The principle that ancestral traits betray themselves is accepted as a truism in common life. Do we see young people rude and stupid? We say, perhaps, "No wonder; their grandfather was a drunken, worthless lout."
Do we see a family of the poorest cla.s.s clever, and industrious, and refined? We say, "They come of a good stock." When we speak in this way, we reason from the common experience of mankind, that children resemble their ancestors. Similarly, when zoologists find an embryo starting its existence from one cell, they say, "No wonder; its ancestors were unicellular." And when they find it a.s.suming a two-layered form, they say, "Its ancestors were two-layered creatures." So certain are zoologists of the existence of an ancestral two-layered form, the parent at once of the existing Coelenterata and of the higher forms, that Professor Haeckel has given it a special name--Gastraea. The two-layered young stage of higher creatures, when it has a free-swimming existence, is called a Gastrula (Fig. 6). Both names, meaning stomach-animal, refer to the structure, which is, in a still simpler form, that of _Hydra_--a two-layered bag of cells, of which the inner layer, lining the cavity, performs the work of digestion. The lowest of the Vertebrata, the Lancelet (see p. 140), has a larva of this kind. The same reasoning which suggests the existence of an ancestral Gastraea-animal, suggests that of an ancestral Planula-animal; for the two-layered animals, on their part, present us with a mon.o.blastic larva of the form already described (p. 34), called a Planula. Hence it is that zoologists look with such eagerness for forms, of which it can be said that they consist of one layer of cells only. The name Planula signifies "wandering animal," because the Planula larva swims about by means of cilia.
[Ill.u.s.tration: FIG. 6.--Diagrammatic representation of a typical Gastrula, or two-layered larval form, highly magnified; optical section, longitudinal. _Ec_, Ectoderm or skin layer; _En_, Endoderm or stomach layer; _m_, mouth leading into the enteric cavity. The dots are the nuclei of the cells.]
[Ill.u.s.tration: FIG. 7.--Diagrammatic representation of a typical Trochosphere, or ciliated larva, considerably magnified. _M_ is the mouth; the stomach and intestine are seen showing through the transparent body.]
Mention has been made above of larval forms. It is perhaps advisable to explain clearly what is meant by this term. It is a matter of every-day knowledge that in some animals the young form presents an appearance and structure very different from that of the grown-up form, and adapted for a different mode of life; the commonest instances are the caterpillar of the b.u.t.terfly and the tadpole of the frog. We are apt to think of these creatures as somewhat exceptional in this respect. But the zoologist, in viewing the whole range of the animal kingdom, finds a vast number of animals with larvae, differing much from the adult, and adapted for a different mode of life. It is, in fact, a very common arrangement; but often these larvae are very minute, perhaps absolutely microscopic, therefore only known to the scientific observer. The two familiar instances we have named are fortunately big enough to be known to everyone. Now it is an axiom with modern zoologists (as has been explained above), that the history of the individual is a summary of the history of its ancestors; larval forms are therefore of special interest in this connection. A very wide-spread form of larva, more advanced in its structure than the little Gastrula that has been already named, has received the name of Trochosphere or Wheel-ball (Fig. 7), because it swims round and round, by means of cilia, usually distributed in bands.
Its inner or stomach-layer, forms a definite alimentary ca.n.a.l, and is separated by a very simple mesoderm from the outside ciliated layer, which presents certain differences in form, according as the creature belongs to one group of animals or to another. The main characters of the Trochosphere are, however, the same in very widely differing groups.
These little larvae give rise to one of the most eagerly debated problems of zoology. Are we to suppose that animals which possess a Trochosphere larva are all descended from one common ancestor? Or are we to think that the Trochosphere is a form of body very convenient for the necessities of juvenile existence in the sea, and therefore independently evolved by animals which are not directly related to each other? Some authorities take the latter view; the former is perhaps more widely accepted, and has even been expressed by the application of the name Trochophora (Wheel-carriers), as a general term for those groups in which such larvae are found. These include some of the higher worms, which present the typical Trochosphere, the Brachiopoda, and the Polyzoa; while variations of the Trochosphere type are shown by the earliest larvae of Mollusca, the larvae of the Echinoderms, and those of the Hemichordata (see p. 33), the latter bringing us, as it were, within eye-shot of the Vertebrata themselves. It will be seen, therefore, that the range of the Trochosphere larva covers a large portion of the ground occupied by our Grade IV. There is, however, one marked exception: the Arthropoda, which seem to have a prejudice against cilia in any form (since they include but one animal which possess any) have no example of a ciliated larva. Even their simplest larval forms belong to a higher type of structure, in which the sh.e.l.ly, jointed structure characteristic of the group is already indicated.
