CHAPTER IV

POLYPS (CUPLIKE ANIMALS)

THE HYDRA, OR FRESH WATER POLYP

[Illustration: FIG. 33.--A HYDRA.]

SUGGESTIONS.--Except in the drier regions of the United States, 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 colorless. 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.

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. 34.--Forms assumed by Hydra.]

[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 thread-like 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 neighboring 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?

=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 dioxid. Blood vessels are unnecessary for transferring oxygen and food from cell to cell.

[Illustration: FIG. 38.--HYDRAS on pondweed.]

=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, or pollen, of the flower unites with the ovules, or female element, 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.

=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?

[Illustration: FIG. 39.--Longitudinal section of hydra (microscopic and diagrammatic).]

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 fibers. 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 labor=”? 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 _ameboid motion_, that is, they extend pseudopods. They also resemble amebas 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.

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. 40.--HYDROID COLONY, with nutritive (_P_) reproductive (_M_) and defensive (_S_) hydranths.]

=The Branch Polyps= (sometimes called _Cœlenterata_).--The hydra is the _only 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_.

[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.]

=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 each other. When each hydranth of the hydroid devoted itself to a special function of digestion, defense, 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 each other.

As the _sponge_ is the first, lowest, and simplest example of the devotion of _special cells to special purposes_, the hydroid is the first, lowest, and simplest example of the occurrence of _organs_, that is of _special parts of the body_ (groups of cells) _set aside for a special work_.

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?

=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.

[Illustration: FIG. 43.--A JELLYFISH.]

[Illustration: FIG. 44.--A JELLYFISH (medusa).]

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, and 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).

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).

[Illustration: FIG. 45.--CORAL POLYPS (tentacles, a multiple of _six_). Notice hypostome.]

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).]

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. 48.--ORGAN PIPE “Coral” (a coralline).]

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. 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.]

[Illustration: FIG. 51.--SEA ANEMONES.]