CHAPTER IV.
ZOOPHYTES.
Not many years ago our knowledge of the lower forms of life was very imperfect, and it was believed that the gulf between the animal and vegetable kingdoms was bridged over by certain creatures which could not properly be classed in either, because they appeared to unite the characters and organization of each. Such was the case with the sponges, already dealt with, and with the creatures now to be considered. These last were on that account called Zoophytes, or animal plants, a term which we must render to-day as plant-like animals. Some of us have again got to the notion that there is no sharp division between animal and plant-life; but with increased knowledge we have put back the debatable or common ground much lower in the scale of life.
With the whole of the families included in this division of life, I do not propose to deal in the present chapter: the Sea-Anemones and the Sea-Jellies, for instance, being treated in succeeding chapters, for each group deserves and demands a chapter to itself. It is characteristic of the Zoophytes that they form a bag of jelly-like material, with an opening at one end which may be regarded as a mouth, though it is without tongue or teeth, and opens directly into the stomach. Around this mouth are set a number of limb-like organs, called tentacles, which are used for seizing the prey and conveying it within the orifice. Their entire structure is very simple, and apart from primitive muscular and nervous systems, and the possession of stinging threads, which can be quickly extruded through the exterior walls of the body, they appear to be almost innocent of organs. This form of structure is generally referred to as a Polypite, and its appearance has been made familiar by the descriptions and figures of the Hydra or Polyp of our stagnant fresh-water ponds. From their general agreement in structure with the Hydra, the creatures to which much of this chapter will be devoted, are called Hydroid Zoophytes. There are, however, but few species that occur solitarily, like the Hydra. In most cases they are associated in inseparable colonies. The egg of a zoophyte gives rise, it is true, to an organism resembling Hydra, but this individual does not long remain solitary; it produces many buds, which rapidly develop, and in turn produce other buds, so that before long there is a colony that may number its thousands of polypites. However numerous the individuals may be, we may be sure that the colony has been the production of a single egg. One came from that egg, but all the others were produced vegetatively by budding from the original polypite, or as later generations from such bud-originated polyps.
A slight examination of such a colony will show that the polypites themselves are held in association by an investing substance (cœnosarc), which takes the form of a living tube of thin flesh, which adheres to rock or shell or seaweed, acting as a support for the community, and also reproducing the polypites. It consists of two distinct layers, an inner and an outer, and sometimes there is a third layer of a different kind between these two, muscular in character. In most cases the outer wall of this tube secretes a sheath of a substance called chitin, of which the external skeletons of insects are composed. This sheath is known as the polypary, because into it the individual polypite withdraws itself. It is this polypary that the seaside visitor finds attached to weeds or shells, and concludes, from its moss-like aspect, it is a seaweed, and probably adds it to his collection as such.
Now if we get down among the rocks near low-water, and look among the coarse brown weeds, we shall not look long before we find one whose stem and parts of the frond are covered with a plantation of erect-growing “somethings,” that look like the backbones of some small fishes. They are only about an inch in height, very slender, and regularly notched on each side. Some of the specimens have one or two branches, but most of them are simple erect stems. It is known as the Sea Oak Coralline (Sertularia pumila), and if we examine it with our lens, we shall find that each of the notches represents the space between the elegant crystal vases that are arranged symmetrically along each side of the stem. These vases are known as calycles, and in each there stands a polypite, reaching out its upper portion and waving its tentacles. In case of danger the polypite can be withdrawn into the calycle; and certain species have an automatic contrivance for closing the mouth of the vase when they have retreated within. All genera have not these calycles.
Returning to the animal for a moment, it should be explained that its organization is so low that there is no true circulatory system for the renewal of the body, by the carrying of elaborated food from the stomach to distant parts of the body; but by the activity of innumerable eye-lash-like hairs on the surface the whole of the particles of food digested in the stomach are carried all over the system to be then assimilated by different parts.
