash represent
capsules with barbed threads. B=a capsule with the thread discharged,
from the same specimen, × 480. C=capsule with barbed thread, from a
specimen of _H. oligactis_ from Lahore. D=undischarged nettle-cell of
_H. vulgaris_ from Europe (after Nussbaum, highly magnified).
E=discharged capsule of the same (after the same author).
_a_=cnidoblast; _b_=capsule; _c_=thread; _d_=cnidocil. Only the base of
the thread is shown in E.]
A section through the body-wall shows it to consist of the three typical
layers of the coelenterates, viz., (i) an outer cellular layer of
comparatively small cells, the ectoderm; (ii) an intermediate,
structureless or apparently structureless layer, the mesogloea or
"central jelly"; and (iii) an internal layer or endoderm consisting of
relatively large cells. The cells of the ectoderm are not homogeneous.
Some of them possess at their base narrow and highly contractile
prolongations that exercise the functions of muscles. Others are
gland-cells and secrete mucus; others have round their margins delicate
ramifying prolongations and act as nerve-cells. Sense-cells, each of
which bears on its external surface a minute projecting bristle, are
found in connection with the nerve-cells, and also nettle-cells of more
than one type.
The mesogloea is very thin.
The endoderm consists mainly of comparatively large cells with polygonal
bases which can be seen from the external surface of the column in
colourless individuals. Their inner surface is amoeboid and in certain
conditions bears one or more vibratile cilia or protoplasmic lashes.
Nettle-cells are occasionally found in the endoderm, but apparently do
not originate in this layer.
The walls of the tentacles do not differ in general structure from those
of the column, but the cells of the endoderm are smaller and the
nematocysts of the ectoderm more numerous, and there are other minor
differences.
A more detailed account of the anatomy of _Hydra_ will be found in any
biological text-book, for instance in Parker's Elementary Biology; but
it is necessary here to say something more as regards the nettle-cells,
which are of great biological and systematic importance.
A nettle-cell of the most perfect type and the structures necessary to
it consist of the following parts:--
(1) A true cell (the cnidoblast), which contains--
(2) a delicate capsule full of liquid;
(3) a long thread coiled up in the capsule; and
(4) a cnidocil or sensory bristle, which projects from the
external surface of the cnidoblast.
A nerve-cell is associated with each cnidoblast.
In _Hydra_ the nettle-cells are of two distinct types, in one of which
the thread is barbed at the base, whereas in the other it is simple.
Both types have often two or more varieties and intermediate forms
occur, but generally speaking the capsules with simple threads are much
smaller than those with barbed ones. The arrangement of the nettle-cells
is not the same in all species of _Hydra_, but as a rule they are much
more numerous in the tentacles than elsewhere on the body, each large
cell being surrounded by several small ones. The latter are always much
more numerous than the former.
CAPTURE AND INGESTION OF PREY: DIGESTION.
The usual food of _Hydra_ consists of small insect larvæ, worms, and
crustacea, but the eggs of fish are also devoured. The method in which
prey is captured and ingested has been much disputed, but the following
facts appear to be well established.
If a small animal comes in contact with the tentacles of the polyp, it
instantly becomes paralysed. If it adheres to the tentacle, it perishes;
but if, as is often the case, it does not do so, it soon recovers the
power of movement. Animals which do not adhere are generally those (such
as ostracod crustacea) which have a hard integument without weak spots.
Nematocysts of both kinds shoot out their threads against prey with
considerable violence, the discharge being effected, apparently in
response to a chemical stimulus, by the sudden uncoiling of the thread
and its eversion from the capsule. Apparently the two kinds of threads
have different functions to perform, for whereas there is no doubt that
the barbed threads penetrate the more tender parts of the body against
which they are hurled, there is evidence that the simple threads do not
do so but wrap themselves round the more slender parts. Nussbaum (Arch.
mikr. Anat. xxix, pl. xx, fig. 108) figures the tail of a _Cyclops_
attacked by _Hydra vulgaris_ and shows several simple threads wrapped
round the hairs and a single barbed thread that has penetrated the
integument. Sometimes the cyst adheres to the thread and remains
attached to its cnidoblast and to the polyp, but sometimes the thread
breaks loose. Owing to the large mass of threads that sometimes
congregate at the weaker spots in the external covering of an animal
attacked (_e. g._, at the little sensory pits in the integument of the
dorsal surface of certain water-mites) it is often difficult to trace
out the whole length of any one thread, and as a thread still attached
to its capsule is frequently buried in the body of the prey, right up to
the barbs, while another thread that has broken loose from its capsule
appears immediately behind the fixed one, it seems as though the barbs,
which naturally point towards the capsule, had become reversed. This
appearance, however, is deceptive. The barbs are probably connected with
the discharge of the thread and do not function at all in the same way
as those on a spear- or arrow-head, never penetrating the object against
which the projectile is hurled. Indeed, their position as regards the
thread resembles that of the feathers on the shaft of an arrow rather
than that of the barb of the head.
Adhesion between the tentacles and the prey is effected partly by the
gummy secretion of the glands of the ectoderm, which is perhaps
poisonous as well as adhesive, and partly by the threads. Once the prey
is fast and has ceased to struggle, it is brought to the mouth, which
opens wide to receive it, by the contraction and the contortions of the
tentacles, the column, and the peristome. At the same time a mass of
transparent mucus from the gastral cavity envelops it and assists in
dragging it in. There is some dispute as to the part played by the
tentacles in conveying food into the mouth. My own observations lead me
to think that, at any rate so far as _H. vulgaris_ is concerned, they do
not push it in, but sometimes in their contortions they even enter the
cavity accidentally.
When the food has once been engulfed some digestive fluid is apparently
poured out upon it. In _H. vulgaris_ it is retained in the upper part of
the cavity and the soluble parts are here dissolved out, the insoluble
parts such as the chitin of insect larvæ or crustacea being ejected from
the mouth. Digestion is, however, to a considerable extent
intracellular, for the cells of the endoderm have the power of thrusting
out from their surface lobular masses of their cell-substance in which
minute nutritive particles are enveloped and dissolved. The movements of
the cilia which can also be thrust out from and retracted into these
cells, keep the food in the gastral cavity in motion and probably turn
it round so as to expose all parts in turn to digestive action. Complete
digestion, at any rate in the Calcutta form, takes several days to
accomplish, and after the process is finished a flocculent mass of
colourless excreta is emitted from the mouth.
COLOUR.
In _Hydra viridis_, a species that has not yet been found in India, the
green colour is due to the presence in the cells of green corpuscles
which closely resemble those of the cells of certain freshwater sponges.
They represent a stage in the life-cycle of _Chlorella vulgaris_,
Beyerinck[AM], an alga which has been cultivated independently.
[Footnote AM: Bot. Zeitung, xlviii (1890): see p. 49, _antea_.]
In other species of the genus colour is largely dependent on food,
although minute corpuscles of a _dark_ green shade are sometimes found
in the cells of _H. oligactis_. In the Calcutta phase of _H. vulgaris_
colour is due entirely to amorphous particles situated mainly in the
cells of the endoderm. If the polyp is starved or exposed to a high
temperature, these particles disappear and it becomes practically
colourless. They probably form, therefore, some kind of food-reserve,
and it is noteworthy that a polyp kept in the unnatural conditions that
prevail in a small aquarium invariably becomes pale, and that its
excreta are not white and flocculent but contain dark granules
apparently identical with those found in the cells of coloured
individuals (p. 154).
Berninger[AN] has just published observations on the effect of
long-continued starvation on _Hydra_ carried out in Germany. He finds
that the tentacles, mouth, and central jelly disappear, and that a
closed "bladder" consisting of two cellular layers remains; but, to
judge from his figures, the colour does not disappear in these
circumstances.
[Footnote AN: Zool. Anz. xxxvi, pp. 271-279, figs., Oct.
1910.]
BEHAVIOUR.
_Hydra viridis_ is a more sluggish animal than the other species of its
genus and does not possess the same power of elongating its column and
tentacles. It is, nevertheless, obliged to feed more frequently. Wagner
(Quart. J. Micr. Sci. xlviii, p. 586, 1905) found it impossible to use
this species in his physiological experiments because it died of
starvation more rapidly than other forms. This fact is interesting in
view of the theory that the green corpuscles in the cells of _H.
viridis_ elaborate nutritive substances for its benefit. _H. vulgaris_,
at any rate in Calcutta, does not ordinarily capture prey more often
than about once in three days.
All _Hydræ_ (except possibly the problematical _H. rubra_ of Roux, p.
