Elementary Zoology Part 6

FOOTNOTE:

[7] In some Protozoa a number of similar cells temporarily unite to form a colony, but each cell may still be regarded as an individual animal.

CHAPTER XVI

BRANCH PORIFERA: THE SPONGES

THE FRESH-WATER SPONGE (_Spongilla_ sp.)

TECHNICAL NOTE.--Fresh-water sponges may perhaps not be readily found in the neighborhood of the school, but they occur over most of the United States, and careful searching will usually result in the finding of specimens. They are compact, solid-looking ma.s.ses, sometimes lobed, resting on and attached to rocks, logs, timbers, etc., in clear water in creeks, ponds, or bayous. They are creamy, yellowish-brown or even greenish in color and resemble some cus.h.i.+on-like plant far more than any of the familiar animal forms.

They can be distinguished from plants, however, by the fact that there are no leaves in the ma.s.s, nor long thread-like fibres such as compose the ma.s.ses of pond algae (pond sc.u.m). When touched with the fingers a gritty feeling is noticeable, due to the presence of many small stiff spicules. Sponges should be removed entire from the substance they are attached to, and may be taken alive to the laboratory. They die soon, however, and should be put into alcohol before decay begins.

Note the form of the sponge ma.s.s. Is it lobed or branched? Examine the surface for openings. These are of two sizes; the larger are _osteoles_ or _exhalant openings_, while the smaller and more numerous are _pores_ or _inhalant openings_. The sponge-flesh is called _sarcode_. Examine a bit of sarcode under the microscope; note the _spicules_. Have these spicules a regular arrangement? Of what are they composed?

Draw the entire sponge, showing shape and openings; draw some of the spicules.

Embedded in the body-substance, especially near the base, note (if present) numerous small, yellowish, sub-spherical or disk-like bodies, the _gemmules_. These are reproductive bodies. Each gemmule is a sort of internal bud. It is composed of an interior group of protoplasmic cells, enclosed by a crust thickly covered with spicules.

In winter the sponge dies down and the gemmules are set free in the water. In spring the protoplasmic contents issue through an aperture in the crust, called the _micropyle_ or _foraminal opening_, and develop and grow into a new sponge.

For a good account of the fresh-water sponge, see Pott's "Fresh-water Sponges."

A CALCAREOUS OCEAN-SPONGE (_Grantia_ sp.) (fig. 7, D, E, F.)

TECHNICAL NOTE.--For inland schools, specimens preserved in alcohol or formalin must be used. They may be obtained from dealers in naturalists' supplies (see p. 453). Specimens of some species of this genus can be obtained at almost any point on the Atlantic or Pacific coasts of this country.

Examine the external structure of a specimen. Note the elongate, sub-cylindrical form, the attached base, the free end. Note the large exhalant opening, _osteole_ or _osculum_, at the free end; the numerous small inhalant openings elsewhere on the surface (best seen in dried specimens). Note the _spicules_ covering the surface of the body, and the longer ones surrounding the osculum. Cut the sponge in two longitudinally and note the simple cylindrical body-cavity, the _gastric cavity_ or _cloaca_. Note the thickness of the body-wall; note the tubes running through the body-wall from cloaca to external surface. Through these tubes water laden with food enters the gastric cavity, where the food is digested, the water and undigested particles pa.s.sing out through the osculum. Crush a bit of dried sponge, or boil a bit of soft sponge in caustic potash and mount on a gla.s.s slide.

Examine under a microscope and note the abundance of spicules and the variety in their form. Two kinds may always be found, and sometimes three. These spicules are composed of carbonate of lime and can be dissolved by pouring on to them a drop of hydrochloric acid.

Some of the sponges may have buds growing out from them near the base.

These buds are young sponges developed as.e.xually. If allowed to develop fully the buds would have detached themselves from the parent and each would have become a new sponge.

Make drawings showing the form of a whole sponge; the appearance of the inner face of the sponge bisected longitudinally; the shape of the spicules.