When we speak, however, of the occurrence of the Trochosphere throughout a wide range of animal life, it must be understood that its presence is not necessarily uniform throughout a group in which it occurs. Larval forms are adaptations which conform with the conditions of life for the particular animal in question: and nearly related kinds of animal may be without a larva. The Trochosphere larva is, of course, only adapted for aquatic existence, and is necessarily absent in the case of terrestrial forms.
TABLE OF THE CLa.s.sIFICATION OF THE ANIMAL KINGDOM[A]
=Grade I.=--UNICELLULAR ANIMALS. PROTOZOA.
(Intermediate forms, see p. 34.)
=Grade II.=--TWO-LAYERED { SPONGES.
ANIMALS. { COELENTERATA.
=Grade III.=--THREE-LAYERED { PLATYHELMINTHES, OR FLAT-WORMS.
ANIMALS. { VERMES, THE HIGHER FORMS.
{ ARTHROPODA.
{ MOLLUSCA.
=Grade IV.=--COELOMATA, OR { BRACHIOPODA.
THREE-LAYERED ANIMALS WITH A { BRYOZOA.
BODY-CAVITY. { ECHINODERMATA.
{ TUNICATA OR ASCIDIANS. } =Chordata.= { VERTEBRATA. }
[A] In the subsequent tables which show the respective sub-divisions of these chief groups, the larger only of the sub-divisions are named.
When an animal has no free larva, but quits the egg in a form practically identical with that of the adult, the development is said to be "direct." But changes equally startling with those displayed when a larva develops into the adult form, may take place while the young animal is enclosed within the egg itself. To these also zoologists apply the axiom referred to above, that the history of the individual summarises the history of the race. Thus, for example, the Amphibian larva, _e.g._ the tadpole of a frog (p. 153) has gills, which disappear in the adult form: the young reptile, bird, or mammal, which has no larval stage, has gills during a comparatively early stage; and loses them at a later period of its development. In each case zoologists conclude that the animal is descended from a fish-like ancestor, which possessed gills all its life, and that the more immediate ancestors in the family tree, have lost their gills by degrees.
The study of the progressive changes of young forms, whether larval, or enclosed within the egg, is called Embryology, and const.i.tutes, in these days, the major branch of zoological science. That it is of paramount importance to the student of cla.s.sification, engaged upon the sorting of the animal kingdom, will be apparent from what has been stated above.
CHAPTER IV
THE ONE-CELLED ANIMALS OR PROTOZOA
Some idea of the general characteristics of the Protozoa has already been given by the description of _Amoeba_. We may now say something about special groups of the Protozoa, which have minor characteristics of their own.
Amoeba belongs to the cla.s.s Rhizopoda, as has been already stated; but there are many of the Rhizopoda that greatly differ from Amoeba in appearance. The possession of a sh.e.l.l or skeleton gives a special importance to several groups. For, as the reader has no doubt already learnt from an earlier volume in this series, such skeletons or sh.e.l.ls have played an important part in the history of the earth"s surface, building up geological strata of vast extent, by the acc.u.mulation of the sh.e.l.ls left after the decay of the owners" tiny bodies, during long periods of time. The chalk rocks that form the "white cliffs of Albion,"
and that are so widely distributed in other parts of the globe, are formed in this manner; while the ooze of the Atlantic and other oceans, similarly composed of Protozoan _debris_, is now at the present time building up what will be the chalk rocks of future ages. Some of these Protozoans attain a remarkable size, instead of being microscopic, as is the case typically with the one-celled animals. Some forms of the Foraminifera found on the coast of North America measure as much as one-fifth of an inch across, while in warmer seas there are kinds which attain, as did the extinct Nummulite of Egypt, the size of a bean. Two inches across is mentioned as the maximum diameter, however, of either extinct or living forms. The Foraminifera are sometimes named Reticularia, because their pseudopodia interlace.
