THE WORMS AND SOME OF THEIR POSTERITY

The somewhat miscellaneous collection of animals that have been thrown together and termed worms is of the greatest importance for our theory of descent. Indeed, it seems probable that all of the four great groups which we have yet to mention have descended directly from worm ancestors. This, at all events, is the view of Haeckel, although it must be admitted that many other theories have been proposed. Nor can it be taken as a matter for surprise that agreement concerning this part of our history should be hard to reach, for the difficulties which are met in it are many and perplexing.

The worms comprise many greatly divergent groups, and the difference between the lowest and the highest of these has been produced by many important steps in Evolution. Of these groups but few immediately concern us; the first and lowest of those which do, is that of the Turbellarians, a section of the Platodes or flat-worms. The Turbellarians are small or microscopic tongue-shaped organisms, of which the majority of species live on the sea-floor, others however being found in fresh water. The surface of the body is covered uniformly with cilia, which serve, in the smaller forms, as organs of propulsion, while in the larger they appear to have the function of maintaining a flow of fresh water over the surface, and thus of assisting respiration. In some respects there has been little advance from the condition of the Coelenterate. The digestive cavity is a simple or more or less divided sac, communicating with the exterior only by means of the mouth. Unlike the condition of affairs in the Coelenterates and Ctenophora, however, the sex glands do not discharge the reproductive bodies into the digestive cavity, but directly to the exterior by means of a special opening. Each individual has a pair of male and a pair of female reproductive glands, but the eggs are not self-fertilised; nor is the fertilisation of the eggs trusted to chance and the sea-water, as in the lower groups. Instead there is a definite exchange of sperms between two individuals, and the eggs are fertilised before they leave the body. They are also frequently supplied with a store of nutritive material by a pair of special yolk glands. A distinct step of progress can thus be recognised in the arrangements for reproduction. Between the outer skin and the inner digestive layer is developed a considerable mass of cells, forming muscular and connective tissue, etc. It will readily be understood that the development of such thick tissue masses occasions two distinct new difficulties in the animal economy; for where cells are in direct contact neither with the digestive layer nor with the exterior, their nutrition and the removal of their waste products can no longer be efficiently carried on without special devices. Hence on the one hand a circulatory system, for the transport of food materials, and on the other an excretory system, become necessary. The first of these new departures was not destined to be made until the next stage of progress; the Turbellarians seem to have temporarily got over the difficulty, like the Ctenophora, by developing a complex and ramifying digestive cavity. An excretory system, however, makes its appearance here. Indeed, the beginnings of such a system can be seen in the Ctenophora, in which there are small excretory organs opening into the digestive cavity. The corresponding organs in the worms, as in all subsequent types, open directly to the outside. In the Turbellarians these organs, which are termed nephridia, are two in number, and consist of long tubes which branch and ramify throughout the body, the small branches terminating in special excreting cells, and the whole constituting a complete and thorough drainage system. The nervous system consists of one or two small masses of nerve cells termed ganglia in the front region, with a somewhat complex network of nerves connecting them with various parts of the body. There are frequently two pairs of sense organs, probably rudimentary eyes and ears respectively. The main features of the digestive, reproductive, excretory, and nervous system are shown in the figures in Fig. 40.

The Turbellarians probably arose from Ctenophora or from some nearly related form, a view that receives support from the occurrence of several apparently intermediate types. The differences may, in fact, be partly accounted for as adaptations to meet the change of habitat from that of the upper waters to that of the sea floor. A spherical, or pear, or bell shape is suitable enough for a swimming animal, but would be impossible for one that was to crawl. The first change, then, we may imagine, was a flattening, which produced a disc-shaped animal, with the mouth in the centre of the lower aspect and the sense organs in the middle of the upper. Secondly, a definite mode of progression, by which one part of the body continually went first, would be an advantage, as permitting of a better co-ordination of movements, and an elongation of the body in the line of movement would have the effect of diminishing resistance and of making progression easier. Finally the sense organs, like the scouts of an army, would be best in front, and would migrate thither, and the mouth, in order to get the full benefit of the food which the sense organs sought out, would gradually shift to a position beside them. These adaptions, it is obvious, have produced a complete change in the architecture of the animal. Our sea-anemone, or Medusa, or Ctenophore is radially symmetrical. That is to say, its parts are arranged like the spokes of a wheel, and it may be divided into two equal halves by each of several planes passing through the main axis. It has an upper and a lower surface, but no head and tail ends. The lowest of the worms now can be divided into two halves only in one direction, that which separates the right and left sides. They are, in scientific language, bilaterally symmetrical. The change to this type of architecture was a very important step of Evolution, particularly in relation to locomotion. Bilateral symmetry was destined to remain a constant feature of three of the four great groups that evolved from the worms. The star-fishes reverted to the earlier condition.