160) spend the greater part of their time attached by the basal disk to
some solid object, but, especially in early life, _H. vulgaris_ is often
found floating free in the water, and all the species possess powers of
progression. They do not, however, all move in the same way. _H.
viridis_ progresses by "looping" like a geometrid caterpillar. During
each forward movement the column is arched downwards so that the
peristome is in contact with the surface along which the animal is
moving. The basal disk is then detached and the column is twisted round
until the basal disk again comes in contact with the surface at a point
some distance in advance of its previous point of attachment. The
manoeuvre is then repeated. _H. vulgaris_, when about to move, bends
down its column so that it lies almost prone, stretches out its
tentacles, which adhere near the tips to the surface (p. 153), detaches
its basal disk, and then contracts the tentacles. The column is dragged
forward, still lying almost prone, the basal disk is bent downwards and
again attached, and the whole movement is repeated. Probably _H.
oligactis_ moves in the same way.
When _H. viridis_ is at rest the tentacles and column, according to
Wagner, exhibit rhythmical contractions in which those of the buds act
in sympathy with those of the parent. In _H. vulgaris_ no such movements
have been observed. This species, however, when it is waiting for prey
(p. 154) changes the direction of its tentacles about once in half an
hour.
All species of _Hydra_ react to chemical and physical stimuli by
contraction and by movements of the column and tentacles, but if the
stimuli are constantly repeated, they lose the power to some extent. All
species are attracted by light and move towards the point whence it
reaches them. _H. vulgaris_, however, at any rate in India, is more
strongly repelled by heat. Consequently, if it is placed in a glass
vessel of water, on one side of which the sun is shining directly, it
moves away from the source of the light[AO]. But if the vessel be
protected from the direct rays of the sun and only a subdued light falls
on one side of it, the polyp moves towards that side. No species of the
genus is able to move in a straight line. Wilson (Amer. Natural. xxv, p.
426, 1891) and Wagner (_op. cit. supra_) have published charts showing
the elaborately erratic course pursued by a polyp in moving from one
point to another and the effect of light as regards its movements.
[Footnote AO: Mr. F. H. Gravely tells me that this is also
the case as regards _H. viridis_ in England, at any rate if
freshly captured specimens are placed overnight in a bottle
in a window in such a position that the early morning
sunlight falls upon one side of the bottle.]
If an individual of _H. vulgaris_ that contains half digested food in
its gastral cavity is violently removed from its natural surroundings
and placed in a glass of water, the column and tentacles contract
strongly for a few minutes. The body then becomes greatly elongated and
the tentacles moderately so; the tentacles writhe in all directions
(their tips being sometimes thrust into the mouth), and the food is
ejected.
REPRODUCTION.
Reproduction takes place in _Hydra_ (i) by means of buds, (ii) by means
of eggs, and (iii) occasionally by fission.
(a) _Sexual Reproduction._
The sexual organs consist of ovaries (female) and spermaries (male).
Sometimes the two kinds of organs are borne by the same individual
either simultaneously or in succession, but some individuals or races
appear to be exclusively of one sex. There is much evidence that in
unfavourable conditions the larger proportion of individuals develop
only male organs.
In temperate climates most forms of _Hydra_ breed at the approach of
winter, but starvation undoubtedly induces a precocious sexual activity,
and the same is probably the case as regards other unfavourable
conditions such as lack of oxygen in the water and either too high or
too low a temperature.
Downing states that in N. America (Chicago) _H. vulgaris_ breeds in
spring and sometimes as late as December; in Calcutta it has only been
found breeding in February and March. Except during the breeding-season
sexual organs are absent; they do not appear in the same position on the
column in all species.
The spermaries take the form of small mound-shaped projections on the
surface of the column. Each consists of a mass of sperm-mother cells, in
which the spermatozoa originate in large numbers. The spermatozoa
resemble those of other animals, each possessing a head, which is shaped
like an acorn, and a long vibratile tail by means of which it moves
through the water. In the cells of the spermary the spermatozoa are
closely packed together, with their heads pointing outwards towards the
summit of the mound through which they finally make their way into the
water. The aperture is formed by their own movements. Downing (Zool.
Jahrb. (Anat.) xxi, p. 379, 1905) and other authors have studied the
origin of the spermatozoa in great detail.
[Illustration: Fig. 28.--Eggs of _Hydra_ (magnified).
A=egg of _H. vulgaris_ (after Chun). B=vertical section through egg of
_H. oligactis_, form A (after Brauer). C=vertical section through egg of
_H. oligactis_, form B (after Brauer).]
The ovaries consist of rounded masses of cells lying at the base of the
ectoderm. One of these cells, the future egg, grows more rapidly than
the others, some or all of which it finally absorbs by means of lobose
pseudopodia extruded from its margin. It then makes its way by amoeboid
movements between the cells of the ectoderm until it reaches the
surface. In _H. vulgaris_ (Mem. Asiat. Soc. Beng. i, p. 350, 1906) the
egg is first visible with the aid of a lens as a minute star-shaped body
of an intense white colour lying at the base of the ectoderm cells. It
increases in size rapidly, gradually draws in its pseudopodia (the rays
of the star) and makes its way through the ectoderm to the exterior. The
process occupies not more than two hours. The issuing ovum does not
destroy the ectoderm cells as it passes out, but squeezes them together
round the aperture it makes. Owing to the pressure it exerts upon them,
they become much elongated and form a cup, in which the embryo rests on
the surface of the parent. By the time that the egg has become globular,
organic connection has ceased to exist. The embryo is held in position
partly by means of the cup of elongated ectoderm cells and partly by a
delicate film of mucus secreted by the parent. The most recent account
of the oogenesis ("ovogenesis") is by Downing (Zool. Jahrb. (Anat.)
xxvii, p. 295, 1909).
(b) _Budding._
The buds of _Hydra_ arise as hollow outgrowths from the wall of the
column, probably in a definite order and position in each species. The
tentacles are formed on the buds much as the buds themselves arise on
the column. There is much dispute as to the order in which these
structures appear on the bud, and Haacke (Jenaische Zeitschr. Naturwiss.
xiv, p. 133, 1880) has proposed to distinguish two species, _H.
trembleyi_ and _H. roeselii_, in accordance with the manner in which the
phenomenon is manifested. It seems probable, however, that the number of
tentacles that are developed in the first instance is due, at any rate
to some extent, to circumstances, for in the summer brood of _H.
vulgaris_ in Calcutta five usually appear simultaneously, while in the
winter brood of the same form four as a rule do so. Sometimes buds
remain attached to their parents sufficiently long to develop buds
themselves, so that temporary colonies of some complexity arise, but I
have not known this to occur in the case of Indian individuals.
(c) _Fission._
Reproduction by fission occurs naturally but not habitually in all
species of _Hydra_. It may take place either by a horizontal or by a
vertical division of the column. In the latter case it may be either
equal or unequal. If equal, it usually commences by an elongation in one
direction of the circumoral disk, which assumes a narrowly oval form;
the tentacles increase in number, and a notch appears at either side of
the disk and finally separates the column into two equal halves, each of
which is a complete polyp. The division sometimes commences at the base
of the column, but this is very rare. Transverse fission can be induced
artificially and is said to occur sometimes in natural conditions. It
commences by a constriction of the column which finally separates the
animal into two parts, the lower of which develops tentacles and a
mouth, while the upper part develops a basal disk. Unequal vertical
division occurs when the column is divided vertically in such a way that
the two resulting polyps are unequal in size. It is apparently not
accompanied by any great increase in the number of the tentacles, but
probably starts by one of the tentacles becoming forked and finally
splitting down the middle.
The question of the regeneration of lost parts in _Hydra_ cannot well be
separated from that of reproduction by fission. Over a hundred and fifty
years ago Trembley found that if a polyp were cut into several pieces,
each piece produced those structures necessary to render it a perfect
polyp. He also believed that he had induced a polyp that had been turned
inside out to adapt itself to circumstances and to reverse the functions
and structure of the two cellular layers of its body. In this, however,
he was probably mistaken, for there can be little doubt that his polyp
turned right side out while not under his immediate observation. Many
investigators have repeated some of his other experiments with success
in Europe, but the Calcutta _Hydra_ is too delicate an animal to survive
vivisection and invariably dies if lacerated. It appears that, even in
favourable circumstances, for a fresh polyp to be formed by artificial
fission it is necessary for the piece to contain cells of both
cell-layers.
DEVELOPMENT OF THE EGG.
The egg of _Hydra_ is said to be fertilized as it lies at the base of
the ectoderm, through which the fertilizing spermatozoon bores its way.
