Stories of the Universe: Animal Life Part 7

{ ECTOPROCTA, with excretory aperture outside { the ring of tentacles, _e.g._, _Fl.u.s.tra_.

=POLYZOA.= { { ENDOPROCTA with excretory aperture inside { the ring of tentacles.

The Polyzoa include freshwater as well as marine forms. They have a free-swimming larva, which becomes fixed after a time, and gives rise to the adult Colonial forms. The zooids of the latter have each an independent head with a crown of tentacles, called the Lophoph.o.r.e (Crest-carrier); but the fixed ends of their bodies communicate with one another. The hard covering of the colony, which retains its form after the animal is dead, is a kind of hardened skin: the apparent "cells" are the openings through which the individual zooids protrude themselves.

Sometimes certain of the zooids undergo modification for special purposes: in this way are formed the "avicularia," snapping appendages, probably defensive in purpose, so called because they open and shut like a bird's beak. There are two divisions of the Polyzoa, the Ectoprocta and the Endoprocta. Among the latter there is found a form which is not colonial.

_Phoronis_, a curious worm-like animal, which has a larval form called _Actinotrocha_ is sometimes placed in cla.s.sification near the Polyzoa, which it resembles in possessing a crown of tentacles (Lophoph.o.r.e).

CHAPTER XII

THE ECHINODERMATA

Everybody knows the Star-fish and many people know the Sea-Urchin.

An "urchin" is not a name for a naughty little boy, but the French (_oursin_) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog,"

a name very appropriate for a creature armed with p.r.i.c.kles. The Greek word _echinos_ also means a hedgehog, so that the long name given to the group means simply hedgehog-skinned. The p.r.i.c.kles attain their maximum in the Sea-Urchin, but they are well represented in the Star-fish, while in the Sea-cuc.u.mber the general tendency to "p.r.i.c.kliness" is much reduced, and represented only by "spicules" (needles) of sh.e.l.ly stuff underneath the skin of the animal.

The largest and the most beautiful of the Sea-Urchins of the English coast is known as the Purple-tipped Sea-Urchin, on account of the beautiful colour of the spines. It lives on rocky coasts, and during very low tides may be seen at home, although it usually takes care not to stray above the water-line. It is a sh.e.l.ly ball with a flat base; its surface is covered with long spines. Its mouth, which is in the centre of the base, shows five wicked-looking teeth peeping out. The sh.e.l.l is pierced by what look like hundreds of minute pin-holes, arranged in a complicated pattern; these are the holes through which it pokes its feet, which greatly resemble those of a Star-fish, being white suckers with a disc at the end. When thrown out to their full length they are, however, much longer than those of the Starfish, for they are naturally obliged to be thrown out to a distance longer than the length of the animal's own p.r.i.c.kles. When moored by all its feet, extended from all sides of the sh.e.l.ly ball, the animal presents a curious and pretty sight. Large specimens are almost as big as a child's head, but smaller ones are more common. There is a considerable range of variation in colour; not only are various shades of purple found, but also purplish-red and red. The spines are mounted on something resembling a ball and socket joint, with a ring-shaped pad, so that they have a wide range of movement; if any of the spines are touched they are immediately set back over a considerable part of the neighbouring surface.

Other kinds may be found upon a more sandy sh.o.r.e. These are heart-shaped and much lighter in colour. The sh.e.l.l is thinner and of less weight.

These adaptations for lessening the animal's weight enable it to move over sand: the species above described has no occasion for such precautions. When it crawls over rocks and the strong seaweeds that grow on them, there is no fear of its sinking in. The sand-dweller, on the contrary, must take care that it is not swallowed up.

[Ill.u.s.tration: FIG. 36.--The Five-holed Sand Cake, _Mellita pentapora_, a flat sea-urchin from the east coast of tropical North America. _A_, upper surface; =B=, lower surface; _C_, side view.]