[Ill.u.s.tration: FIG. 8.--Fossil Skeletons of Polycystina, from the so-called "Infusorial Earth" of Barbadoes, highly magnified.]
The Foraminifera have sh.e.l.ls composed of carbonate of lime, but there are other forms that build up geological deposits, in which the sh.e.l.l is flinty. The diagram (Fig. 8) shows some fossil sh.e.l.ls of Protozoa from the marl of Barbadoes. These const.i.tute a deposit which was named "Infusorial earth," in the earlier days of microscopic observation, when all Protozoans were spoken of as Infusoria. The name, Infusoria, it must be recollected, is now restricted to a special cla.s.s, to which the forms in question do not belong. These fossil forms were named Polycystina, and are still often spoken of under that name, although the animals that present the peculiar feature of possessing "more than one cyst" now are called Radiolarians. The "cyst" consists of a basket-work supporting skeleton of flint; there may be several, one inside the other, and connected by radial bars. A living species named _Actinomma_ has three such layers of basket-work, one in the outer layer of protoplasm, one in the inner layer, and a central one. It will perhaps be remembered by the reader that the animals of this group, Radiolaria, are forms described in a previous volume of the series, as so curiously a.s.sociated in Symbiosis with the algae known as Yellow Cells.
The famous polishing slate of Bilin in Bohemia consists of flinty Protozoan sh.e.l.ls; it is 14 feet thick, and a cubic inch has been estimated to contain 41,000,000,000 of the sh.e.l.ls.
While the Radiolarians are marine, the Heliozoa, a group in which the skeleton is also present, but not usually so greatly developed, are predominantly fresh-water forms. Both cla.s.ses take their name (Ray-animals, Sun-animals) from the stiff radiating rods of the skeleton.
Strongly to be contrasted with the above groups belonging to the Rhizopoda are the Infusoria proper, which are characterized by the usual possession of cilia. Cilia (literally "eyelashes") are fine hair-like processes of the protoplasm of the cell, which fringe its exterior; by their constant movement they enable the animal to swim, and at the same time they create a current in the water, which washes up to the region of the mouth particles which may serve for food; for these creatures have this very great advantage over Amoeba, and the other forms above referred to, that they possess something which may be called a mouth.
That is to say, there is one particular spot of the surface where particles are taken in. This may seem to be a restriction, when we compare the Infusorian with Amoeba, which is apparently able to take in food at any part of the surface. But it is a restriction which is a.s.sociated with an advantage; the Infusorian cell, namely, has a firm exterior with a definite outline, instead of being soft and mobile all over. The firmer exterior layer of protoplasm, which is in turn covered by a thin cuticle or limiting membrane, is called the cortex or rind.
For this reason the name Corticata is sometimes given to the group, _i.e._, Protozoa with a rind.
_Vorticella_, the Bell Animalcule, is a stalked form living in ditches, which is usually selected as a typical form of the Infusoria. It receives its name, the Whirlpool Animal, from the current which its cilia create in the water. The purpose of this current is to wash food particles into the mouth. a.s.sociated with the Infusoria under the name of Corticata are the Gregarina and some other parasitic forms.
It is interesting to note that the main types of the unicellular animals are repeated again in the cells of different parts of the bodies of multicellular animals. Amoeboid cells, so called because of their mobility and general resemblance to Amoeba, are found in various parts of the higher animals. The lymph corpuscles of vertebrata, and the white corpuscles of vertebrate blood, as well as the blood corpuscles of invertebrates, are among the instances of this. There are cells, on the contrary, such as those that line the mucous tracts, which are of a Vorticella type, so to speak; fixed to their bases, and presenting cilia on the free aspect.
Two things must be noticed before we leave the subject of the Protozoa.
One is, that some forms present the beginning of a multicellular condition. Several units sometimes join together, and in this way a complex object may be formed, in which there are several nuclei; or the original unit may keep on growing till it consists of many successive portions, and in some of them a fresh nucleus may arise. This occurs in some of the Foraminifera.
The next thing to be noticed is, that there are a number of organisms which const.i.tute a debateable ground, and are claimed now by the botanist, and now by the zoologist. While the latter insists on calling them Protozoa (Primitive Animals) the former would have them Protophyta (Primitive Plants). The fact is that in these organisms of the first grade, the distinction between "plant" and "animal" has not become a hard and fast line; and the disputed forms may be best described as links between the two. The chemistry of nutrition is probably more to be relied upon as a distinction, than the difference of structure. It is here that the two groups, plants and animals, start upon different roads, and many of the differences in structure must be regarded as the direct result of the fundamental difference in the mode of nutrition.