As soon as the egg has emerged from the cells of its parent it begins to
split up in such a manner as to form a hollow mass of comparatively
large equal cells. Smaller cells are separated off from these and soon
fill the central cavity. Before segmentation begins a delicate film of
mucus is secreted over the egg, and within this film the larger cells
secrete first a thick chitinous or horny egg-shell and within it a
delicate membrane. Development in some cases is delayed for a
considerable period, but sooner or later, by repeated division of the
cells, an oval hollow embryo is formed and escapes into the water by the
disintegration of the egg-shell and the subsequent rupture of the inner
membrane. Tentacles soon sprout out from one end of the embryo's body
and a mouth is formed; the column becomes more slender and attaches
itself by the aboral pole to some solid object.
ENEMIES.
_Hydra_ seems to have few natural enemies. Martin (Q. J. Micr. Sci.
London, lii, p. 261, 1908) has, however, described how the minute worm
_Microstoma lineare_ attacks _Hydra "rubra"_ in Scottish lochs, while
the larva of a midge devours _H. vulgaris_ in considerable numbers in
Calcutta tanks (p. 156).
COELENTERATES OF BRACKISH WATER.
Marine coelenterates of different orders not infrequently make their way
or are carried by the tide up the estuaries of rivers into brackish
water, and several species have been found living in isolated lagoons
and pools of which the water was distinctly salt or brackish. Among the
most remarkable instances of such isolation is the occurrence in Lake
Qurun in the Fayûm of Egypt of _Cordylophora lacustris_ and of the
peculiar little hydroid recently described by Mr. C. L. Boulenger as
_Moerisia lyonsi_ (Q. J. Micr. Sci. London, lii, p. 357, pls. xxii,
xxiii, 1908). In the delta of the Ganges there are numerous ponds which
have at one time been connected with estuaries or creeks of brackish
water and have become isolated either naturally or by the hand of man
without the marine element in their fauna by any means disappearing (p.
14). The following species have been found in such ponds:--
(_a_) _Hydrozoa._
(1) _Bimeria vestita_, Wright (1859).
Hincks, Hist. Brit. Hydr. Zooph. p. 103, pl. xv, fig. 2 (1868);
Annandale, Rec. Ind. Mus. i, p. 141, fig. 3 (1907).
This is a European species which has also been found off S. America. It
occurs not uncommonly in the creeks that penetrate into the Ganges delta
and has been found in pools of brackish water at Port Canning. The
Indian form is perhaps sufficiently distinct to be regarded as a
subspecies. The medusoid generation is suppressed in this genus.
(2) _Syncoryne filamentata_, Annandale (1907).
Annandale, Rec. Ind. Mus. i, p. 139, figs. 1, 2 (1907).
Both hydroid and medusæ were found in a small pool of brackish water at
Port Canning. The specific name refers to the fact that the ends of the
rhizomes from which the polyps arise are frequently free and elongate,
for the young polyp at the tip apparently takes some time to assume its
adult form.
(3) _Irene ceylonensis_, Browne (1905).
Browne, in Herdman's Report on the Pearl Fisheries of
Ceylon, iv, p. 140, pl. iii, figs. 9-11 (1905); Annandale,
Rec. Ind. Mus. i, p. 142, fig. 4 (1907).
The medusa was originally taken off the coast of Ceylon, while the
hydroid was discovered in ponds of brackish water at Port Canning. It is
almost microscopic in size.
The first two of these species belong to the order Gymnoblastea
(Anthomedusæ) and the third to the Calyptoblastea (Leptomedusæ).
(b) _Actinozoa._
(4) _Sagartia schilleriana_, Stoliczka (1869).
_S. schilleriana_, Stoliczka, Journ. As. Soc. Beng. (2)
xxxviii, p. 28, pls. x, xi (1869); _Metridium
schillerianum_, Annandale, Rec. Ind. Mus. i, p. 47, pl. iii
(1907).
This sea-anemone, which has only been found in the delta of the Ganges,
offers a most remarkable instance of what appears to be rapid adaptation
of a species to its environment. The typical form, which was described
in 1869 by Stoliczka from specimens taken in tidal creeks and estuaries
in the Gangetic area and in the ponds at Port Canning, is found attached
to solid objects by its basal disk. The race (subsp. _exul_), however,
that is now found in the same ponds has become elongate in form and has
adopted a burrowing habit, apparently owing to the fact that the bottom
of the ponds in which it lives is soft and muddy.
In addition to these four species a minute hydroid belonging to the
order Gymnoblastea and now being described by Mr. J. Ritchie has been
taken in the ponds at Port Canning. It is a very aberrant form.
FRESHWATER COELENTERATES OTHER THAN HYDRA.
_Hydra_ is the only genus of coelenterates as yet found in fresh water
in India, but several others have been discovered in other countries.
They are:--
(1) _Cordylophora lacustris_, Allman (1843).
Hincks, Hist. Brit. Hydr. Zooph. p. 16, pl. iii, fig. 2
(1868).
This is a branching hydroid that does not produce free medusæ. It forms
bushy masses somewhat resembling those formed by a luxuriant growth of
_Plumatella fruticosa_ (pl. iii, fig. 1) in general appearance. _C.
lacustris_ is abundant in canals, rivers, and estuaries in many parts of
Europe and has recently been found in the isolated salt lake
Birket-el-Qurun in the Fayûm of Egypt.
(2) _Cordylophora whiteleggei_, v. Lendenfeld (1887).
Zool. Jahrb. ii, p. 97 (1887).
A species or race of much feebler growth; as yet imperfectly known and
only recorded from fresh water in Australia.
_Cordylophora_ is a normal genus of the class Hydrozoa and the order
Gymnoblastea; the next four genera are certainly Hydrozoa, but their
affinities are very doubtful.
(3) _Microhydra ryderi_, Potts (1885).
Potts, Q. J. Micr. Sci. London, l, p. 623, pls. xxxv, xxxvi;
Browne, _ibid._ p. 635, pl. xxxvii (1906).
This animal, which has been found in N. America and in Germany,
possesses both an asexual hydroid and a sexual medusoid generation. The
former reproduces its species by direct budding as well as by giving
rise, also by a form of budding, to medusæ that become sexually mature.
The hydroid has no tentacles.
(4) _Limnocodium sowerbii_, Lankester (1880).
Lankester, Q. J. Micr. Sci. London, xx, p. 351, pls. xxx,
xxxi (1880); Fowler, _ibid._ xxx, p. 507, pl. xxxii (1890).
There is some doubt as to the different stages in the life-cycle of this
species. The medusa has been found in tanks in hot-houses in England,
France and Germany, and a minute hydroid closely resembling that of
_Microhydra ryderi_ has been associated with it provisionally.
(5) _Limnocodium kawaii_, Oka (1907).
Oka, Annot. Zool. Japon. vi, p. 219, pl. viii (1907).
Only the medusa, which was taken in the R. Yang-tze-kiang, is as yet
known.
(6) _Limnocnida tanganyikæ_, Bohm (1889).
R. T. Günther, Ann. Nat. Hist. (6) xi, p. 269, pls. xiii,
xiv (1893).
Only the medusa, which is found in Lake Tanganyika, Lake Victoria Nyanza
and the R. Niger, has been found and it is doubtful whether a hydroid
generation exists.
(7) _Polypodium hydriforme_, Ussow (1885).
Morph. Jahrb. xii, p. 137 (1887).
Two stages in this peculiar hydroid, which is found in the R. Volga, are
known, (_a_) a spiral ribbon-like form parasitic on the eggs of the
sterlet (_Acipenser ruthenus_), and (_b_) a small _Hydra_-like form with
both filamentous and club-shaped tentacles. The life-history has not yet
been worked out[AP].
[Footnote AP: Since this was written, Lippen has described a
third stage in the life-history of _Polypodium_ (Zool. Anz.
Leipzig, xxxvii, Nr. 5, p. 97 (1911)).]
II.
HISTORY OF THE STUDY OF HYDRA.
Hydra was discovered by Leeuwenhoek at the beginning of the eighteenth
century and had attracted the attention of several skilful and accurate
observers before that century was half accomplished. Among them the
chief was Trembley, whose "Mémoires pour servir à l'histoire d'un genre
de Polype d'eau douce"* was published at Paris 1744, and is remarkable
not only for the extent and accuracy of the observations it enshrines
but also for the beauty of its plates. Baker in his work entitled "An
attempt towards a natural history of the Polyp"* (London, 1743) and
Rösel von Rosenhof in the third part of his "Insecten-Belustigung"
(Nurenberg, 1755) also made important contributions to the study of the
physiology and structure of _Hydra_ about the same period. Linné
invented the name _Hydra_, and in his "Fauna Sueica" and in the various
editions of his "Systema Naturæ" described several forms in a manner
that permits some of them to be recognized; but Linné did not
distinguish between the true _Hydra_ and other soft sessile
Coelenterates, and it is to Pallas ("Elenchus Zoophytorum," 1766) that
the credit properly belongs of reducing the genus to order. It is a
tribute to his insight that three of the four species he described are
still accepted as "good" by practically all students of the
Coelenterates, while the fourth was a form that he had not himself seen.