Within the circle of tentacles is the mouth, which is sometimes cut into lobes, and is generally borne upon a very mobile proboscis, which may be withdrawn or protruded, and in some genera takes a trumpet-shape; in others it is conical. In the winter the cœnosarc may frequently be found with all its calycles empty; and it might then be supposed that the zoophyte is dead and only its skeleton remains. But this is not necessarily so, and a closer inspection may convince us that the organism is alive. In spring it will furnish its calycles with new polypites, and all will go merrily again. At certain seasons buds of a peculiar structure are formed, which develop into polypites, whose function it is to produce eggs, instead of catching and digesting food for the colony. These are known as gonophoræ, and sometimes they remain where they were produced, simply bursting to discharge their contents. In other cases they detach themselves from the parent colony at a certain stage in their development, and float off, having all the appearance of minute jelly-fishes. Some of these, instead of remaining small, attain an enormous size, so that it is difficult to credit their origin to the so-called coralline upon which they were produced, and of which in turn they are really the egg-bearers. The eggs they scatter will develop into plant-like growths such as they were produced by; from the edge of their jelly-umbrella and from its handle, buds are given off, which open as jelly-fish like itself.
Growth proceeds rapidly among these creatures, and if a balk of timber be immersed in the sea, it is not long ere there is a fine forest in miniature upon its surface, and that forest will consist of some of these corallines. The species are generally distributed along our coasts, but a few are local. Thus the finest of all the British species of Sertularia—Diphasia pinnata—is found only on the coasts of Devon and Cornwall, where most other species attain their maxima of beauty and luxuriance. Its relative Diphasia alata, as well as Calycella fastigiata and Aglaophenia tubulifera, have been found in Britain, only in Cornwall, Shetland, the Hebrides, and on the west coast of Scotland. On the other hand, certain species belong to the north, and such species as Salacia abietina, the Sea-fir, and Sertularia tricuspidata are not found on our shores below the north-east coast. Sertularia fusca is similarly confined, so far as our seas are concerned, to the east coast of Scotland and the north-east of England; and Thuiaria thuja, found on the east coast, is rare in Devon and Cornwall; whilst the species of Aglaophenia are plentiful on our south-west and north-west coasts, and rarely seen on the north-east.
Although some species are distinctly deep-water forms, necessitating the dredge for their capture, the vast majority inhabit the littoral and laminarian zones. Among the littoral species are many of the rarer forms, and some of these are found only on special species of seaweeds, or on the shells of particular mollusks. Mr. Hincks, whose beautiful work on the “Hydroid Zoophytes” you must see, gives some very good advice as to collecting in the littoral zone. He recommends my own favourite plan of lying flat beside the rock-pool, and bringing the eye close to the water. “He should bring his eye to the edge of the pool, and look down the side, so as to catch the outline of any zoophytes that may be attached to it amidst the tufts of minute Algæ. He must not be content with a hasty glance, but look and look again until his eye is familiar with the scene, and may accurately discriminate its various elements. And let him watch for the shadows; for in following them he will often secure the reality. I have frequently detected the tiny Campanulariæ and Plumulariæ in this way, by means of the images of their frail forms which the light had sketched on the rock beneath them. For tools, the hunter must have his stout, flat, sharp-edged, collecting knife, a long-armed and substantial forceps, and a varied array of bottles, ranging from the homœopathic tube to the pickle-jar. If his choice of ground be good, and his patience proof, and his eye quick, he will have an ample reward for his labour in the rich spoil of beauty which he will bear away, even if he should not hit upon any novelty; but amongst the minute zoophytes there is still, I have no doubt, much to be done in the discovery of new forms, as there certainly is in working out thoroughly the history of those that are known.” I hope that in the foregoing remarks I have made it quite clear that our Sea Oak Coralline is not an individual but a community of individuals—a community on the strictest of co-operative principles, in which the good fortune accruing to one of the polypites by food falling in its way, is shared by all alike; for a polypite cannot digest it and retain it to its own selfish use, instead, it goes to the nutriment of the commonwealth.