There are Sea-Urchins that carry their precautions against sinking to an extreme degree. These are the Shield-Urchins or Clypeastridae, so-called from their flat shape; they include the American forms popularly known as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of these flattened forms adapted for moving over fine sand and ooze, and literally "as flat as a pancake." The mouth is approximately in the centre of the lower surface, _B_; the upper surface, _A_, shows a rosette pattern on the top of the sh.e.l.l. This is formed by the rows of holes for the very minute tube feet. In the English Sea-Urchin above described, which is one of the group called (for that reason) Regulares, the rows of holes are uniformly continued all along the rounded sides of the body down to the neighbourhood of the mouth. Here they are much restricted, forming merely a rosette at the top of the sh.e.l.l: hence they are described as circ.u.mscript or "petaloid." The excretory aperture is shown in the photograph as a smaller dot on one side of the mouth, while in the Echinus, on the contrary, it is at the top of the sh.e.l.l.

The five odd-looking, elongated holes are a curious individual peculiarity of this Sea-Urchin. It has already been explained that the Shield-Urchins are flattened in order to distribute their weight; these holes are a contrivance for still further reducing the weight in comparison with the area. This is when the animal is lying quiet at the bottom of the water, but when it moves about what effect will the presence of the holes produce? Flattened animals are usually supposed to derive an advantage from the fact that they sink more slowly through depths of water; as in lying upon the ground, their weight is distributed, and they float, as it were, in the same stratum of water without sinking further down. This creature, on the contrary, has apparently feared lest it should move too slowly when it moves in a vertical direction, and it presents us with an arrangement by means of which its sinking through water is facilitated. Water will pa.s.s readily through the five holes as the animal goes either up or down, and the resistance of the whole flat area to the water is thus reduced and vertical movement rendered more easy. Thus, by one and the same contrivance, the animal has lessened its weight when lying quiet, and diminished the resistance it meets with when it moves. The distribution of the holes, moreover, is such as to regulate the animal's position in sinking, and to prevent it from falling "headlong." For although the creature has, strictly speaking, no "head," yet the end nearest the mouth is the thickest and heaviest part of the "cake," and would naturally tend downwards. This tendency is counteracted by the fact that the thicker end is unperforated, while the thinner and lighter end has a large central hole to diminish its resistance and enable it to sink more rapidly.

Adapted for living in sand rather than on rocks, but not so extreme in the peculiarity of their form as the Shield-Urchins, are the Heart-Urchins, already referred to, s.h.a.ggy-looking creatures whose fine yellowish-white spines give them almost the appearance of being clothed with fur. The excretory aperture is at the narrow end of the "heart,"

and the mouth at one side of the lower surface towards the wide end. The complicated apparatus of teeth found in other Sea-Urchins is absent in these. They are abundant on sandy sh.o.r.es. During the severe winter of 1894-5, when the Mersey at Liverpool was frozen nearly for one memorable day, and filled with floating ice for many more, I saw the sh.o.r.e beyond New Brighton heaped all along with a bank, often two feet across, of the common Heart-Urchin. These, which afforded a fine feast for the hungry sea-gulls, had been killed by the intense cold, and afterwards washed ash.o.r.e by the tide. The vast numbers of this creature which exist on that coast were thus unexpectedly brought to light.

These animals are sometimes described as "burrowing" creatures, because they live covered in sand. The term is rather misleading. Far from wishing to burrow, they spend their lives in a constant struggle with sand that closes over them only too readily; and their whole structure is adapted to prevent their sinking in a quick-sand.

We began our chapter with the Sea-Urchins, because they are the most important members of the group to which they give their name; but there are forms belonging to the Echinodermata that are more familiar to the ordinary observer--the Starfishes. Those who take an interest in the cultivation of the oyster find them far too familiar--for the starfish is the oyster's deadliest foe, not even excepting man.

The common Starfish, _Asterias rubens_, may constantly be found among stones, about low-tide mark. Its manner of walking is peculiar and characteristic. On the under surface of each ray are rows of white sucker-like tube-feet, which can either be drawn in or pushed out. By doing each alternately the animal walks. First the feet are extended to their full length; then the terminal sucking disc of each catches hold of the ground. Then the feet are again retracted, while their discs still cling; the effect of this is, naturally, to pull the ray onwards.

This process is repeated again and again, until some appreciable degree of movement is effected. The tube-feet are in connection with a system of vessels filled with fluid, known as the Water-vascular System of the Starfish. The fluid is driven on by muscular contractions until the feet are fully extended, and again driven back when the feet are retracted.

The Water-vascular System is a structure common to all Echinoderms; and vessels of a comparable character are found in some worms.