In the nineteenth century the freshwater polyp became a favourite object
of biological observation and was watched and examined by a host of
observers, among the more noteworthy of whom were Kleinenberg, Nussbaum,
and Brauer, who has since the beginning of the present century made an
important contribution to the taxonomy of the genus.
BIBLIOGRAPHY OF HYDRA.
_Hydra_ has been examined by thousands of students in biological
laboratories all over the civilized world, and the literature upon it is
hardly surpassed in magnitude by that on any other genus but _Homo_. The
following is a list of a few of the more important general memoirs and
of the papers that refer directly to Asiatic material. A systematic
bibliography is given by Bedot in his "Matériaux pour servir a
l'Histoire des Hydroïdes," Rev. Suisse Zool. xviii, fasc. 2 (1910).
(a) _General._
1743. BAKER, "An attempt towards a natural history of the Polyp"*
(London).
1744. TREMBLEY, "Mémoires pour servir à l'histoire d'un genre de polypes
d'eau douce"* (Paris).
1755. RÖSEL VON ROSENHOF, "Insecten-Belustigung: iii, Hist. Polyporum."
1766. PALLAS, "Elenchus Zoophytorum."
1844. LAURENT, "Rech. sur l'Hydre et l'Eponge d'eau douce" ("Voy. de la
Bonite, Zoophytologie").
1847. JOHNSTON, "A History of the British Zoophytes" (2nd edition).
1868. HINCKS, "History of British Hydroid Zoophytes."
1872. KLEINENBERG, "Hydra. Eine Anatomisch Entwicklungsgeschichtliche
Untersuchung."
1882. JICKELI, "Der Bau der Hydroidpolypen," Morph. Jahrb. viii, p. 373.
1887. NUSSBAUM, "Ueber die Theilbarkeit der lebendigen Materie. II.
Mittheilung. Beiträge zur Naturgeschichte des Genus Hydra," Arch. mikr.
Anat. Bonn, xxix, p. 265.
1891. BRAUER, "Über die Entwicklung von Hydra," Zeitschr. wiss. Zool.
Leipzig, lii, p. 169.
1892. CHUN, "Coelenterata (Hohlthiere)," in Bronn's Thier-Reichs II (2).
1905. DOWNING, "The spermatogenesis of Hydra," Zool. Jahrb. (Anat.) xxi,
p. 379.
1908. BRAUER, "Die Benennung und Unterscheidung der Hydra-Arten," Zool.
Ann. xxxiii, p. 790.
1909. FRISCHHOLZ, "Biologie und Systematik im Genus Hydra," Braun's
Annal. Zool. (Würzburg) iii, p. 105.
1910. BERNINGER, "Über Einwirkung des Hungers auf Hydra," Zool. Anz.
xxxvi, p. 271.
(b) _Asiatic References._
1894. RICHARD, "Sur quelques Animaux inférieurs des eaux douces du
Tonkin (Protozoaires, Rotifères, Entomostracés)," Mém. Soc. zool.
France, vii, p. 237.
1904. VON DADAY, "Mikroskopische Süsswasserthiere aus Turkestan," Zool.
Jahrb. (Syst.) xix, p. 469.
1906. ANNANDALE, "Notes on the Freshwater Fauna of India. No. IV. _Hydra
orientalis_ and its bionomical relations with other Invertebrates," J.
Asiat. Soc. Bengal (new series), ii, p. 109.
1906. ANNANDALE, "The Common _Hydra_ of Bengal: its Systematic Position
and Life History," Mem. As. Soc. Bengal, i, p. 339.
1907. ANNANDALE, "Notes on the Freshwater Fauna of India. No. X. _Hydra
orientalis_ during the Rains," J. Asiat. Soc. Bengal (new series), iii,
p. 27.
1907. ANNANDALE, "Notes on the Freshwater Fauna of India. No. XI.
Preliminary Note on the occurrence of a Medusa (_Irene ceylonensis_,
Browne) in a brackish pool in the Ganges Delta and on the Hydroid Stage
of the species," J. Asiat. Soc. Bengal (new series), iii, p. 79.
1907. WILLEY, "Freshwater Sponge and Hydra in Ceylon," Spolia Zeylan.
Colombo, iv, p. 184.
1908. ANNANDALE, "Observations on specimens of _Hydra_ from Tibet, with
notes on the distribution of the genus in Asia," Rec. Ind. Mus. ii, p.
311.
1910. POWELL, "Lessons in Practical Biology for Indian Students"
(Bombay).
1910. LLOYD, "An Introduction to Biology for Students in India"
(London).
GLOSSARY OF TECHNICAL TERMS USED IN PART II.
_Aboral_ (or _basal_) The disk by means of which a free polyp
_disk_ attaches itself to external objects.
_Cnidoblast_ The living cell of the nematocyst or
nettle-cell (_q. v._).
_Cnidocil_ A minute bristle that projects on the
surface in connection with a nettle-cell
(_q. v._).
_Column_ The upright or potentially upright
part of a polyp (_q. v._).
_Ectoderm_ The external cell-layer of the body-wall.
_Endoderm_ The internal cell-layer of the body-wall.
_Green (chlorophyll) Minute green bodies contained in cells
corpuscles_ of polyps or other animals and
representing a stage in the life-history
of an alga (_Chlorella_).
_Mesogloea_ The intermediate, gelatinous layer of
the body-wall.
_Nettle-cell (nematocyst)_ A cell capsule full of liquid in which
an eversible thread is coiled up.
_Oral disk_ The eminence that surrounds the mouth
and is surrounded by tentacles.
_Peristome_ See "oral disk."
_Polyp_ An individual coelenterate of simple
structure that is fixed temporarily or
permanently by one end of a more
or less cylindrical body and possesses
a mouth at the other end.
_Tentacles_ Filamentous outgrowths (in _Hydra_
hollow) of the body-wall round the
mouth.
LIST OF THE INDIAN HYDRIDA.
Class HYDROZOA.
Order ELEUTHEROBLASTEA.
Family HYDRIDÆ.
Genus HYDRA, _Linné_ (1746).
24. _H. vulgaris_, Pallas (1766).
25. _H. oligactis_, Pallas (1766).
Order ELEUTHEROBLASTEA.
Naked hydrozoa which reproduce their kind by means of buds or eggs, or
by fission, without exhibiting the phenomena of alternation of
generations.
Family HYDRIDÆ.
HYDRAIDÆ, Johnston, Hist. Brit. Zooph. (ed. 2) i, p. 120
(1847).
HYDRIDÆ, Hincks, Hist. Brit. Hydroid. Zooph. p. 309 (1868).
Small Eleutheroblastea in which the mouth is surrounded by hollow
tentacles. Permanent colonies are not formed, but reproduction by
budding commonly takes place.
Genus HYDRA, _Linné_.
TYPE, _Hydra viridis_, Linné.
Freshwater polyps which produce eggs with hard chitinous shells.
Although habitually anchored by the end of the body furthest from the
mouth to extraneous objects, they possess considerable powers of
locomotion. They are extremely contractile and change greatly from time
to time in both form and size.
Only three well-established species of the genus, which is universally
distributed and occurs only in fresh or brackish[AQ] water, can be
recognized, namely, _H. viridis_, Linné (=_H. viridissima_, Pallas), _H.
vulgaris_, Pallas (=_H. grisea_, Linné), and _H. oligactis_, Pallas
(=_H. fusca_, Linné). The two latter occur in India, but _H. viridis_
does not appear to have been found as yet anywhere in the Oriental
Region, although it is common all over Europe and N. America and also in
Japan. The distribution of _H. vulgaris_ is probably cosmopolitan, but
there is some evidence that _H. oligactis_ avoids tropical districts,
although, under the name _Hydra fusca_, it has been doubtfully recorded
as occurring in Tonquin[AR].
[Footnote AQ: A small form of _H. viridis_ (var. _bakeri_,
Marshall) is found in brackish water in England.]
[Footnote AR: Richard, Mém. Soc. zool. France, vii, p. 237
(1894).]
The three species may be distinguished from one another by the following
key:--
[I. Colour leaf-green; the cells contain green
(chlorophyll) corpuscles of definite form.
A. Tentacles comparatively stout, habitually
shorter than the column, which is cylindrical.
Egg-shell without spines, ornamented
with a reticulate pattern _viridis_.]
II. Colour never leaf-green; no chlorophyll
corpuscles present in the cells.