A COMMERCIAL SPONGE

TECHNICAL NOTE.--For the study of the skeleton of an ocean-sponge with more complex body buy several common small bath-sponges without large holes running entirely through them. The teacher should have also a few specimens of small marine sponges preserved in alcohol or formalin. Such specimens should be part of the laboratory equipment (see account of laboratory equipment, p. 450), and can be readily and cheaply obtained from dealers in naturalists' supplies.

The bath-sponge or slate-sponge consists simply of the hard parts or skeleton of a sponge animal. In life all of the skeleton is enclosed or covered by a soft, tough ma.s.s composed of layers of cells. Note the many openings on the surface of the sponge. Crush a bit of the skeleton and examine it under the microscope. Note that it is composed of fine fibres of a tough, h.o.r.n.y substance called _spongin_, instead of tiny distinct calcareous spicules.

OTHER SPONGES

The sponges are fixed, plant-like aquatic animals. The members of a single family live in fresh water, being found in lakes, rivers, and ca.n.a.ls in all parts of the world. All the other sponges, and there are several thousand species known, live in the ocean. They are to be found at all depths, some in shallow water near the sh.o.r.e and others in deeper water, even to the deepest depths yet explored. They are found in all seas, though especially abundantly in the Atlantic Ocean and Mediterranean Sea.

=Form and size.=--The shape of the simplest sponges is that of a tiny vase or nearly cylindrical cup, hollow and attached at its base. At the free end there is a large opening. But there is a great deal of variety in the form and size of different sponges. There is, indeed, much variation in the shape and general character of different individuals of the same species. Unlike most other animals, sponges are fixed, and the character of the surface to which a sponge is attached has much influence upon its shape. If this surface is rough and uneven the sponge may follow in its growth the sinuosities of the surface and so become uneven and distorted in shape. At best, only a few kinds of sponges have any very even and symmetrical shape. Most of them are very unsymmetrical and grow more like a low compact bushy plant than like the animals we are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high, while the largest may be over a meter (39 in.) in height. In color living sponges may be red, purple, orange, gray, and sometimes blue.

Most sponges have the whole body of one color.

[Ill.u.s.tration: FIG. 12.--The skeleton of a "gla.s.s" sponge (skeleton composed of siliceous spicules) from j.a.pan. (From specimen.)]

=Skeleton.=--A very few sponges have no skeleton at all. The others have a skeleton or hard parts composed of interwoven fibres of the tough, h.o.r.n.y substance called spongin, or of hosts of fine needles or spicules of silica or of carbonate of lime. The siliceous skeletons of some of the so-called gla.s.s-sponges (fig. 12) are very beautiful. The lime and siliceous sponge spicules exhibit a great variety of outline, some being anchor-shaped, some cross-shaped, and some resembling tiny spears or javelins.

=Structure of body.=--The skeleton of a sponge whether composed of interlacing fibres or of short spicules is always invisible from the outside when the sponge is alive. It is embedded in, or clothed by, the soft, fleshy part of the body. The soft part of the sponge is composed simply of two layers of cells, one const.i.tuting the external surface of the body, and the other lining the interior cavities and ca.n.a.ls of the body. Between these two cell-layers there is a ma.s.s of soft gelatinous substance all through which protoplasm ramifies, and in which are embedded numerous scattered cells. There are, as seen in the case of _Spongilla_ and _Grantia_, no systems of organs such as characterize the higher animals. No heart, lungs, alimentary ca.n.a.l, nervous system, organs of locomotion, eyes, ears, or other organs of special sense; the sponge has none of these. It is simply an aggregate of cells, arranged in two layers, and supported usually by a skeleton of h.o.r.n.y fibres or calcareous or siliceous spicules. Its body is usually shapeless, unsymmetrical and without front or back, right or left. It is not to be wondered at that sponges were for a long time believed to be plants.

=Feeding habits.=--The sponges feed on minute bits of animal or plant substance and on the microscopic unicellular plants or animals which float in the water which bathes their bodies. The water entering the sponge-body through the various openings of the surface is moved along by the waving or las.h.i.+ng of the flagella of the cells which line the ca.n.a.ls, and these currents of water bear with them the tiny organisms which are taken up by these same cells and digested. The incoming currents of water meet in the central cavity or cavities of the body and pa.s.s out through the large opening called the osculum at the free end of the vase-like body, or if the body is branched, through the large openings at the tips of these branches.