The next class of worms with which we have to deal is that of the Rotifera. In their general structure, and in their excretory and sensory-nervous systems, the Rotifers do not differ essentially from the Turbellarians. They do differ, however, in that the digestive cavity has a second opening to the exterior, at the end opposite to the mouth. The advantage of this arrangement, which was retained in the subsequent stages of Evolution, is obvious, for it renders possible a much more regular and thorough digestive process. Instead of the food passing in, and the undigested remains passing out, by the same opening, and instead of the contents of the digestive cavity being a general mixture of food material in all stages of digestion, there is now a regular stream of food passing through the cavity in one direction, and being digested as it goes. A near relative of the Rotifers is shown in Fig. 42.

Thirdly, we must briefly allude to the Nemertines. These are a group of flattened thread-like worms of very variable size, found both in fresh and salt water. The most notable advance in this group is to be seen in the occurrence of a special circulatory system. It has already been indicated that the gastric cavity of the lower forms has the double function of digestion and of the transport of nutritive substances to the various parts of the body. In the Nemertines the second of these functions is carried out by the blood system, which consists of two or three vessels that run parallel throughout the length of the body and anastomose at either end. There is no indication of any enlarged or specially contractile portion of any of these, no indication, that is to say, of a heart. The blood conveys not only nutritive substances, but also, as in the higher animals, oxygen. Some Nemertines have indeed red blood, containing true hÆmoglobin, which is well known as the oxygen-carrying material in the vertebrates. A typical Nemertine is shown in Fig. 44, and a diagram showing some features of the anatomy in Fig. 45. It will be seen that the nervous system is of the same type as in the worms already described. There are two pairs of sense organs, one pair being eyes, and the other probably having the function of gauging the chemical nature of the water. The Nemertines possess a peculiar organ in a snout or proboscis, which they can protrude or withdraw into a special sac. The snout is armed with a sharp sting, and forms an effective weapon whether against the creature's enemies or its prey.

About this stage of Evolution, the exact point being somewhat difficult to fix, there appears the body cavity. This, which is altogether distinct from the digestive cavity, is a familiar feature of the anatomy of the higher animals. In it are suspended the heart and lungs and the whole of the digestive organs and glands. The question of the origin of the body cavity and the blood system is a very difficult one, and a thorough theoretical discussion would take us too far.

Before proceeding to the question of the origin of the vertebrates, we may pause briefly to consider the other groups to which the worms appear to have given rise. First of these we may take the Echinoderms, which include the well-known star-fishes and sea-urchins, and the very beautiful feather stars. As already indicated, it is believed that the radial symmetry, which is so characteristic of this group, is not a primitive feature, but that, in fact, the Echinoderms are descended from bilaterally symmetrical ancestors. One reason for this view is that the larval or immature form is always markedly bilaterally symmetrical. In an ordinary star-fish, which we may take as typical of the group, the mouth is in the middle of the lower aspect, and the excretory opening of the digestive cavity in the upper side just opposite. There is no blood system, or excretory organs, and no concentration of nerve cells into any form of brain. Eyes, however, are present, and sensitiveness to light may be easily demonstrated. The most remarkable feature of the group is the water-vascular system, consisting of a series of radial canals, one in each ray, which join a circular one situated in the central portion of the body. The system of canals communicates with the exterior by means of a sieve-like plate on the upper surface, and it is kept full of water by the continual pumping action of cilia on the walls of the tube which leads down from the sieve plate.

The ordinary star-fish is carnivorous, and lives largely on ordinary mussels, which it bridges over with its arms, and opens by a steady and long-continued pulling, the soft parts being then sucked up by the partially protruded stomach. A few types of Echinoderms are shown in Figs. 46, 47, 48.

The group of the Mollusca includes such common forms as cuttle-fishes, whelks, slugs, snails, mussels, and oysters. These, it will be observed, comprise marine, freshwater, and land forms. The molluscs, like the next two groups with which we have to deal, have made a conquest of the land, though in the present instance it cannot be regarded as very complete. The anatomy of the group shows much variation, and only a few of the leading features can be alluded to. The digestive system is highly developed. The mouth is provided with a jaw or jaws, and with a tongue-like ribbon, which is covered with rows of teeth, like a file, and by whose action the food is torn and disintegrated. A gullet leads from the mouth to a stomach, which is followed by an intestine. Salivary glands and a large hepatic gland or liver are present. Respiration occurs partly through the skin, but special organs also exist for this function, gills in the water forms, and a lung cavity in those which breathe air. There is a well-developed blood system, and generally a heart; the blood is pumped direct from the heart to the general body tissues, and returns to it by way of the kidneys or nephridia, which purify it of waste materials, and the respiratory organs, where it is freed of carbon dioxide and supplied with oxygen. The nervous system varies greatly, but a pair of cerebral ganglia—a brain—is usually present. There is a particularly keen sense of smell, and taste and hearing may also easily be shown to exist. Some forms are blind, from which condition there is a regular series of stages of development of the eye, up to forms in which it becomes a highly perfected organ, with cornea, iris, lens, and retina. The close similarity between this and the ordinary vertebrate eye, which must have evolved quite separately, is one of the strangest coincidences of Evolution. Thus in many ways the molluscs are to be regarded as highly specialised types. But in two important directions, in intelligence and in their arrangements for locomotion, they stand as a group on a low plane of development. Figs. 49, 50, and 51 illustrate some of the forms met with in the group. The origin of the molluscs, as well as that of the Echinoderms, is wrapped in obscurity. That each group is derived from some form of worm is probable, yet some zoologists hold even such a general statement as this to be lacking in support.