Some of these Hydroid Zoophytes, though sharing the communist character, are much simpler in form, and we shall find a common example ready to our hand on almost anything in the way of stone or shell removed from a rock-pool. It is a minute creature, as stout as a “short white” pin, and about a third of the length, white or pinkish; a number of them spring in a row from a creeping stem of firmer substance, in which are well-defined tubular openings, in which the upright bodies stand. These answer to the calycles of Sertularia, just as the upright bodies agree with the polypites of that genus. The name of this creature is Clava multicornis, and it may conveniently be called the Many-horned Club. It gets its name Clava from the shape of the polypite which thickens towards the top, and then tapers off again to the summit, where its mouth is situated. It has a number of tentacles, varying from ten to forty, according to age, but these do not form a regularly disposed crown round the mouth; instead, they are placed anyhow on the thickened part of the polypite. The name multicornis refers to these many-horns or tentacles. An advance on this type is seen in Coryne pusilla, a much larger but equally common inhabitant of our rock-pools, in which the tentacles are knobbed, and are arranged in a series of more definite whorls.
There is another group which is more likely to be confounded with the Sertularians by those who are content with hasty glances at things; but species of the one group may be readily distinguished from the other by the aid of a simple lens. The Sertularians, as we have seen, have the calycles arranged symmetrically on each side of the axis. The Plumularians, as the other group are called, have their calycles arranged along one side only of stem and branches. The Sertularians are frequently spoken of as Sea-firs, the arrangement of the calycles giving some species a very close resemblance to the branches of fir-trees. In the Plumularians, the resemblance much more nearly approaches a feather.
Hincks, describing Plumularia cornucopiæ says:—“In the present species a conspicuous band of opaque white encircles the body, like a girdle, a little below the tentacles, and adds much to the beauty of a colony in full life and activity, when its many polypites are in eager pursuit of prey, stretching themselves forward, and casting forth their flower-like wreaths, now suddenly clasping their arms together, and then as suddenly flinging them back; now holding them motionless, the tips elegantly recurved, and then on some alarm shrinking into half their size, and folding them together like flowers closing their petals when the sun has gone.”
In addition to the calycles in which the polypites live, there are special reproductive chambers as in the Sertularians. In this species (P. cornucopiæ) “they assume the shape of an inverted horn, and are formed of material translucent as the finest glass. Each one of them, in fact, is a little crystal cornucopia, in which is lodged one of the reproductive members of the commonwealth, a class totally distinct from that which is charged with the function of alimentation. These graceful receptacles are several times larger than the calycles, from the base of which they spring, singly or in pairs, and within them the ova are produced and the embryos matured which are to give rise to new colonies.”
One of this group, the Lobster-horn or Sea-beard (Antennularia antennina), shown at the back of the illustration of acorn-shells on page 183, has the calycles arranged in whorls all around the axis, which produces a very singular appearance, not at all unlike the antennæ of some of the larger crustacea.
In the Creeping Bell (Calycella syringa) so common on seaweeds, etc., the calycles are more bell-shaped, and the mouth of the bell is fringed with a series of large triangular teeth, similar to the peristome of many moss-fruits. When the polypite withdraws into his calycle, these teeth bend inwards, and so close the opening.
Many of the forms of Jelly-fish to be described in the next chapter, though they are described with separate names, are now known to be merely stages in the history of some of these Hydrozoa or Hydroid Zoophytes—the developed free-swimming gonophoræ previously mentioned.
A singular member of the group has the form of a jelly-fish, but does not act as one. This was formerly named Lucernaria, but is now known as Haliclystus octoradiatus. It was thought to swim like a jelly-fish, but it really creeps. Its form is like a ladies’ sunshade that, instead of being the ordinary umbrella shape, tapers off to the stick at the top. What would be the ferrule of the sunshade is the footstalk of Haliclystus. By this footstalk it attaches itself to a weed, say, and hangs down its eight arms with their connecting web, and by means of a little knob on the edge of the web alternating with its “arms,” it is able to take hold until it has “looped” like a geometer caterpillar, by bringing its footstalk forward and taking fresh hold. The extremities of the eight arms (or ribs of the sunshade) are ornamented with tassels of tentacles, and it uses these after the manner of a sea-anemone when it wishes to secure food. It, in fact, has some of the peculiarities of both jelly-fish and anemone, though it will not act quite consistently with either character. I have found it on Laminaria and other weeds at low water, and a few months since I picked one off the plumage of a dead guillemot, that had been drowned in a storm and afterwards washed ashore.