The same currents of water bring also oxygen for the sponge's breathing and carry away the carbonic acid gas given out by the body-cells.

As a German naturalist has said, the one necessary condition for the life of a sponge is the streaming of water through its body. All sponges have a system of ca.n.a.ls for this water-current and all have means, in the waving flagella or cilia with which these ca.n.a.ls are lined, for producing these currents. When a live sponge is put into a vessel of water, currents are immediately set up, and they always flow into the body through the many fine openings and out of the body through the osculum.

=Development and life-history.=--Although the sponge in its adult condition is permanently attached by its base to the sea-bottom or to some rock or sh.e.l.l, when it is first born it is an active free-swimming creature. The sponges reproduce in two ways, as.e.xually and s.e.xually. The as.e.xual mode of reproduction of the fresh-water sponge by gemmules has already been described. The ocean sponges also reproduce as.e.xually either by forming interior gemmules or external buds. In this latter method a bud forms on the outer surface of the body which increases in size and finally grows into a new sponge individual. In some species this new sponge does not become separated from the body of the mother, but remains attached to it like a branch to a tree-trunk. By the continued production of such non-separating individuals, a colony of sponges is formed which has the general appearance of a branching plant.

In other species the new sponge formed by the development and growth of a bud falls off and becomes a distinct separate individual.

In the s.e.xual mode of reproduction, male or sperm-cells and female or egg-cells are developed in the same individual. The sperm-cells are motile and swim about in the cavities and ca.n.a.ls of the sponge-body until they find egg-cells, which they fertilize. The fertilized eggs begin to develop and pa.s.s through their first stages in the sponge-body. Finally the embryo sponge, which is usually a tiny oval or egg-shaped ma.s.s of cells, escapes from the body of the parent into the water. The young sponge has some of its outer cells provided with cilia, and by means of these it swims about. After a while it comes to rest on the ocean-floor or on some rock or sh.e.l.l, attaches itself, and begins to take on the form and character of the parent. It leads hereafter a fixed sedentary life.

=The sponges of commerce.=--The sponge-skeletons which are the "sponges" that we use all belong to a few species, not more than half a dozen. Most of the commercial sponges come from the Mediterranean Sea, though some come from the Bahama Islands, some from the Red Sea, and a few from the coasts of Greece, Asia Minor, and Africa. The commercial sponges do not live in very deep water; they are usually found not deeper than 200 feet. The living sponges are collected by divers, or are dragged up by men in boats using long-poled hooks, or dredges. "When secured they are exposed to the air for a limited time, either in the boats or on sh.o.r.e, and then thrown in heaps into the water again in pens or tanks built for the purpose. Decay thus takes place with great rapidity, and when fully decayed they are fished up again, and the animal matter beaten, squeezed, or washed out, leaving the cleaned skeleton ready for the market. In this condition after being dried and sorted, they are sold to the dealers, who have them trimmed, re-sorted and put up in bales or on strings ready for exportation. There are many modifications of these processes in different places, but in a general way these are the essential-steps through which the sponge pa.s.ses before it is considered suitable for domestic purposes. Bleaching-powders or acids are sometimes used to lighten the color, but these unless very delicately handled injure the durability of the fibres."

=Cla.s.sification.=--The sponges are cla.s.sified according to the character of the skeleton. In one group are put all those sponges which have a skeleton of calcareous spicules, and this group is called the Calcarea.

All other sponges are grouped as Non-Calcarea, the members of this group either having no skeleton at all, or having a skeleton composed of siliceous spicules or of spongin fibres. According to the absence or presence of a skeleton and the character of the skeleton when it exists the Non-Calcarea are subdivided into smaller groups.

CHAPTER XVII

BRANCH CLENTERATA: THE POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES

The structure and life-history of an example of the polyps (the Fresh-water Hydra, _Hydra_ sp.) has been studied in Chapters X and XI.

