POLYPS (CUPLIKE ANIMALS)
THE HYDRA, OR FRESH-WATER POLYP
[Illustration:
FIG. 33.—A HYDRA.
]
SUGGESTIONS.—Except in the drier regions of North America the hydra can
usually be found by careful search in fresh-water ponds not too
stagnant. It is found attached to stones, sticks, or leaves, and has a
slender, cylindrical body from a quarter to half an inch long, varying
in thickness from that of a fine needle to that of a common pin. The
green hydra and the brown hydra, both very small, are common species,
though hydras are often white or colourless. They should be kept in a
large glass dish filled with water. They may be distinguished by the
naked eye but are not studied satisfactorily without a magnifying glass
or microscope. Place a living specimen attached to a bit of wood in a
watch crystal filled with water, or on a hollowed slip, or on a slip
with a bit of weed to support the cover glass, and examine with hand
lens or lowest power of microscope. Prepared microscopical sections,
both transverse and longitudinal, may be bought of dealers in
microscopic supplies. One is shown in Fig. 39.
[Illustration:
FIG. 34.—Forms assumed by Hydra.
]
Is the hydra’s =body= round or two-sided? (Fig. 35.) What is its
_general shape_? Does one individual keep the same shape? (Fig. 34.) How
does the length of the threadlike _tentacles_ compare with the length of
the hydra’s body? About how many tentacles are on a hydra’s body? Do all
have the same number of tentacles? Are the tentacles knotty or smooth?
(Fig. 35.) The hydra is usually extended and slender; sometimes it is
contracted and rounded. In which of these conditions is the base (the
foot) larger around than the rest of the body? (Fig. 34.) Smaller? How
many _openings_ into the body are visible? Is there a depression or an
eminence at the base of the tentacles? For what is the _opening_ on top
of the body probably used? Why are the tentacles placed at the top of
the hydra’s body? Does the _mouth_ have the most convenient location
possible?
[Illustration:
FIG. 35.—HYDRA (much enlarged).
]
The conical projection bearing the _mouth_ is called _hypostome_ (Fig.
34). The mouth opens into the _digestive cavity_. Is this the same as
the general body cavity, or does the stomach have a wall distinct from
the _body cavity_? How far down does the body cavity extend? Does it
extend up into the tentacles? (Fig. 39.)
If a _tentacle is touched_, what happens? Is the body ever bent? Which
is more sensitive, the columnar body or the tentacles? In searching
for hydras would you be more likely to find the tentacles extended or
drawn in? Is the hypostome ever extended or drawn in? (Fig. 34.)
=Locomotion.=—The round surface, or disk, by which the hydra is
attached, is called its foot. Can you move on one foot without hopping?
The hydra moves by alternately elongating and rounding the foot. Can you
discover other ways by which it moves? Does the hydra always stand upon
its foot?
[Illustration:
FIG. 36.—NETTLING CELL. II. discharged, and I. not discharged.
]
=Lasso Cells.=—Upon the tentacles (Fig. 35) are numerous cells provided
each with a threadlike process (Fig. 36) which lies coiled within the
cell, but which may be thrown out upon a water flea, or other minute
animal that comes in reach. The touch of the lasso paralyzes the prey
(Fig. 37). These cells are variously called lasso cells, nettling cells,
or thread cells. The thread is hollow and is pushed out by the pressure
of liquid within. When the pressure is withdrawn the thread goes back as
the finger of a glove may be turned back into the glove by turning the
finger outside in. When a minute animal, or other particle of food comes
in contact with a tentacle, how does the tentacle get the food to the
mouth? By bending and bringing the end to the mouth, or by shortening
and changing its form, or in both ways? (Fig. 34, _C_.) Do the
neighbouring tentacles seem to bend over to assist a tentacle in
securing prey? (Fig. 34, _C_.)
[Illustration:
FIG. 37.—HYDRA capturing a water flea.
]
=Digestion.=—The food particles break up before remaining long in the
stomach, and the nutritive part is absorbed by the lining cells, or
endoderm (Fig. 39). The indigestible remnants go out through the mouth.
The hydra is not provided with a special vent. Why could the vent not be
situated at the end opposite the mouth?
[Illustration:
FIG. 38.—HYDRAS on the under surface of pondweed.
]
=Circulation and Respiration.=—Does water have free access to the body
cavity? Does the hydra have few or nearly all of its cells exposed to
the water in which it lives? From its structure, decide whether it can
breathe like a sponge or whether special respiratory cells are necessary
to supply it with oxygen and give off carbon dioxide. Blood vessels are
unnecessary for transferring oxygen and food from cell to cell.
=Reproduction.=—Do you see any swellings upon the side of the hydra?
(Fig. 34, A.) If the swelling is near the tentacles, it is a _spermary_;
if near the base, it is an _ovary_. A sperm coalesces with or fertilizes
the ovum after the ovum is exposed by the breaking of the ovary wall.