A. Tentacles capable of great elongation but
when the animal is at rest never very much
longer than the column, which is cylindrical
when the gastral cavity is empty.
Largest nettle-cells almost as broad as
long. Egg-shell bearing long spines most
of which are divided at the tips _vulgaris_, p. 148.
B. Tentacles, even when the animal is at rest,
much longer than the column, the basal
part of which, even when the gastral
cavity is empty, is constricted. Largest
nettle-cells considerably longer than
broad. Egg-shell smooth or bearing
short, simple spines _oligactis_, p. 158.
24. Hydra vulgaris, _Pallas_.
Polypes de la seconde espèce, Trembley, Mém. pour servir à
l'histoire d'un genre de polypes d'eau douce*, pl. i, figs.
2, 5; pl. vi, figs. 2, 8; pl. viii, figs. 1-7; pl. xi, figs.
11-13 (1744).
Rösel von Rosenhof, Insecten-Belustigung, iii, Hist.
Polyporum, pls. lxxvi, lxxvii, lxxix-lxxxiii (1755).
? _Hydra polypus_, Linné, Fauna Suecica, p. 542 (1761).
_Hydra vulgaris_, Pallas, Elenchus Zoophytorum, p. 30
(1766).
? _Hydra attenuata_, _id_., _ibid_. p. 32.
_Hydra grisea_, Linné (Gmelin), Systema Naturæ (ed. 13), p.
3870 (1782).
_Hydra pallens_, _id_., _ibid_. p. 3871.
_Hydra vulgaris_, Ehrenberg, Abhandl. Akad. Wiss. Berlin,
1836, p. 134, taf. ii.
_Hydra brunnea_, Templeton, London's Mag. Nat. Hist. ix, p.
417 (1836).
_Hydra vulgaris_, Laurent, Rech. sur l'Hydre at l'Éponge
d'eau douce (Voy. de la Bonite, Zoophytologie), p. 11, pl.
i, pl. ii, figs. 2, 2'' (1844).
_Hydra vulgaris_, Johnston, Hist. British Zoophytes (ed. 2),
i, p. 122, pl. xxix, fig. 2 (1847).
_Hydra vulgaris_, Hincks, Hist. British Hydroid Zoophytes,
i, p. 314, fig. 41 (1868).
_Hydra aurantiaca_, Kleinenberg, Hydra, p. 70, pl. i, fig.
1, pl. iii, fig. 10 (1872).
_Hydra trembleyi_, Haacke, Zool. Anz. Leipzig, ii, p. 622
(1879).
_Hydra grisea_, Jickeli, Morph. Jahrb. viii, p. 391, pl.
xviii, fig. 2 (1883).
_Hydra grisea_, Nussbaum, Arch. mikr. Anat. Bonn, xxix, p.
272, pl. xiii, pl. xiv, figs. 33, 37, 47 (1887).
? _Hydra hexactinella_, v. Lendenfeld, Zool. Jahrb. Jena,
ii, p. 96, pl. vi, figs. 13, 14 (1887).
? _Hydra hexactinella_, _id_., Proc. Linn. Soc. N. S. Wales,
x, p. 678, p. xlviii, figs. 1-4 (1887).
_Hydra grisea_, Brauer, Zeit. wiss. Zool. Leipzig, lii, p.
169 (1891).
_Hydra grisea_, Chun, in Brönn's Thier-Reichs, ii (2), pl.
ii, figs. 2_b_, 2_c_, 5 (1892).
_Hydra grisea_, Downing, Zool. Jahrb. (Anat.) Jena, xxi, p.
381 (1905).
_Hydra orientalis_, Annandale, J. Asiat. Soc. Bengal, (new
series) i, 1905, p. 72.
_Hydra orientalis_, _id._, _ibid._ (new series) ii, 1906, p.
109.
_Hydra orientalis_, _id._, Mem. Asiat. Soc. Bengal, i, p.
340 (1906).
? _Hydra orientalis_, Willey, Spol. Zeylan. Colombo, iv, p.
185 (1907).
_Hydra grisea_, Weltner, Arch. Naturg. Berlin, lxxiii, i, p.
475 (1907).
_Hydra vulgaris_, Brauer, Zool. Anz. xxxiii, p. 792, fig. 1
(1908).
_Hydra orientalis_, Annandale, Rec. Ind. Mus. ii, p. 312
(1908).
_Hydra grisea_, Frischholz, Braun's Zool. Annal. (Würzburg),
iii, pp. 107, 134, &c., figs. 1 and 10-17 (1909).
_Hydra grisea_, _id._, Biol. Centralbl. Berlin, xxix, p. 184
(1909).
_Hydra vulgaris_, Brauer, Die Süsswasserfauna Deutschlands,
xix, p. 192, figs. 336-338 (1909).
_Hydra pentactinella_, Powell, Lessons in Practical Biology
for Indian Students, p. 24 (Bombay, 1910).
Phase _orientalis*_, Annandale.
_Colour_ variable; in summer usually pale, in winter either deep orange,
dull brown, or dark green. The cells do not contain spherical or oval
coloured bodies.
[Illustration: Fig. 29.--_Hydra vulgaris_, from Calcutta (phase
_orientalis_).
A=winter brood; B=summer brood, the same individual in an expanded and a
contracted condition. B is more highly magnified than A.]
_Column_ slender and capable of great elongation, normally almost
cylindrical, but when containing food often shaped like a wine-glass.
The surface is thickly set with nettle-cells the cnidocils of which give
it an almost hirsute appearance under the microscope. When extended to
the utmost the column is sometimes nearly 30 mm. (1-1/5 inches) long,
but more commonly it is about half that length or even shorter.
_Tentacles_ usually 4-6, occasionally 8. They are always slender except
when they are contracted, then becoming swollen at the base and slightly
globular at the tip. If the animal is at rest they are not very much
longer than the body, but if it is hungry or about to move from one
place to another they are capable of very great extension, often
becoming like a string of minute beads (the groups of nettle-cells)
strung on an invisible wire.
_Nettle-cells._ The capsules with barbed threads (fig. 27, p. 131) are
very variable in size, but they are invariably broad in proportion to
their length and as a rule nearly spherical. In a _Hydra_ taken in
Calcutta during the winter the largest capsules measured (unexploded)
0.0189 mm. in breadth and 0.019 in length, but in summer they are
smaller (about 0.012 mm. in breadth). Smaller capsules with barbed
threads always occur. The barbed threads are very long and slender. At
their base they bear a circle of stout and prominent spines, usually 4
in number; above these there are a number of very small spines, but the
small spines are usually obscure. Malformed corpuscles are common. The
capsules with unbarbed threads are very nearly as broad at the distal as
at the proximal end; they are broadly oval with rounded ends.
_Reproductive organs._ The reproductive organs are confined to the upper
part of the body. In India eggs (fig. 28, p. 137) are seldom produced.
They sometimes appear, however, at the beginning of the hot weather. In
form they are spherical, and their shell bears relatively long spines,
which are expanded, flattened and more or less divided at the tip. The
part of the egg that is in contact with the parent-polyp is bare.
Spermaries are produced more readily than ovaries; they are mammillate
in form and number from 4 to 24. Ovaries and spermaries have not been
found on the same individual.
_Buds_ are confined to a narrow zone nearer the base than the apex of
the column. Rarely more than 2 are produced at a time, and I have never
seen an attached bud budding. In winter 5 tentacles are as a rule
produced simultaneously, and in summer 4. In the former case a fifth
often makes its appearance before the bud is liberated.
In Calcutta two broods can be distinguished, a cold-weather brood, which
is larger, stouter, and more deeply coloured, produces buds more freely,
has larger nematocysts, and as a rule possesses 6 tentacles; and a
hot-weather brood, which is smaller, more slender and paler, produces
buds very sparingly, has smaller nematocysts, and as a rule possesses
only 4 or 5 tentacles. Only the cold-weather form is known to become
sexually mature. There is evidence, however, that in those parts of
India which enjoy a more uniform tropical climate than Lower Bengal,
polyps found at all times of year resemble those found in the hot
weather in Calcutta, and sometimes produce spermatozoa or eggs.
I have recently had an opportunity of comparing specimens of the
Calcutta hot-weather form with well-preserved examples of _H. vulgaris_,
Pallas (=_H. grisea_, Linn.), from England. They differ from these
polyps in very much the same way as, but to a greater degree than they
do from the winter phase of their own race, and I have therefore no
doubt that _H. orientalis_ is merely a tropical phase of Pallas's
species. My description is based on Indian specimens, which seem to
differ, so far as anatomy is concerned, from European ones in the
following points:--
(1) The sexes are invariably distinct;
(2) the nematocysts are invariably smaller.
