Stories of the Universe: Animal Life Part 8

The group, as a whole, is sometimes considered to present evidence of having degenerated from a higher type; but whatever else may be doubtful or obscure in its history, the nature of the larval notochord is quite clear and certain; zoologists have never had any doubt about its nature since the first few years after its discovery.

Ascidians are not at all uncommon animals on the English coast. Some of them may be met with on stones near low-water mark, and I have often seen them on the sh.e.l.ls of oysters sold in the shops--for there the town-dwelling naturalist may often find a good many interesting things without much trouble. They are like little lumps of tough jelly; of various colours, according to the kind, red being the most common, and of very indefinite shape. You may see some of the colonial kinds forming pretty star-shaped patterns, attached to various objects, such as stones and the larger seaweeds.

The place of Ascidians in cla.s.sification was a puzzle, until their relationship with Vertebrates was discovered. At one time they were placed with the Mollusca. Now they are grouped, together with the Vertebrata and some other creatures that remain to be spoken of, under the name of Chordata, or animals possessing a Notochord.

Some of the Ascidians present what has been already described in other types (p. 57) as "alternation of generations." The discovery of this fact was made by the poet Adelbert von Chamisso. Some of his verses are known to English readers, for whom they were translated by Mary Howitt, a poetess whose writings were popular with our grandmothers, and deserved to be so. This is not the only case in which a poet has been also a zoologist: Goethe studied the science, and framed a theory regarding the vertebrate skull, which he regarded as consisting of a series of vertebrae. In this he was less fortunate than the Italian poet; for while Chamisso's observations were correct, and were confirmed by subsequent writers, Goethe's theory of the skull is anything but correct. It was made worse, too, by the speculations of subsequent writers, who attempted to follow it into detail, with the result of demonstrating its absurdity.

CHAPTER XIV

THE VERTEBRATA

We have spoken of the Notochord as a structure which precedes the formation of the spinal column in Vertebrates. This needs a little more definite explanation. We all know that the spinal column of vertebrates is formed to protect the spinal cord. This protection is, however, an afterthought, so to speak, of the vertebrate structure; the lowest of all vertebrates is quite without it; and in the lower groups of fishes we may trace various steps of its formation. But in these cases where the spinal column is absent or incomplete, there is a large and well-developed notochord; and in the embryo of higher vertebrates, when the spinal column has not yet begun to be formed, the notochord is equally a conspicuous feature. It runs from the region known as the mid-brain, to the end of the tail, and lies throughout just beneath the spinal cord. Whatever its original use in the animal body may have been, it undoubtedly acts now as a support to the spinal cord, and indeed to the whole body. Bones, we must explain, do not exist either in the lower vertebrate, or in the early embryo. In the latter they are formed by degrees. The spinal cord and the notochord each begin to be surrounded by rings of cartilage or gristle, which by degrees is changed into bone. The rings surrounding the notochord, however, gradually encroach upon it and obliterate it. The place where it has been becomes the Centrum, or most solid part of each vertebra. The notochord at first is continuous, and has no division into successive parts; but when the bony spinal column is developed, it consists of a series of successive vertebrae. Each of them is made up of several parts, which by degrees become consolidated into the vertebrae.

[Ill.u.s.tration: FIG. 41.--_A_, The Notochord of Vertebrates. Section, considerably magnified, through the middle of an embryo one inch long, of Acanthias, one of the Spiny Dog-fishes allied to the sharks. 1, Section through Spinal Cord; 2, Section through Notochord; below it lies a bean-shaped s.p.a.ce, which is a section through a large blood-vessel; _sk_, epiblast or skin; _me_, mesoblast or middle layer of the body; the dots represent the nuclei of its transparent cells. The intestine, _i_, lined with hypoblast, is traversed by a spiral valve, and surrounded by the horse-shoe shaped body-cavity. _B_, Diagram indicating the position of the Notochord in the vertebra of an adult Common Dog-fish (_Scyllium canicula_). 1, "Neural arch" of the vertebra, consisting of processes of bone enclosing the central nervous system, or spinal cord; 2, bony centrum of the vertebra, hollowed out into a cup, in which lies a soft pad, the remains of the notochord.]

The lowest member of the vertebrate group, separated in fact from the true vertebrates and placed in a lower division all by itself, is the little animal called the Lancelet or Amphioxus. It is often spoken of as a "fish"; but it is only by a stretch of our courtesy that it can receive that name, being an animal of a much lower form than the fishes.