Sometimes the sperm from one hydra unites with the ovum of another
hydra. This is called _cross-fertilization_. The same term is applied to
the process in plants when the male element, developed in the pollen of
the flower, unites with the female element of the ovule of the flower on
another plant. The hydra, like most plants and some other animals, is
hermaphrodite, that is to say, both sperms and ova are produced by one
individual. In the autumn, eggs are produced with hard shells to
withstand the cold until spring. Sexual reproduction takes place when
food is scarce. Asexual generation (by budding) is common with the hydra
when food supply is abundant. After the bud grows to a certain size, the
outer layer of cells at the base of the bud constricts and the young
hydra is detached.
[Illustration:
FIG. 39.—Longitudinal section of hydra (microscopic and diagrammatic).
]
=Compare the sponge and the hydra= in the following respects:—many
celled, or one-celled; obtaining food; breathing; tubes and cavities;
openings; reproduction; locomotion. Which ranks higher among the
metazoa? The metazoa, or many-celled animals, include all animals except
which branch?
Figure 39 is a _microscopic view_ of a vertical section of a hydra to
show the =structure of the body wall=. There is an outer layer called
the _ectoderm_, and an inner layer called the _endoderm_. There is
also a thin supporting layer (black in the figure) called the
_mesoglea_. The mesoglea is the thinnest layer. Are the cells larger
in the endoderm or the ectoderm? Do both layers of cells assist in
forming the reproductive bud? The ectoderm cells end on the inside in
contractile tails which form a thin line and have the effect of muscle
fibres. They serve the hydra for its remarkable changes of shape. When
the hydra is cut in pieces, each piece makes a complete hydra,
provided it contains both endoderm and ectoderm.
In what ways does the hydra show “=division of labour=”? Answer this
by explaining the classes of cells specialized to serve a different
purpose. Which cells of the hydra are least specialized? In what
particulars is the plan of the hydra different from that of a simple
sponge? An ingenious naturalist living more than a century ago,
asserted that it made no difference to the hydra whether the ectoderm
or the endoderm layer were outside or inside,—that it could digest
equally well with either layer. He allowed a hydra to swallow a worm
attached to a thread, and then by gently pulling in the thread, turned
the hydra inside out. More recently a Japanese naturalist showed that
the hydra could easily be turned inside out, but he also found that
when left to itself it soon reversed matters and returned to its
natural condition, that the =cells are really specialized= and each
layer can do its own work and no other.
=Habits.=—The hydra’s whole body is a hollow bag, the cavity extending
even into the tentacles. The tentacles may increase in number as the
hydra grows but seldom exceed eight. The hydra has more active motion
than locomotion. It seldom moves from its place, but its tentacles are
constantly bending, straightening, contracting, and expanding. The body
is also usually in motion, bending from one side to another. When the
tentacles approach the mouth with captured prey, the mouth (invisible
without a hand lens) opens widely, showing five lobes or lips, and the
booty is soon tucked within. A hydra can swallow an animal larger in
diameter than itself.
The =endoderm= cells have _amœboid motion_, that is, they extend
pseudopods. They also resemble amœbas in the power of _intra-cellular
digestion_; that is, they absorb the harder particles of food and digest
them afterwards, rejecting the indigestible portions. Some of these
cells have _flagella_ (see Fig. 39) which keep the fluid of the cavity
in constant motion.
Sometimes the =hydra moves= after the manner of a small caterpillar
called a “measuring worm,” that is, it takes hold first by the foot,
then by the tentacles, looping its body at each step. Sometimes the body
goes end over end in slow somersaults.
[Illustration:
FIG. 40.—HYDROID COLONY, with nutritive (_P_) reproductive (_M_) and
defensive (_S_) hydranths.
]
The _length_ of the extended hydra may reach one half inch. When
touched, both tentacles and body contract until it looks to the unaided
eye like a round speck of jelly. This shows _sensibility_, and a few
small star-shaped cells are believed to be _nerve cells_, but the hydra
has not a nervous _system_. Hydras show their liking for light by moving
to the side of the vessel or aquarium whence the light comes.
[Illustration:
FIG. 41.—“PORTUGUESE MAN-O’-WAR” (compare with Fig. 40). A floating
hydroid colony with long, stinging (and sensory) streamers.
Troublesome to bathers in Gulf of Mexico. Notice balloon-like float.
]
=The Branch Polyps= (sometimes called _Cœlenterata_).—The hydra is the
chief _fresh-water representative_ of this great branch of the animal
kingdom. This branch is characterized by its members having only one
opening to the body. The polyps also include the salt water animals
called _hydroids_, _jellyfishes_, and _coral polyps._
=Hydroids.=—Figure 40 shows a _hydroid_, or group of hydra-like growths,
one of which eats and digests for the group, another defends by nettling
cells, another produces eggs. Each hydra-like part of a hydroid is
called a _hydranth_. Sometimes the buds on the hydra remain attached so
long that a bud forms upon the first bud. Thus three generations are
represented in one organism. Such growths show us that it is not always
easy to tell what constitutes an individual animal.