I have seen in Burma an abnormal individual with no tentacles. Its buds,
however, possessed these organs.
TYPE. None of the older types of _Hydra_ are now in existence. That of
_H. orientalis_ is, however, in the collection of the Indian Museum.
GEOGRAPHICAL DISTRIBUTION.--_H. vulgaris_ is common in Europe and N.
America and is probably found all over tropical Asia. The following are
Indian and Ceylon localities:--BENGAL, Calcutta and neighbourhood
(_Annandale_, _Lloyd_); Adra, Manbhum district (_Paiva_), Rampur Bhulia
on the R. Ganges (_Annandale_); Chakradharpur, Chota Nagpur
(_Annandale_); Pusa, Bihar (_Annandale_); Puri, Orissa (_Annandale_):
MADRAS, sea-beach near Madras town (_Henderson_): BOMBAY, island of
Bombay (_Powell_): BURMA, Mandalay, Upper Burma, and Moulmein, N.
Tenasserim (_Annandale_): CEYLON, Colombo and Peradeniya (_Willey_,
_Green_). Dr. A. D. Imms tells me that he has obtained specimens that
probably belong to this species in the Jumna at Allahabad.
BIOLOGY.--In India _H. vulgaris_ is usually found, so far as my
experience goes, in stagnant water. In Calcutta it is most abundant in
ponds containing plenty of aquatic vegetation, and seems to be
especially partial to the plant _Limnanthemum_, which has floating
leaves attached to thin stalks that spring up from the bottom, and to
_Lemna_ (duckweed). Dr. Henderson, however, found specimens in a pool of
rain-water on the sea-shore near Madras.
There is evidence that each of the two broods which occur in Lower
Bengal represents at least one generation; probably it represents more
than one, for tentacles are rarely if ever produced after the animal has
obtained its full size, and never (or only owing to accident) decrease
in number after they have once appeared. The winter form is found
chiefly near the surface of the water, especially on the roots of
duckweed and on the lower surface of the leaves of _Limnanthemum_; but
the summer form affects deeper water in shady places, and as a rule
attaches itself to wholly submerged plants. The latter form is to be met
with between March and October, the cold-weather form between October
and March, both being sometimes found together at the periods of
transition. In the unnatural environment of an aquarium, however,
individuals of the winter form lose their colour and become attenuated,
in these features resembling the summer form, even in the cooler months.
Buds produced in these conditions rarely have more than five tentacles
or themselves produce buds freely after liberation.
The buds appear in a fixed order and position, at any rate on
individuals examined in winter; in specimens of the summer form the
position is fixed, but the order is irregular. Each quadrant of the
column has apparently the power of producing, in a definite zone nearer
the aboral pole than the mouth, a single bud; but the buds of the
different quadrants are not produced simultaneously. If we imagine that
the quadrants face north, south, east, and west, and that the first bud
is produced in the north quadrant, the second will be produced in the
east quadrant, the third in the south, and the fourth in the west. It is
doubtful whether more than four buds are produced in the lifetime of an
individual, and apparently attached buds never bud in this race. The
second bud usually appears before the first is liberated, and this is
also the case occasionally as regards the third, but it is exceptional
for four buds to be present at one time. About three weeks usually
elapse between the date at which the bud first appears as a minute
conical projection on the surface of the parent and that at which it
liberates itself. This it does by bending down, fixing itself to some
solid object by means of the tips of its tentacles, the gland-cells of
which secrete a gummy fluid, and then tearing itself free.
Although it is rare for more than two buds to be produced
simultaneously, budding is apparently a more usual form of reproduction
than sexual reproduction. Individuals that bear eggs have not yet been
found in India in natural conditions, although males with functional
spermaries are not uncommon at the approach of the hot weather. The few
eggs that I have seen were produced in my aquarium towards the end of
the cold weather. Starvation, lack of oxygen, and too high a temperature
(perhaps also lack of light) appear to stimulate the growth of the male
organs in ordinary cases, but perhaps they induce the development of
ovaries in the case of individuals that are unusually well nourished.
The spines that cover the egg retain débris of various kinds upon its
surface, so that it becomes more or less completely concealed by a
covering of fragments of dead leaves and the like even before it is
separated from the polyp. Its separation is brought about by its falling
off the column of the parent. Nothing is known of its subsequent fate,
but probably it lies dormant in the mud through the hot weather. Eggs
are sometimes produced that have no shells. This is probably due to the
fact that they have not been fertilized.
Reproduction by fission occurs rarely in the Indian _Hydra_, but both
equal and unequal vertical fission have been observed. In the case of
equal fission the circumoral area lengthens in a horizontal direction,
and as many extra tentacles as those the polyp already possesses make
their appearance. The mouth then becomes constricted in the middle and
notches corresponding to its constriction appear at either side of the
upper part of the column. Finally the whole animal divides into two
equal halves in a vertical direction. I have only seen one instance of
what appeared to be unequal vertical fission--that of a polyp consisting
of two individuals still joined together by the basal disk, but one
about half the size of the other. Each had three well-developed
tentacles, and in addition a minute fourth tentacle. This was situated
on the side opposed to that of the other individual which bore a similar
tentacle. Transverse fission has not been observed. The Indian _Hydra_
is a very delicate animal as compared with such a form as _H. viridis_,
and all attempts to produce artificial fission without killing the polyp
have as yet failed.
Young individuals are often, and adults occasionally, found floating
free in the water, either with the mouth uppermost and the tentacles
extended so as to cover as large an area as possible or with the aboral
pole at the surface. In the former case they float in mid-water, being
of nearly the same specific gravity as the water, and are carried about
by any movement set up in it. In the latter case, however, the base of
the column is actually attached to some small object such as the cast
skin of a water-flea or to a minute drop of mucus originally given out
by the polyp's own mouth; the tentacles either hang downwards or are
spread out round the mouth, and the animal is carried about by wind or
other agencies acting on the surface.
In addition to this passive method of progression the polyp can crawl
with considerable rapidity. In doing so it bends its column down to the
object along which it is about to move in such a way that it lies almost
parallel to the surface, the basal disk, however, being still attached.
The tentacles are then extended and attach themselves near the tips to
the surface a considerable distance away. Attachment is effected by the
secretion of minute drops of adhesive substance from gland-cells. The
basal disk is liberated and the tentacles contract, dragging the column,
which still lies prone, along as they do so. The basal disk again
affixes itself, the tentacles wrench themselves free, the surface of
their cells being often drawn out in the process into pseudopodia-like
projections, which of course are not true pseudopodia[AS] but merely
projections produced by the mechanical strain. The whole action is then
repeated. The polyp can also pull itself across a space such as that
between two stems or leaves by stretching out one of its tentacles,
fixing the tip to the object it desires to reach, pulling itself free
from its former point of attachment, and dragging itself across by
contracting the fixed tentacle. The basal disk is then turned round and
fixed to the new support.
[Footnote AS: See Zykoff, Biol. Centralbl. xviii, p. 272
(1898), and Annandale, Rec. Ind. Mus. i, p. 67 (1907).]
The Indian polyp, like all its congeners, is attracted by light, but it
is more strongly repelled by heat. Probably it never moves in a straight
line, but if direct sunlight falls on one side of a glass aquarium, the
polyps move away from that side in a much less erratic course than is
usually the case. If conditions are favourable, they often remain in one
spot for weeks at a time, their buds congregating round them as they are
set free. In a natural environment it seems that regular migrations take
place in accordance with changes in temperature, for whereas in cool
weather many individuals are found adhering to the lower surface of the
floating leaves of _Limnanthemum_, few are found in this position
immediately after a rise in the thermometer. If the rise is only a small
one, they merely crawl down the stems to the end of which the leaves are
attached, but as soon as the hot weather begins in earnest, the few that
survive make their way to the deepest and most shady part of the pond.
In captivity the polyps seek the bottom of any vessel in which they are
contained, if sunlight falls on the surface of the water.
The chief function of the tentacles is that of capturing prey. The
Indian polyp feeds as a rule in the early morning, before the day has
become hot. In an aquarium at any rate, the tentacles are never more
than moderately extended during the night. If the polyp is hungry, they
are extended to their greatest length in the early morning, and if prey
is not captured, they sometimes remain in this condition throughout the
day. In these circumstances they hang down or stand up in the water
closely parallel to one another, and often curved in the middle as if a
current were directed against them. Prey that comes in contact with one
of them has little chance of escape, for nematocysts from all the
tentacles can be readily discharged against it. Approximately once in
half an hour the direction of the tentacles is changed, but I have been
unable to observe any regular rhythmical movements of the tentacles or
any correlation between those of a parent polyp and the buds still
attached to it.