There is an important group of incrusting organisms that you will find represented on almost the first specimen of Fucus you pick up, and which you may be tempted to class with these zoophytes; but they occupy a much higher position in the scale of life. I refer to the Sea-mats, the Sea-scurfs, the Bird’s-head Coralline, and allied forms, whose proper designation is Marine Polyzoa. They are more nearly allied to the mollusks, the structure approaching towards that of the Lamp shells. They are associated in colonies (zoaria), but there is no connecting cœnosarc as in the Hydrozoa, although there is communication between the chambers by wisps of animal matter. Each chamber of the Sea-mat marks the habitation of a complete individual, who catches, eats, and digests for himself alone, not for the colony. These chambers are of a horny, persistent character, secreted of course by the polypide; with a small opening through which the creature protrudes its mouth and fringe of tentacles. Its body consists of a thin bag filled with a clear fluid, in which can be traced the gullet enlarging into a simple stomach, contracting again into the intestine. There are muscles by means of which the upper part of the sac with the mouth and tentacles are withdrawn inside the lower part. Add to this a nerve-ganglion beside the gullet, sexual organs within the sac, and the polypide is fully described.
The original founder of the colony was produced from an egg, and was for a time a restless larva, swimming and creeping and whirling around by the aid of cilia. Finally it settles down on weed or stone, and becomes anchored; drops its cilia and develops its horny chamber and its crown of tentacles. Having reached its full degree of growth, it buds at the sides, and originates other creatures like itself. Just as the solitary daisy root or chrysanthemum throws out what the gardener terms suckers, and soon becomes the centre of a clump of similar plants; so the solitary Sea-mat soon becomes only one in a symmetrically arranged colony, containing hundreds of individuals, all produced by budding from the original egg-produced polypide.
Some of these colonies have a number of queer adjuncts, which bear a startling likeness to the head of a bird of prey, with moveable jaws, that are for ever snapping. These have, of course, given rise to many theories to account for them; but it appears now to be generally accepted that the “bird’s-head” is a specialised member of the zoarium who serves some purpose, probably of defence, or of scavenging, that is of advantage to the whole colony. In some species, this differentiation of individuals takes the form of a long whip-like process, constantly lashing, instead of the snapping jaws. The forms of the marine polyzoa are very varied, but we shall be unable to do more than indicate a few of them here, leaving the reader to make wider acquaintance with a most interesting group by studying the species in Hincks’s British Marine Polyzoa.
The Sea-mat (Flustra foliacea) is a deep-water form, whose colonies take the shape of fronds, resembling Fucus serratus in outline; but it is thrown up on the beach in great quantities, and it will be one of the first things you will find on the shore, especially if you rout about among the weeds washed up by every tide. Creeping over these flat frond-like masses you will probably find other species that take a more branching form, such as the common Creeping Coralline (Scrupocellaria reptans), or the more bushy Bird’s-head Coralline (Bugula avicularia). The Tufted Ivory Coralline (Crisea eburnea) has tubular chambers of ivory whiteness; it is of branching habit, and occurs on some of the red seaweeds. The Foliaceous Coralline (Membranipora pilosa) runs in very narrow ribbons, covered with a “pile” of bristles, up the stems of various weeds; and many another of the nearly two hundred and fifty British species will be sure to fall to the patient and sharp-eyed investigator.
The horny cell in which the polypide resides is really its own cuticle or outer skin, to which it is inseparably attached. If careful examination be made, it will be found that at the mouth of the so-called cell the horny material suddenly changes its character and becomes a very fine and delicate tissue, capable of the greatest freedom of movement and folding such as is absolutely impossible with the horny portion. This remarkable change of character in the two portions of the same cuticle allows the anterior portion of the polypide, with its crown of tentacles, to be suddenly and completely withdrawn out of danger, just as easily as the tip of a glove-finger can be withdrawn into its lower portion.
The tentacles that encircle the mouth of the polypide are hollow, and covered with ever-waving cilia, whose beating causes currents of water to set in towards the animal’s mouth, bringing food with them. These tentacles appear to be also the only sense organs possessed by the polypide, and to serve the further purpose of gills. None of the Polyzoa to which we here make reference possesses a heart or blood-vessels.