How does the Starfish know where it is going? Underneath each ray, near the tip, is a little feeler (or tentacle) and a little eye spot. By means of these it gets an idea where each ray is going to; and, since it often moves but one ray at a time, this is sufficient for it. When necessary, however, the several rays can act in concert with one another.

The rayed form of the Starfishes led to their being at first included in the group of Radiate Animals, along with the tentacle-bearing Coelenterata; but it has long been recognised that they are animals of much higher structure. Their very larvae can barely be brought into comparison with animals so simple as the true "radiates."

The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes.

In these the arms are thin and sharply defined from the little central disc, instead of sloping gently out of it, as in the Starfishes. The rapid wriggling movements of the arms have gained for them their very appropriate name. They are also called Brittle Stars, because the arms break off easily, sometimes at the will of the animal. Several kinds of them are common on our sh.o.r.es, although they are not so common as the ordinary Starfishes. Fig. 37 shows the general form of a Brittle Star.

[Ill.u.s.tration: FIG. 37.--A Brittle-Star, _Ophiopteris antipodum_.]

[Ill.u.s.tration: FIG. 38. A Sea-Cuc.u.mber, _Cuc.u.maria Planci_, from Naples, natural size.]

The Sea-Cuc.u.mbers, Holothuroidea, are another group of Echinodermata that are represented on our own coasts; by small specimens, however, while the Pacific Ocean furnishes instances of larger size--the Trepangs--which are used by the Chinese as articles of food. The name Sea-Cuc.u.mber is given in fanciful comparison to a small Gherkin; presumably one that has been very badly pickled--for the colour of the animal is brownish and by no means green. The mouth of a Sea-Cuc.u.mber is surrounded by a circlet of tentacles (partially indicated in the diagram, Fig. 38). The body is elongated and crawls along: the "star"

shape, so characteristic of the Echinoderms, is scarcely to be recognised except in cross section, where the longitudinal rows of tube-feet are seen to outline a pentagon. The skeleton of the Sea-Cuc.u.mber is of a very meagre description. Instead of forming a rounded case, as in the Sea-Urchin, it consists only of loose pieces of very small size, situated below the skin. The Starfishes are intermediate in this respect. Their "skeleton" consists of a vast number of pieces or "ossicles," which are of fair size, but are not closely united, as in the Sea-Urchin. They are, however, so numerous and so well knit, that the skeleton of a dead Starfish presents the complete outward form of the animal. It must be noted that the ordinary skeleton of the Sea-Urchin is only _apparently_ exterior. As is the case with the ossicles of the Starfish and Sea-Cuc.u.mber, the skin lies outside, and the hard particles belong to the middle layer, or mesoderm. In this the skeleton of Echinoderms differs from the "sh.e.l.l" of a crab or lobster, which is formed by a hardening of the skin itself.

[Ill.u.s.tration: FIG. 39.--_A_, Head of a Stone Lily or Encrinite, _Encrinus liliformis_, a fossil from the Muschelkalk of Brunswick, natural size. _B_, Rock with stalks of encrinites. _C_, Section of a stalk.]

The Crinoidea, Encrinites or Stone-Lilies, form another group of the Echinodermata. Though still represented by living forms, they attained their maximum development in past ages. The English "Mountain Limestone"

of the Carboniferous period is full of their fossilized remains, which form a marble often used for ornamental purposes. The so-called "Stone Lily" consists of a "head" comparable with the body of a Star-fish or other Echinoderm, which is borne at the end of a long fixed stalk. The marble above named owes its ornamental appearance to the presence of these stalks, often very long, and cut through at every possible angle.

The Crinoids have their living representative in English Seas, _Antedon_, the Feather-Star (Fig. 40). On the side opposite the mouth, where, in the Encrinite, the stalk would be, there are a group of elongated processes called cirrhi, by means of which the animal can attach itself to stones or seaweeds. When not thus fixed, it swims about, by moving its fringed arms, each of which is forked. It will be seen that when the animal is fixed by its cirrhi, it stands mouth upwards, so that its position compared with that of the Starfish or Sea-Urchin is upside down. The young of the Feather-Stars have stalks by which they are fixed, like the Encrinites; but afterwards the stalk is lost.