The prey consists chiefly of the young larvæ of midges (Chironomidæ) and
may-flies, but small copepod and phyllopod crustacea are also captured.
As soon as the prey adheres firmly to the tentacles and has become
paralysed it is brought to the mouth by their contracting strongly and
is involved in a mass of colourless mucus extruded from the digestive
cavity. Partly by the contraction of muscle-fibres in the body-wall and
partly by movements of the mouth itself assisted by the mucus, which
apparently remains attached to the walls of the cavity, the food is
brought into the mouth. If it is at all bulky, it remains in the upper
part of the cavity, the gland-cells pouring out a digestive fluid upon
it and so dissolving out soluble substances. A large share of the
substances thus prepared falls down to the bottom of the cavity and are
there digested by the endoderm cells. The insoluble parts of the food
are, however, ejected from the mouth without ever reaching the base of
the cavity.
The colour of the polyp appears to be due mainly to the results of
digestion. Brown or orange individuals recently captured in a pond and
kept in favourable conditions take three or four days to digest their
food, and the excreta ejected from the mouth then take the form of a
white flocculent mass. If, however, the same individuals are kept for
long in a glass aquarium, they lose their colour, even though they feed
readily. Digestion is then a much more rapid process, and the excreta
contain minute, irregular, coloured granules, which appear to be
identical with those contained in the endoderm cells of individuals that
have recently digested a meal fully. Starved individuals are always
nearly colourless. It seems, therefore, that in this species colour is
due directly to the products of digestion, and that digestion does not
take place so fully in unfavourable conditions or at a high temperature
as it does in more healthy circumstances. The dark green colour of some
polyps is, however, less easily explained. I have noticed that all the
individuals which have produced eggs in my aquarium have been of this
colour, which they have retained in spite of captivity; whereas
individuals that produced spermatozoa often lost their colour completely
before doing so, sometimes becoming of a milky white owing to the
accumulation of minute drops of liquid in their endoderm cells. Even in
green individuals there is never any trace in the cells of coloured
bodies of a definite form.
The Indian polyp, unlike European representatives of its species, is a
very delicate little animal. In captivity at any rate, three
circumstances are most inimical to its life: firstly, a sudden rise in
the temperature, which may either kill the polyp directly or cause it to
hasten its decease by becoming sexually mature; secondly, the lack of a
free current of air on the surface of the aquarium; and thirdly, the
growth of a bacterium, which forms a scum on the top of the water and
clogs up the interstices between the leaves and stems of the
water-plants, soon killing them. If adult polyps are kept even in a
shallow opaque vessel which is shut up in a room with closed shutters
they generally die in a single night; indeed, they rarely survive for
more than a few days unless the vessel is placed in such a position that
air is moving almost continuously over its surface. The bacterium to
which I allude often almost seals up the aquarium, especially in March
and April, in which months its growth is very rapid. Strands of slime
produced by it surround the polyp and even enter its mouth. In this
event the polyp retracts its tentacles until they become mere
prominences on its disk, and shrinks greatly in size. The colouring
matter in its body becomes broken up into irregular patches owing to
degeneracy of the endoderm cells, and it dies within a few hours.
_Hydra_ in Calcutta is often devoured by the larva of a small midge
(_Chironomus fasciatipennis_, Kieffer) common in the tanks from November
to February. In the early stages of its larval life this insect wanders
free among communities of protozoa (_Vorticella_, _Epistylis_, &c.) and
rotifers on which it feeds, but as maturity approaches begins to build
for itself a temporary shelter of one of two kinds, either a delicate
silken tunnel the base of which is formed by some smooth natural
surface, or a regular tube the base of which is fixed by a stalk
situated near the middle of its length to some solid object, while the
whole surface is covered with little projections. The nature of the
covering appears to depend partly on that of the food-supply and partly
on whether the larva is about to change its skin.
I had frequently noticed that tunnels brought from the tank on the under
surface of _Limnanthemum_ leaves had a _Hydra_ fixed to them. This
occurred in about a third of the occupied shelters examined. The _Hydra_
was always in a contracted condition and often more or less mutilated.
By keeping a larva together with a free polyp in a glass of clean water,
I have been able to observe the manner in which the polyp is captured
and entangled. The larva settles down near the base of its column and
commences to spin a tunnel. When this is partially completed, it passes
a thread round the polyp's body to which it gives a sharp bite. This
causes the polyp to bend down its tentacles, which the larva entangles
with threads of silk, doing so by means of rapid, darting movements; for
the nettle-cells would prove fatal should they be shot out against its
body, which is soft. Its head is probably too thickly coated with chitin
to excite their discharge. Indeed, small larvæ of this very species form
no inconsiderable part of the food of the polyp, and, so far as my
observations go, a larva is always attacked in the body and swallowed in
a doubled-up position.
When the _Hydra_ has been firmly built into the wall of the shelters and
its tentacles fastened down by their bases on the roof, the larva
proceeds, sometimes after an interval of some hours, to eat the body,
which it does very rapidly, leaving the tentacles attached to its
shelter. The meal only lasts for a few minutes; after it the larva
enjoys several hours' repose, protected by remains of its victim, which
retain a kind of vitality for some time. During this period it remains
still, except for certain undulatory movements of the posterior part of
the body which probably aid in respiration. Then it leaves the shelter
and goes in search of further prey. Its food, even when living in a
tunnel, does not consist entirely of _Hydra_. I have watched a larva
building its shelter near a number of rotifers, some of which it
devoured and some of which it plastered on to its tunnel.
The tubular shelters occasionally found are very much stouter structures
than the tunnels, but are apparently made fundamentally of the same
materials; and structures intermediate between them and the tunnels are
sometimes produced. The larva as a rule fastens to them branches
detached from living colonies of Vorticellid protozoa such as
_Epistylis_[AT].
[Footnote AT: Further particulars regarding the life-history
of this larva will be found on pp. 114 and 115, J. Asiat.
Soc. Bengal, ii (n. s.) 1906.]
Of animals living in more or less intimate relations with the polyp, I
have found two very distinct species of protozoa, neither of which is
identical with either of the two commonly found in association with
_Hydra_ in Europe, _Trichodina pediculus_ and _Kerona polyporum_. On two
occasions, one in January and the other at the beginning of February, I
have seen a minute colourless flagellate on the tentacles of the
Calcutta polyp. On the first occasion the tentacles were completely
covered with this protozoon, so that they appeared at first sight as
though encased in flagellated epithelium. The minute organism was
colourless, transparent, considerably larger than the spermatozoa of
_Hydra_, slightly constricted in the middle and rounded at each end. It
bore a long flagellum at the end furthest from its point of attachment,
the method of which I could not ascertain. When separated from the polyp
little groups clung together in rosettes and gyrated in the water. On
the other occasion only a few individuals were observed. Possibly this
flagellate was a parasite rather than a commensal, as the individual on
which it swarmed was unusually emaciated and colourless, and bore
neither gonads nor buds. The larger stinging cells were completely
covered by groups of the organism, and possibly this may have interfered
with the discharge of stinging threads.
The other protozoon was _Vorticella monilata_, Tatem, which has been
found, not in association with _Hydra_, in Europe and S. America. In
Calcutta I have only seen it attached to the column of the polyp, but
probably it would also be found, if carefully looked for, attached to
water-weeds.
Especially in the four-rayed stage, the polyp not infrequently attaches
itself to shells of _Vivipara_, and, more rarely, to those of other
molluscs. It is doubtful whether this temporary association between
_Hydra_ and the mollusc is of any importance to the latter. Even when
the polyp settles on its body and not on its shell (as is sometimes the
case) the _Vivipara_ appears to suffer no inconvenience, and makes no
attempt to get rid of its burden. It is possible, on the other hand,
that the _Hydra_ may protect it by devouring would-be parasites; but of
this there is no evidence[AU].
[Footnote AU: In the Calcutta tanks operculate molluscs such
as _Vivipara_ are certainly more free from visible attack
than non-operculate species. This is the case for instance,
as regards the common aquatic glowworm (_Luciola_ sp.),
which destroys large numbers of individuals of _Limnophysa_,
_Limnæus_, &c. If it has been starved for several days in an
aquarium it will attack an operculate form, but rarely with
success. Similarly _Chætogaster bengalensis_ attaches itself
exclusively to non-operculate forms. In the one case the
polyp could do very little against an adversary with so
stout an integument as the insect, while, in the other, it
is doubtful whether the worm does any harm to its host. The
polyp would afford very little protection against the
snail's vertebrate enemies or against what appears to be its
chief foe, namely, drought. As the water sinks in the tank
non-operculate species migrate to the deeper parts, but
_Vivipara_ and _Ampullaria_ close their shells, remain where
they are, and so often perish, being left high and dry,
exposed to the heat of the sun.]