[Illustration:
FIG. 42.—The formation of many free-swimming jellyfishes from one
fixed hydra-like form. The saucer-like parts (_h_) turn over after
they separate and become like Fig. 43 or 44. Letters show sequence
of diagrams.
]
_Hydroids_ may be conceived _to have been developed_ by the failure of
budding hydras to separate from the parent, and by the gradual formation
of the habit of living together and assisting one another. When each
hydranth of the hydroid devoted itself to a special function of
digestion, defence, or reproduction, this group lived longer and
prospered; more eggs were formed, and the habits of the group were
transmitted to a more numerous progeny than were the habits of a group
where members worked more independently of one another.
As the _sponge_ is a simple example of the devotion of _special cells to
special purposes_, the hydroid is a primitive and simple example of the
occurrence of _organs_, that is of _special parts of the body set aside
for a special work_.
[Illustration:
FIG. 43.—A JELLYFISH.
]
How many mature hydranths are seen in the hydroid shown in Fig. 40? Why
are the defensive hydranths on the outside of the colony? Which
hydranths have no tentacles? Why not?
[Illustration:
FIG. 44.—A JELLYFISH (medusa).
]
=Jellyfish.—Alternation of Generations.—Medusa.=—With some species of
hydroids, a very curious thing happens.—The _hydranth that is to produce
the eggs falls off_ and becomes independent of the colony. More
surprising still, its appearance changes entirely and instead of being
hydra-like, it becomes the large and complex creature called _jellyfish_
(Fig. 43). But the _egg of the jellyfish_ produces a small _hydra-like
animal_ which gives rise by budding to a _hydroid_, and the cycle is
complete.
The bud (or reproductive hydranth) of the hydroid does not produce a
hydroid, but a jellyfish; the egg of the jellyfish does not produce a
jellyfish, but a hydroid. This is called by zoologists, _alternation of
generations_. A _complete individual_ is the life from the germination
of one egg to the production of another. So that an “individual”
consists of a hydroid colony fixed in one place together with all the
jellyfish produced from its buds, which may now be floating miles away
from it in the ocean. Bathers in the surf are sometimes touched and
stung by the long, streamer-like tentacles of the jellyfish. These, like
the tentacles of the hydra, have nettling cells (Fig. 41).
[Illustration:
FIG. 45.—CORAL POLYPS (tentacles, a multiple of _six_). Notice
hypostome.
]
The umbrella-shaped free-swimming jellyfish is called a _medusa_ (Fig.
44).
=Coral Polyps.=—Some of the salt water relatives of the hydra produce
buds which remain attached to the parent without, however, becoming
different from the parent in any way. The _coral polyps_ and
_corallines_ are examples of _colonies_ of this kind, possessing a
common stalk which is formed as the process of multiplication goes on.
In the case of coral polyps, the separate animals and the flesh
connecting them secrete within themselves a hard, _limy, supporting
structure known as coral_. In some species, the coral, or stony part, is
so developed that the polyp seems to be inserted in the coral, into
which it withdraws itself for partial protection (Fig. 45).
The _corallines_ secrete a smooth stalk which affords no protection, but
they also secrete a coating or sheath which incloses both themselves and
the stalk. The coating has apertures through which the polyps protrude
in order to feed when no danger is near (Fig. 46). The red “corals” used
for jewelry are bits of stalks of corallines. The corallines (Figs. 47,
48) are not so abundant nor so important as the coral polyps (Figs. 45,
49).
[Illustration:
FIG. 46.—RED CORALLINE with crust and polyps (_eight_ tentacles).
]
[Illustration:
FIG. 47.—SEA FAN (a coralline).
]
[Illustration:
FIG. 48.—ORGAN PIPE “Coral” (a coralline).
]
Colonies of coral polyps grow in countless numbers in the tropical seas.
The coral formed by successive colonies of polyps accumulates and builds
up many islands and important additions to continents. The Florida
“keys,” or islands, and the southern part of the mainland of Florida
were so formed.
[Illustration:
FIG. 49.—UPRIGHT CUT through coral polyp × 4.
_ms_, mouth; _mr_, gullet; _ls_, _ls_, fleshy partitions (mesenteries)
extending from outer body wall to gullet (to increase absorbing
surface); _s_, _s_, shorter partitions; _mb_, _fb_, stony support
(of lime, called coral); _t_, tentacles.
]
[Illustration:
FIG. 50.—SEA ANEMONE.
]
The =Sea Anemone=, like the coral polyp, lives in the sea, but like the
fresh-water hydra, it _deposits no limy support for its body_. The
anemone is much larger than the hydra and most coral polyps, many
species attaining a height of several inches. It _does not form
colonies_. When its arms are drawn in, it looks like a large knob of
shiny but opaque jelly. Polyps used to be called _zoophytes_
(_plant-animals_), because of their flower-like appearance (Figs. 50,
51).
[Illustration:
FIG. 51.—SEA ANEMONES.
]