It was discovered in 1834, in the Mediterranean, and described as a fish; but it had previously been discovered in 1778, by a German naturalist who described it as a slug. The latter was misled by its external shape. He had not the advantage of the modern methods of preparing animals for examination under the microscope; in these days, Amphioxus is cut into successive slices along its whole length, and each of these carefully magnified, so that no detail of structure is lost.

The Amphioxus burrows in the sea-sand; it lies buried in it, with its mouth just uncovered. Its food consists of microscopic vegetable organisms. Its distribution is very wide; it is found in both the Atlantic and Pacific waters. It occurs most abundantly in the salt-water lakes of Sicily, and in the Gulf of Naples. The specimen first seen, in 1778, came from the coast of Cornwall. There are eight species; the one which is found in the English Channel is the _Amphioxus lanceolatum_, also found in the Mediterranean and on the sh.o.r.es of North America.

The cla.s.ses of the Vertebrata are Fishes, Amphibia, Reptiles, Birds and Mammals. We used to learn that of these, fishes had gills, and Amphibia gills for a time; but, to be strictly accurate, we must say that fishes have gills, and _all_ the rest of the Vertebrata have gills for a time. There is no exception to this rule, not even among the highest vertebrates of all. But in those vertebrates which stand higher in the scale of life than Amphibia, viz., Reptiles, Birds, and Mammals, these gills are never brought into use. They only exist in the early embryo, and afterwards disappear, giving rise by their modification to other structures.

Strange to say, one of these structures is the ear. This takes its origin from one of the gill-"clefts" or s.p.a.ces. The Eustachian tube, which communicates between the ear and the nose, is part of this cleft; and the little bones which are inside the ear represent the bones of that gill-cleft. For, in fishes, bones support each gill, and are connected together to form a complex arrangement. In the higher vertebrates, which possess gills only in the embryo, this gill-skeleton is much modified, and persists as a bone, the hyoid bone supporting the tongue.

The gills of vertebrates, arranged in successive pairs along the throat, are "perforating gills"; that is to say, they consist essentially of holes or s.p.a.ces which pa.s.s right through the wall of the throat.

If we were to seek for a general character of the vertebrates, besides those mentioned above, that they all possess a notochord and gills, we might also find it in the character of the skin. Fishes, Reptiles, Birds and Mammals, all agree in this, that they have a special clothing of the skin--scales, feathers and fur, respectively. These three kinds of structure, although so widely differing in appearance, are practically formed all in the same way, viz., by alternate ingrowths and outgrowths of the skin; the ingrowth forming the root of the scale, hair or feather, and the outgrowth its projecting part. If these infoldings and outgrowths of the skin could be straightened out into a plane surface, the skin of a small vertebrate would cover an enormous area. The above list excludes the Amphibia: in this cla.s.s, it should be mentioned, the scales have been lost, and are only found in one group.

The scales of Fishes were at one time proposed as a basis of cla.s.sification: large groups being characterized respectively by the possession of plain rounded scales (cycloids), scales fringed at the posterior end (ctenoid, or comb-like); placoid scales, consisting of bony plates, and ganoid scales, large plates covered with shiny enamel.

These distinctions, however, were not found useful as a guide in cla.s.sification. The diagram shows the elaborate scales of the common sole.

[Ill.u.s.tration: FIG. 42.--Scales of the Common Sole, highly magnified.]

Let us now consider some other creatures that resemble vertebrates in some ways, and help to form the group of Chordata. Balanoglossus is one of them, the Acorn-tongue Animal. This odd name is given to it on account of a structure which is called (like the elephant's trunk) a Proboscis; this may be compared with a tongue, so far as its use goes, for it is thrust out to catch prey and again drawn in. It is oval in shape, and therefore fancifully compared to an acorn. It is highly sensitive, being richly supplied with nerves. The creature is to all intents and purposes a kind of worm; and, like many of the higher worms, it has a larva with bands of cilia. This larva, which is better represented in some species than in others, was originally described under the name of _Tornaeria_. It is considered to resemble, in some degree, the larva of Echinoderms; on this hint, some zoologists have sought to establish a connection between Vertebrates and Echinoderms, and have been able to find other points of comparison besides the one named. It remains to be seen whether this suggestion will lead to further results. It may be added that the larva of Balanoglossus has also been compared with that of Phoronis (p. 122), thus a.s.suming a relationship with the Polyzoa, and through them with the Brachiopoda. It appears, therefore, that the subject of the possible relationships of the Vertebrata is one of the greatest complexity. The last named theory, however, has been adversely criticised by very high authority.