OTHER POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES

TECHNICAL NOTE.--The teacher should have, if possible, several pieces of coral and a few specimens of Clenterates in alcohol or formalin, which will show the external character, at least, of these animals (see account of laboratory equipment, p. 450). If the school is on the coast, the pupils should be shown the sea-anemones of the tide-pools.

The animals which are included in the branch Clenterata are, at least in living condition, unfamiliar to most of us. Like the sponges, they are almost all inhabitants of the ocean; a few, like _Hydra_, live in fresh water. Like the sponges, too, most of the members of this branch are fixed, and in their general appearance suggest a plant rather than an animal. The name zoophytes, or plant-animals, which is often applied to these animals is based on this superficial resemblance. But many of the Clenterates lead an active free-swimming life. This is true of the jellyfishes which float or swim about on or near the surface of the ocean. Many of the zoophytes spend part of their life in an active free-swimming condition before settling down, becoming attached and thereafter remaining fixed. In localities near the seash.o.r.e many animals belonging to this great group can be readily found and observed. The beautiful sea-anemones with their slowly-waving tentacles, the fine many-branched truly plant-like hydroids with their hosts of little buds, and the soft colorless ma.s.ses of jelly, the jellyfishes, which are cast up on to the beaches by the waves are all animals belonging to the branch Clenterata.

=General form and organization of body.=--The general or typical plan of body-structure for the Clenterata, these animals which come next to the sponges in degree of complexity, can best be understood by imagining the typical cylindrical or vase-like body of the simple sponges to be modified in the following way: The middle one of the three layers of the body-wall not to be composed of scattered cells in a gelatinous matrix, but to be simply a thin non-cellular membrane; the body-wall not to be pierced by fine openings or pores, but connected with the outside only by the single large opening at the free end, and this opening to be surrounded by a circlet of arm-like processes or tentacles, which are continuations of the body-wall and similarly composed. Such a body-structure, which we saw well shown by _Hydra_, is the fundamental one for all polyps, sea-anemones, corals, and jellyfishes. The variety in shape of the body and the superficial modifications of this type-plan are many and striking, but after all the type-plan is recognizable throughout the whole of this great group of animals.

The two chief body-shapes represented in the branch are those of the polyps on the one hand, and the jellyfishes or medusae on the other. The polyp-shape is that of a tube with a basal end blind or closed, attached to some firm object in the water and with the free end with an opening, the mouth-opening. At this mouth-end there is a circlet of movable, very contractile tentacles. The mouth may open directly into the interior of the body, which interior may be called the digestive cavity, or it may lead into a simple short tube produced by the inv.a.g.i.n.ation or bending in of the body-wall, which may be looked on as the simplest kind of sophagus. This sophageal tube opens into the body-cavity or digestive cavity. This cavity may be incompletely divided by longitudinal part.i.tions which project from the sides into the cavity.

The jellyfish or medusoid body-form corresponds in general to an umbrella or bell. Around the edge of this umbrella are disposed numerous threads or tentacles (corresponding to the circlet of tentacles in the polyp). The mouth-opening is at the end of a longer or shorter projection which hangs down from the middle of the under side of the umbrella. The interior body-cavity or digestive cavity extends out into the umbrella-shaped part of the body, usually in the condition of ca.n.a.ls radiating from the centre and a connecting ca.n.a.l running around the margin of the umbrella.

=Structure.=--Although the Clenterata show little indication of the complex composition of the body out of organs, as it exists among the higher animals, yet they do show an unmistakable advance on the simple, almost organless body of the sponges. This is chiefly shown by the differentiation among the cells which compose the body. In the polyps and jellyfishes some of the cells are specialized to be unmistakable muscle-cells, some to be nerve-cells and fibres, and so on. A very simple nervous system consisting of small groups of nerve-cells connected by nerve-fibres exists. Some very simple special sense-organs may occur. The digestive system, although in the simpler Clenterates consisting merely of the cylindrical body-cavity enclosed by the body-wall and opening by the single hole at the free end of the body, in some is rather complex and is composed of different parts. Those Clenterates which are not fixed but lead an active, free-swimming life, viz., the jellyfishes or medusae, are the most highly organized.