[Ill.u.s.tration: FIG. 40.--A Feather-Star, _Antedon bifida_, British Seas, three-quarters of the natural size. The short threads in the middle are the cirrhi.]

Among fossil Echinoderms there are two groups of stalked forms which have no living representatives. These are the Cystoidea and the Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx or head, which is comparable, with the body of the free Echinoderms.

The Sea-Urchins possess a swimming larval stage, which goes through remarkable changes after pa.s.sing out of the two-layered (Gastrula) form. It becomes provided with cilia, which are arranged in bands, and outgrowths of peculiar form are established in the case of the Sea-Urchins, while the larvae of the other groups also present characteristic shapes. Within the larva the adult form develops, the outside of the larva being finally thrown off.

In the young Feather-Star, a subsequent stage of the young animal has a stalk, by which, like the Encrinite, it is fixed. This animal therefore is at first free-swimming, afterwards fixed, and again free in its final stage--a remarkable series of changes.

These queer-shaped things, the Sea-Urchins and their allies, are perhaps the last creatures amongst which we should think of looking for relations of the Worms. Yet the earliest stages of the larva are considered to present a certain amount of resemblance to the Wheel-ball larva, which has been referred to elsewhere (pp. 42 and 72). Still more startling fact, these larvae have been compared to that of _Balanoglossus_, the lowest member of the Chordata, and a relation of the Vertebrates themselves (see p. 143).

TABLE SHOWING THE CLa.s.sIFICATION OF THE ECHINODERMATA

{ ECHINOIDEA, OR SEA-URCHINS.

{ ASTEROIDEA, OR STAR-FISHES.

=ECHINODERMATA.= { OPHIUROIDEA, OR BRITTLE-STARS.

{ CRINOIDEA, OR FEATHER-STARS AND { STONE-LILIES.

{ HOLOTHUROIDEA, OR SEA-CUc.u.mBERS.

CHAPTER XIII

THE CHORDATA

The older zoologists used to speak of Vertebrata and Invertebrata as animals with a back-bone and animals without one, and everyone thought it a very natural way of dividing up the animal kingdom. It never occurred to anyone that it was possible to bridge the interval between them and find a link between the two. But now the Vertebrata have been compelled to give up their aristocratic pretensions, and own that they have risen from the ranks of the common people of the animal world; in other words, that they are descended from the Invertebrates. Their family secrets have been published to the world, and now everybody knows that they have poor relations. But how many, and how nearly related?

This we do not accurately know; consequently the whole zoological world for many years has concentrated all its energies on attempts to find out the truth about the matter.

A great sensation was caused by the first discovery of a poor relation of the vertebrates among the Ascidians, or Leather-bottle animals. These are named from their shape and texture, for they have a leathery skin.

Now some of these Ascidians have larvae with a tail; and in the tail there is a long cord-like structure, which in many essential particulars resembles the cord which precedes the back-bone in the vertebrate embryo. This structure is called the Notochord (a string down the back).

The credit of this great discovery belongs to Russia; for the presence of the Notochord in the Ascidian larva was discovered by A. Kowalevsky, in 1866.

To the present generation of zoological students, the Chordate affinities of Ascidians are part of the ABC of knowledge; and it is hardly possible for them to realise that it is only thirty years ago since the idea was so new that Huxley, in his "Text-book of the Vertebrata," only alluded to it in a footnote. Would-be zoological critics, at a somewhat later period, met the theory with ridicule, for want of better argument. For critics include not only "those who have failed in literature and art," but also those who have failed in science.

The majority of the Ascidians are sessile animals, which fix themselves, like Sea-Anemones, to some object when they have pa.s.sed their earliest stages of growth; and although there are many forms that swim freely, most authorities are inclined to believe that these have arisen by adaptation, and that the kinds that are fixed when adult are the original type of the group.

Anything more unlike what we should expect to find as a relative of the vertebrates could not possibly be imagined. What has been written about these little animals by various observers would make a whole series of volumes of the size of this one, so many are the puzzles afforded by their internal structure. The arrangement of their organs is in many respects very unsymmetrical. Their most striking peculiarity, perhaps, is the nature of the gills. These form a kind of basket-work, consisting of minute holes with intermediate supports; and they are a.s.sociated with a special cavity outside them called the Atrial chamber. Into this the gills pa.s.s the sea-water which they have breathed.