We have not, however, explained yet what is the claim of Balanoglossus to be grouped with the Chordata. This consists in the fact that a certain part a.s.sociated with the interior of the proboscis has been identified, from its structure, mode of origin, and relations with the nerves, as a notochord. Balanoglossus also agrees with the true vertebrates in possessing successive pairs of perforating gills (see p.

142), which are especially noticeable in the young animal. The presence of this feature is important, in view of the fact that some authorities have sought to throw doubt on the genuineness of the notochord of Balanoglossus.

Balanoglossus is not without relations, some of which have been recently discovered, while others have been known for some time, although their affinities were not at first recognised. Among these the most remarkable are sessile forms which have received the names respectively of _Cephalodiscus_ and _Rhabdopleura_. Both produce buds and form a colony, and in both a notochord has been distinguished. The former was procured from the Straits of Magellan, while the latter makes its dwelling-place in a nearer region, having been found off the Shetland Islands, and off the Lofoden Islands. Cephalodiscus, which is a very curious creature, receives its name from a disc placed at the head end. The use of this structure is believed to be as follows. The units of the colony live inside a common system of tubes, which they secrete; each unit, when adult, is independent, and can move about inside the tubes; the disc is used as a means of attachment to successive spots of the tube-wall, as the animal wanders from place to place. Above the disc are twelve plume-like tentacles covered with cilia, which create a current in the water surrounding the head, and wash food particles into the mouth.

That these creatures are but distant relations of the true vertebrates is a fact expressed by the names under which they are grouped in cla.s.sification. Those forms which we have just described have received the name of Hemichordata--that is to say, Chordata which have but half a notochord, since the notochord is very restricted in extent; while the Ascidians are grouped under the name of Urochordata, or Chordata which only possess a notochord in the tail. The name of Adelochorda, "with an obscure chord," is sometimes applied to the Hemichordata.

TABLE SHOWING THE CLa.s.sIFICATION OF THE CHORDATA

{ HEMICHORDATA, BALANOGLOSSUS, &c.

=CHORDATA.= { UROCHORDATA, THE ASCIDIANS.

{ VERTEBRATA, THE BACK-BONED ANIMALS.

Let us return now to the Vertebrate. A character common to all the groups of the Vertebrata is the possession of teeth. Readers of the previous volumes of this series will recollect that, even among birds, instances of the possession of teeth may be found among fossil forms, although they are absent in the birds of the present day. In all the other divisions of the Vertebrata, the presence of teeth is the rule, their absence an exception so rare that we may easily note the chief instances of it. Among Amphibia, there are Toads that have no teeth; among Reptiles, the Tortoises and Turtles have none; among Mammals, teeth are wanting in _Echidna_, the Spiny Ant-eater; and in the Ant-eaters and the Whalebone Whales they are absent in the adult, although present in early embryonic life.

The majority of people, if asked to give a definition of the meaning of teeth, would reply that they are hard structures that grow in the jaw.

But this is an idea that requires very considerable modification from a scientific point of view. In the first place, they are found in other places besides the jaws; and in the second place, they are by rights structures originally belonging to the skin. Both these important facts must be ill.u.s.trated by reference to the Fishes, which exhibit the primitive types of teeth.

In fishes, not only are teeth found on the jawbone, but sometimes also on other bones which border upon the cavity of the mouth; they are found on the palatine bone, or roof-plate of the mouth, and, still more strange, upon bones which belong to the "hyoid apparatus," or skeleton of the gills (see above). The latter may form a set of throat-teeth, which are used for grinders, while the jaw-teeth are used for biting.

Among the Carps, the jaw-teeth are reduced, and the fish depends upon its throat-teeth only. In the Wra.s.ses, one pair of the bones that bear throat-teeth (the inferior pharyngeal bones) are fused, so as to form a stronger apparatus: and from this circ.u.mstance, the group of Fishes to which they belong has been given the name of Pharyngognathi, fishes possessing throat-jaws. They have, however, biting teeth as well, in the true jaws. The grinding teeth are apparently used for consuming the food in a leisurely manner when once it has been taken into the mouth.

A curious circ.u.mstance in connection with these "throat-jaws" is, that they produce musical sounds. Fishes have other means, however, of producing a voice--usually by means of the swimming-bladder and muscles in connection with it. Probably they are able, to some extent, to effect communication with each other in this way.