The following very instructive remarks on the subject are taken from Professor Hertwig"s valuable book "The Biological Problem of To-Day,"[B]
pp. 111, 112.
[B] "The Biological Problem of To-Day, Preformation or Epigenesis," by Professor O. Hertwig. Translated by P. C. Mitch.e.l.l. Heinemann, 1896.
"The different mode of nutrition of animals results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy pa.s.sage for solid particles, or they are clothed only by a thin membrane, through which solutions of slightly diffusible organic colloids may pa.s.s. Therefore, unlike plants, multicellular animals display a compact structure with internal organs adapted to the different conditions which result from the method of nutrition peculiar to animals. A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a s.p.a.ce within its body, into which solid organic food-particles are carried and digested thereafter in a state of solution, to be shared by the single cells lining the cavity. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. In its further complication the animal organisation proceeds along the same lines. The system of internal hollows becomes more complicated by the specialisation of secreting surfaces, and by the formation of an alimentary ca.n.a.l, and of a body-cavity separate from the alimentary ca.n.a.l. In plants it is the external surface that is increased as much as possible. In animals, in obedience to their different requirements, increase takes place in the internal surface. The specialisation of plants displays itself in organs externally visible--in leaves, twigs, flowers, and tendrils. The specialisation of animals is concealed within the body, for the internal surface is the starting-point for the formation of the organs and tissues."
TABLE SHOWING THE CLa.s.sIFICATION OF THE PROTOZOA
=Grade I.= { RHIZOPODA, OR { GYMNOMYXA.
UNICELLULAR ANIMALS. =PROTOZOA= { { INFUSORIA, OR { CORTICATA.
CHAPTER V
THE COELENTERATA
Next after the animals that consist of one cell only we have to consider the group of animals among which the lower kinds, at any rate, consist of a number of cells arranged in two layers. The representative of this group that the reader is most likely to meet with is the Sea-Anemone, the Coral animal probably he will be content to know from pictures.
Everybody who has been accustomed to take a little interest in natural history, remembers the use of the old-fashioned term "Zoophyte." It was a name given to animals like those named above, which have a flower-like appearance, due to the possession of a set of petal-like arms or tentacles, placed all round the mouth; its literal meaning was animal plant, in allusion to the flower-like form. The great French zoologist, Cuvier, gave the group name Radiata to animals of this kind. This name is now not much used, because we have learnt to emphasize other peculiarities possessed by these animals, as well as that of radial symmetry, viz., their two-layered body-wall and simple digestive s.p.a.ce (see p. 36). The group called Radiata by Cuvier, included, too, a number of animals which are widely separated from the "Zoophytes" in modern systems of cla.s.sification.
Sea-Anemones may be found on almost every rocky part of the English sh.o.r.es. Look for them in pools towards low-tide mark; if uncovered by the water, they will be found with the arms drawn in, so that the animal looks merely like a small round k.n.o.b of shiny opaque coloured jelly; if covered by the water, they will usually be found open, that is to say, with the arms (often called Tentacles) spread out. In the middle of the circle of arms is the mouth; and the apparent "flower" possesses an excellent appet.i.te, as will readily be seen if any unfortunate little shrimp or sea-snail should come within reach of the arms. The latter will then at once contract upon it, and draw it into the mouth. Touch any of the common Sea-Anemones, and you will find that it is firmly fixed to the rock; at an early period of life it becomes fixed, and practically it remains always in one place, although a slight movement of the base is sometimes possible. Hence the advantage of the "radial"
structure, for the arms reach equally in all directions round that most important centre of activity, the mouth. The most common kind of Sea-Anemone is of a dull dark red colour, and small in size; but others are large and brilliant in colouring. No uncoloured drawing would convey much idea of their beauty: the reader should consult the works of the late P. Gosse, an authority on Sea-Anemones, in whose books many beautiful ill.u.s.trations will be found.
A much smaller animal than the Sea-Anemone is found in fresh water and is called _Hydra_. Its arms or tentacles are longer in proportion to its body, especially in one species, than is the case in the Sea-Anemones.