Our third great group is that of the Arthropods (literally 'jointed footed'), including the Crustaceans (crabs, lobsters, shrimps, etc.), spiders and mites, centipedes and insects. The Arthropods are sometimes classed together with their ancestors, the ringed worms (such as the common earth-worm), as Articulata, a name which refers to a very obvious feature, the repetition of similar segments in a regular series from front to rear. This is perhaps most apparent in the ringed worms and centipedes, but it is to be seen in all members of the group. This same [69]
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[71]tendency to reduplication of parts in a regular series may be observed in the vertebrates, as we shall see. Slight indications of it are also to be found in the Nemertines. Numerous theories have been proposed which derive the vertebrates from some of the Articulata—from the ringed worms or the Crustaceans, and even from the air-breathing members; and at first sight such theories seem attractive, for in some of their more obvious characters there is a certain resemblance between the two groups. But there are also many and fundamental differences, and few zoologists have accepted any hypothesis of this type. We may briefly allude to some of these differences.

In the Arthropods, where the body consists of hard and soft parts, the 'skeleton' is an external one, and encloses the soft parts. Respiration occurs by means of the skin or of gills, or, in air-breathing forms, by 'trachea,' which are small branching tubes opening on the sides of the body. But in no case has the mouth or the digestive tract any connection with the respiratory system, a condition of affairs very different from that obtaining in the vertebrates. The nervous system consists of a brain, situated above the gullet, a nerve ring round the latter, and a double nerve cord running along the body, below the digestive canal. This is obviously the opposite position to that occupied by the main nerve cord in the vertebrates, an important point of difference.

The Arthropods are an extraordinarily successful group. A multitude of forms of Crustaceans populate the waters, and they are excelled in numbers and variety only by the insects upon land. While the individual size appears to be somewhat strictly limited, probably by the nature of the respiratory and blood systems, many types show exceedingly high development in various directions—in intelligence, in social and parental instincts, etc. The insects are of course to be regarded as the highest Articulata, and have, like the highest vertebrates, the mammals and birds, almost completely forsaken the water for the dry land and the air. An interesting member of the Articulata, from the standpoint of the Evolution theory, is the Peripatus, shown with a ringed worm on Fig. 52 (3). It gains its interest for us from the fact that, while classed as an Arthropod, it stands very nearly half-way between the ringed worms (Fig. 52 (1)) and the true Arthropoda, and thus forms a solitary link between the two types. In Fig. 52 (2) and Figs. 53 to 58 are shown a number of types of Arthropods.

We must now go back and take up the main thread of our story. The next stage that falls to be described is that of a highly interesting group of worms known as Enteropneusta, a name signifying 'gut breathers.' This group contains a very small number of worm forms, which are to be found burrowing in the sand of the sea floor. A typical example is the Balanoglossus, a worm of some four inches in length, whose general appearance is illustrated in Fig. 59. The creature has, as will be observed, a large muscular snout or proboscis, behind which follows a small portion called the collar, and behind this again the long body. The most noteworthy feature of the group, as the name implies, is the respiratory system. The mouth, which is situated in the region of the 'collar,' leads into a gullet, which is partially divided into an upper and a lower canal by means of two inwardly projecting longitudinal folds, one on either side. Only the lower of these canals is used as a food passage; the [73]
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[77]upper communicates with the outside by means of a large number of transverse slits on its sides. Water is continually being taken in by the mouth and passed along this upper canal, to reach the outside by way of the gill slits, and in doing so it passes over the gills, where the blood is circulating in fine capillary vessels. Here the blood is supplied with oxygen from the water, and is at the same time relieved of carbonic oxide. This, it will be observed, is the same method of respiration as that of the fishes. Behind the last gill slit the digestive canal becomes a simple tube, with two digestive glands or liver sacs. There are two main blood vessels, the larger running along above the digestive canal, and the smaller below it, the two being connected by means of numerous branches. There is a swelling of the dorsal vessel—a heart—at its forward extremity, in the base of the proboscis. The nervous system is peculiar; it consists of two nerve cords, the smaller below the gut and the larger above it—the latter therefore occupying a position similar to that of the spinal cord in the vertebrates. Thus in two respects, in its respiratory and nervous systems, Balanoglossus must be regarded as a highly extraordinary member of the worm group, and in both its peculiarities it shows an approach to the vertebrate. There can be little doubt as to the position of this group as an important connecting link between the ordinary worms and the vertebrates. Fig. 60 illustrates the main features of the anatomy.

As to the origin of the Enteropneusta, opinions are somewhat divided. Their blood system and their development would seem to suggest a descent from the ringed worms. On the other hand, their possession of a snout, and their very slight indication of division into segments, would seem to separate them from the group mentioned and to connect them rather with the Nemertines. The latter view is perhaps the more probable.

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