It has already been stated that teeth, in their primitive form, are to be regarded as skin-structures. Certain fish, which are looked upon as ancestral types, have, dispersed throughout the skin, a number of bony plates, or granules (placoid scales), more or less formidable, and tipped with a hard enamel-like substance. Teeth are regarded as but a special form of these. But if they are skin-structures, how come they in the mouth and throat? Because the mouth and throat are lined by an ingrowth from the external skin; the origin and growth of this is seen in the embryo.

In the Mammalia the teeth, though restricted in number, attain the greatest possible variety of form, so that the jaws of different but allied species may be distinguished by their teeth.

Let us now return to the lowest vertebrate of all, which has a large notochord and no bones. This is the _Amphioxus_, the Lancelet. Amphioxus has no bones whatever, and no head, in the sense in which we usually employ that term; that is to say, most of the structures which we see in the vertebrate head are undeveloped. The peculiarities of the structure of Amphioxus are many. Among them may be named the curious gills: these form a sort of basket-work along the sides of the throat, which at first sight bears little resemblance to the gills of fishes, and reminds us of those of Ascidians. The gills lead also, as in Ascidians, to another cavity, the Atrial chamber. This basket-work is formed, however, by the subdivision of the primary pairs of gills. These are very numerous, ninety pairs being sometimes named as the number. They cut up the wall of the throat to such an extent, that additional supporting bars are needed to strengthen it; and, by the formation of these, both in parallel and in transverse directions to the primary part.i.tions, the "basket-work" is produced, as the growth of the animal proceeds.

The primitive nature of the notochord is, however, perhaps the most striking feature of Amphioxus. The chord pa.s.ses to the front of the animal's snout--head it can hardly be called--instead of ending in the middle of the brain, as in true vertebrates, for there is, indeed, no "brain" of any extent to lie in front of it; and the notochord, together with the spinal cord itself, have no other protection than a fibrous sheath. The spinal column is thus entirely absent, except so far as it may be regarded as represented by this thin sheath.

The Lancelet also differs from the true vertebrates, in that it has no limbs. There is a fringing fin along the body, but it is not comparable with the fins of fishes. It differs also in possessing no teeth.

In one respect, however, the Lancelet reminds us of a fish: and that is in the arrangement of its muscles; these form a successive series of overlapping ma.s.ses on each side of the body, as in a fish.

The development of the Lancelet presents us with an instance of the two-layered larva, or Gastrula. This shows that Amphioxus is a comparatively primitive type. But it has been suspected that it is less primitive than it looks, and that it has degenerated from some higher form, owing to its preferring a dull mode of existence, half-buried in sand or mud.

There is a huge gap between the Lancelet and the true vertebrates. The lowest form of the latter is _Ammocoetes_, the larva of the Lamprey (_Petromyzon_). The latter, even in the adult form, has no true limbs, though there are fringing fins. The notochord sheath is supplemented, however, by cartilage bars which are equivalent to the beginnings of the vertebrae of the back-bone. The gills are very different from those of other true vertebrates, and it has no jaws. Teeth it has, however, on the tongue and the lining of the mouth. Probably this creature is greatly altered by adaptation to its peculiar mode of life, so that no certain conclusions can be drawn from it regarding the structure of primitive fishes. It has a sucking mouth, by means of which it hangs on to fishes, while it rasps away their flesh with its rough tongue. When not thus engaged, it hangs on to a stone by means of its suctional mouth, thus fixing itself at rest. The Hag-fish, _Myxine_, in many respects similar, devours dead fishes chiefly. The Hag-fish is found on English coasts: so is the Marine Lamprey; while two freshwater forms are found in streams.

Leaving the Cyclostomata, as the above fishes are called, we reach the true fishes, which have limbs and scales. Something has already been said regarding their teeth and gills. The Cartilaginous fishes, in which most part of the skeleton remains gristle and does not become transformed into bone, include the Sharks, Rays, and Dog-fishes, all savage animals with strong teeth. The common spotted Dog-fish of our own sh.o.r.es is familiar to everybody: fishermen regard it with disgust, as it is not eatable. The Rays are flattened fishes, which live at the bottom of rather deep water, and attain enormous size even on our own coasts.

The Thornback Skate is covered with p.r.i.c.kles (placoid scales). All these fishes are grouped under the name of Elasmobranchii, the Strap-gilled, so called from the structure of the gill-arches.