The association, however, is undoubtedly useful to _Hydra_. The mud on
the shells of _Vivipara_ taken on floating objects shows that in cool
weather the snail comes up from the bottom to the surface, and it
probably goes in the opposite direction in hot weather. Moreover, the
common Calcutta species (_V. bengalensis_) feeds very largely, if not
exclusively, on minute green algæ. It therefore naturally moves towards
spots where smaller forms of animal and vegetable life abound and
conditions are favourable for the polyp. The polyp's means of
progression are limited, and the use of a beast of burden is most
advantageous to it, for it can detach itself when it arrives at a
favourable habitat. If specimens are kept in water which is allowed to
become foul, a very large proportion of them will attach themselves to
any snails confined with them. Under natural conditions they would thus
in all probability be rapidly conveyed to a more suitable environment.
In the tanks it is far commoner to find young four-rayed polyps on
_Vivipara_ than individuals with five or six rays; but the adults of the
species are far less prone to change their position than are the young.
The Calcutta _Hydra_, especially in spring, exhibits a distinct tendency
to frequent the neighbourhood of sponges and polyzoa, such as _Spongilla
carteri_ and the denser forms of _Plumatella_. Possibly this is owing to
the shade these organisms provide.
25. Hydra oligactis, _Pallas_.
Polypes de la troisième espèce, Trembley, Mém. hist.
Polypes,* pl. i, figs. 3, 4, 6; pl. ii, figs. 1-4; pl. iii,
fig. 11; pl. v, figs. 1-4; pl. vi, figs. 3-7, 9, 10; pl.
viii, figs. 8, 11; pl. ix (1744).
Rösel von Rosenhof, Insekt.-Belustigung, iii, Hist. Polyp.,
pls. lxxxiv-lxxxvi (1755).
_Hydra socialis_, Linné, Fauna Sueica, p. 542 (1761).
_Hydra oligactis_, Pallas, Elench. Zooph. p. 29 (1766).
? _Hydra attenuata_, _id._, _ibid._ p. 32.
_Hydra fusca_, Linné, Syst. Nat. (ed. 13), p. 3870 (1782).
_Hydra oligactis_, Johnston, Brit. Zooph. i, p. 124, fig. 27
(p. 120) (1847).
_Hydra oligactis_, Hincks, Hist. Brit. Hydr. Zooph. i, p.
315, fig. 42 (1868).
_Hydra roeselii_, Haacke, Jena Zeitschr. Naturwiss. xiv, p.
135 (1880).
? _Hydra rhætica_, Asper, Zool. Anz. 1880, p. 204, figs.
1-3.
_Hydra vulgaris_, Jickeli (_nec_ Pallas), Morph. Jahrb.
viii, p. 391, pl. xviii, fig. 3 (1882).
_Hydra fusca_, Nussbaum, Arch. mikr. Anat. Bonn, xxix, p.
273, pl. xiv, figs. 34-36, pl. xv, figs. 48-51, &c. (1887).
_Hydra fusca_, Brauer, Zeit. wiss. Zool. Leipzig, lii, p.
177, pl. xi, figs. 2, 5, 6; pl. xii, fig. 6 (1891).
_Hydra_ sp. ? _id._, _ibid._ pl. xi, figs. 3, 3a, 4, 7, 8;
pl. xii, figs. 1, 2, 5-13.
_Hydra fusca_, Chun in Brönn's Thier-Reichs, ii (2), pl. ii,
figs. 2(_a_), 4, 6 (1892).
_Hydra monoecia_, Downing, Science* (5) xii, p. 228.
_Hydra fusca_, _id._, Zool. Jahrb. (Anat.) xxi, p. 382
(1905).
_Hydra dioecia_, _id._, _ibid._ pl. xxiii, figs. 6, 7, &c.
_Hydra fusca_, Hertwig, Biol. Centralbl. xxvi, p. 489
(1906).
_Hydra oligactis_, Brauer, Zool. Anz. xxxiii, p. 792, fig. 2
(1908).
_Hydra polypus_, _id._, _ibid._
_Hydra fusca_, Frischholz, Ann. Zool. (Würzburg), iii, p.
114, figs. 2-9 (1909).
_Hydra oligactis_, Brauer, Süsswasserfauna Deutschl. xix, p.
193, figs. 339-341 (1909).
_Hydra polypus_, _id._, _ibid._ figs. 342-344.
This species differs from _H. vulgaris_ in the following characters:--
(1) Even when the gastral cavity is empty, the basal part of
the column is distinctly more slender than the upper part;
(2) even when the animal is at rest, the tentacles are much
longer than the column;
(3) the nettle-cells of both types are usually smaller and
more uniform in size than in the other species; those with
barbed threads (fig. 27, p. 131) are always flask-shaped and
somewhat narrower in proportion to their length, while those
with simple threads are pointed or almost pointed at their
distal end;
(4) the stinging threads of the more complex form are
comparatively stout and short;
(5) there are comparatively few nettle-cells in the column;
(6) the egg-shell is nearly smooth or covered more or less
completely with short, simple spines (fig. 28, p. 137).
_H. oligactis_ is usually a more vigorous form than _H. vulgaris_ and,
in spite of its name, has often a considerable number of tentacles. The
few Indian specimens examined have, however, been small and have not had
more than six tentacles. I have not seen an Indian specimen with more
than two buds, but European specimens sometimes produce a great many,
and as the daughter buds do not always separate from the parent until
they have themselves produced buds, temporary colonies of some
complexity arise; Chun figures a specimen with nineteen daughter and
granddaughter buds[AV].
[Footnote AV: Pallas writes as regards this "pulcherrime
vegetantem varietatem" with his usual critical insight, "Vix
tamen peculiaris speciei nomine salutanda videtur." It is
probably the _Hydra socialis_ of Linné.]
In Europe and N. America there appear to be two races or phases of the
species. To avoid ambiguity they may be called form A and form B and
described as follows:--
Form A is of vigorous growth. It is as a rule dioecious, and
its reproductive organs may be borne practically at any
level on the surface of the column. Its eggs are spherical
and as a rule covered almost uniformly with spines.
Form B is smaller and has smaller and more variable
nettle-cells. Its reproductive organs are borne only on the
distal third or at the base of its column and it is often
monoecious. The lower surface of its egg is flattened,
adherent, and devoid of spines.
The larger form (A) was originally named _Hydra monoecia_ by Downing,
who in 1904 expressed a wish to substitute for the specific name, which
had been given through inadvertence, the more appropriate one _dioecia_.
As, however, it appears to be the commoner of the two in northern
Europe, we may regard it as probably being the one named _Hydra
oligactis_ by Pallas and therefore may accept it as the _forma typica_
of that species. According to Brauer (1908) the smaller form is Linné's
_Hydra polypus_; but the original description of the "species" hardly
bears out this view. As reproductive organs have not yet been found in
Indian specimens, it is impossible to say to which of the two forms they
belong.
A red form of _H. oligactis_ occurs in Tibet in the lake Rham-tso, at an
altitude of about 15,000 feet and has been reported from various small
lakes in mountainous parts of Europe. It is probably the form called
_Hydra rhætica_ by Asper, but his figures are lacking in detail and
appear to have been drawn from specimens in a state of partial
contraction. _H. rubra_, Lewes (Ann. Mag. Nat. Hist. (3) v, p. 71,
1860), may also be identical with this form. Roux, indeed, states that
_H. rubra_ is only found living unattached at considerable depths (Ann.
Biol. lacustre ii, p. 266, 1907); but this statement does not accord
with the fact that Lewes's specimens were found in ponds on Wimbledon
Common.
TYPE not in existence.
GEOGRAPHICAL DISTRIBUTION.--_H. oligactis_ is widely distributed in
Europe and N. America, but in India has only been found in and near the
city of Lahore in the Punjab.
BIOLOGY.--This species was found by Major J. Stephenson, I.M.S., in the
basin of a fountain at Lahore and in an ornamental canal in the Shalimar
Gardens on the outskirts of the same city. Nothing is known as regards
its habits in this country. In N. America, according to Downing, form B
breeds in September and October and form A from October to December. The
eggs of form B remain attached to the parent until the two cellular
layers are formed and then drop off, whereas those of form A are fixed
by the parent to some extraneous object, its column contracting until
they are in a favourable position for attachment.
The colour of Indian examples of _H. oligactis_ apparently resembles
that of the Calcutta winter brood of _H. vulgaris_ so far as visual
effect is concerned, but I have noticed in specimens from Lahore and the
neighbourhood that very minute spherical bodies of a dark green colour
are present in the endoderm cells.