The majority of familiar fishes, such as the herring, mackerel, cod and sole, belong to the group of _Teleostei_, or Bony Fishes, in which, by contradistinction from the last group, as much of the skeleton as possible becomes bone. Nevertheless, traces of the notochord persist in the back-bone of these fishes. Break the back-bone across, of a cod or a sole, and you will find between adjacent sides of the centra, or middle parts of the vertebrae, a pad of gristly substance. This is the remaining substance of the notochord, which finds room between the cup-shaped sides of the centra. When the centrum, instead of being biconcave, is solid, as in the higher Vertebrata, the notochord is obliterated by its encroachment.

The Amphibia, familiarly represented by Frogs and Toads, receive their name, "adapted for both lives," from the fact that they usually divide their lives between land and water. They are, from one point of view, the most interesting of the cla.s.ses of the Vertebrata, for they form a dividing line between the lower and upper Chordata. Below we have Hemichordata, Ascidians, Amphioxus, Fishes; all water-dwellers, breathing by gills. Above, we have Reptiles, Birds, Mammals, air-breathers, never possessing gills, except for a short time, as rudiments in the embryo, not brought into use. They are linked by the Amphibia, in which we see the larva a water-dweller, breathing by gills; the adult, an air-breather, adapted for life on land, and obliged to come to the surface to breathe, even when it pa.s.ses its time in the water. The individual Amphibian tells us the past history of the higher groups; once they had gills--but growing older, they lost them.

Fig. 43 shows us an outline sketch of Amphibian larvae; we should require an enlarged diagram of an earlier stage, to show the gills, which are external and projecting at first, but afterwards are overgrown by the skin with the exception of an orifice on each side. The diagram shows the gradual change of form. The tails in these tadpoles will presently be lost, for they belong to the Anura, or tail-less order of Amphibia (Frogs and Toads). The tailed Amphibians, Urodela, are represented in Great Britain by the Newts, _Triton_, popularly called Efts. Belonging to the Tailed Amphibians also, is the Axolotl, a creature found in the lakes of Mexico, and in those of the Rocky Mountains. It may or may not retain its gills; and forms with gills, and forms without, may be found in the same lake, each capable of laying eggs. The two forms were at first described under two different generic names: but when specimens of the gill-bearing _Siredon_, kept in confinement, lost their gills, it was seen that they became _Amblystoma_. There are other cases of larval forms that produce young, and this curious occurrence is known as "paedogenesis."

[Ill.u.s.tration: FIG. 43.--Tadpoles, three-quarters of their natural size.

_A_ to _D_, different stages of the Tadpole of the Common Toad, from Epping Forest, England. _E_, Tadpole of _Pelodytes punctatus_, dorsal view.]

The Amphibia include the curious creatures called Caeciliae (blind animals), or Gymnophiona. They are snake-like in form, and are without limbs; they burrow underground. Their real place in cla.s.sification was not found out at first, but they were cla.s.sed, by a wrong shot, with the Reptiles. They are interesting as being the only Amphibians that have scales. These are very minute, embedded in the skin, and arranged in transverse rings. The name Gymnophiona, naked serpents, is therefore doubly inapplicable: for they are not serpents, and not scaleless.

The Reptiles and Birds at first sight seem to be widely different. The latter are the warmest blooded of all vertebrates, the former are coldblooded. The one wear feathers, the other scales. Nevertheless, there is an intimate connection between them; the reader has doubtless already learned from other sources the facts about their relationship, so we will not here do more than recall a few of these facts. One is, that the birds of earlier times had teeth in their beaks, and possessed jointed tails. Another, that the Reptiles of earlier times included forms that were able to fly. A third notable fact is the presence of claws on the wings of some birds, showing that the wing of the bird was not always wholly specialised for use in flight.

We owe to Professor Huxley, the recognition of the close relationship of Birds and Reptiles, and the name Sauropsida (Reptile-like animals), under which both are included. They agree in being air-breathers and never having gills, except the rudiments present in the early embryo: this distinguishes them from Amphibia. They agree in having the skull set on to the back-bone by a single articulating surface or condyle; and thus differ alike from Amphibia and from Vertebrata. They agree in having the red corpuscles of the blood nucleated; and in this differ from the Mammalia, in which the red corpuscles are non-nucleated discs.

From a popular point of view, we may say that the striking distinction between birds and reptiles lies in beauty and ugliness. Even in their eggs, the reptiles display no love for adornment, no colouring or pattern. Fig. 44 shows the eggs of some reptiles.

[Ill.u.s.tration: FIG. 44.--Eggs of Reptiles, half the natural size.

_A_, of African Cobra. _B_, of Common English Snake. _C_, of Common English Lizard, _Lacerta agilis_. _D_, of Elephantine Tortoise. _E_, of Crocodile.]