Invertebrate Animals

When we reach invertebrate animals, we enter an immense series of different animals, the most numerous in nature, the most curious and the most interesting with respect to the affinities in the observable differences of organic structure and faculties.

One is convinced, by observing their condition, that to bring them successively into existence, nature proceeded gradually from simplest to the most complex.

Nature intended to reach a planned organic structure which would permit the greatest improvement (that of the vertebrate animals), a plan very different from the one which she was probably forced to create to reach that goal, one senses that among these numerous animals we should run into not a single system in which the organic structure is improved progressively but various very distinct systems, each of which must have come about at the point where each organ of primary importance started to come into being.

When nature succeeds in creating a special organ for digestion (as in the polyps), for the first time she gave a particular and constant form to the animal furnished with it.

The infusorians, where she started it all, could not possess either the faculty which this organ provides or the style and shape of its organic structure in order to develop its functions.

Later, when nature established a special organ for respiration, as she varied this organ to improve it and to accommodate it to the environmental circumstances of the animals, she diversified the organic structure according to what the existence and the development of the other special organs successively required.

When, after that, nature succeeded in producing the nervous system, it was at once possible to create a system of muscles, and from then on it was necessary to have strong points of attachment for the muscles, the paired parts making up a symmetric form. The result of that was different types of organic structure, because of the environmental circumstances and the acquired parts, which could not have arisen previously.

Finally, when nature had attained sufficient movement in the fluids contained by the animal, so that the circulation could be organized, that once again resulted in important particular features in the organic structure which distinguishes it from organic systems in which circulation does not occur at all.

To notice the basis of what I have just laid out and to set out evidence for the degradation and the simplification in the organic structure (since we are following the order of nature in the reverse direction), let us run quickly through the different classes of invertebrate animals.

MOLLUSKS

As one goes down the graduated ladder which forms the series of animals, the fifth rank necessarily belongs to the mollusks. For although we place them a degree lower than the fish, because they have no vertebral column, these are nevertheless the best organized of the invertebrates. The breathe by gills, but very diversified ones, in their form and size, or in their location on the inside or the outside of the animal, according to the genera and the habits of the races which these genera include. They all have a brain, nerves which do not have ganglia, that is, which do not display a row of ganglions along the length of a longitudinal marrow, arteries and veins, and one or a several one-chambered hearts. These are the only known animals which possess a nervous system but not a spinal column or a longitudinal marrow with ganglia..

The gills basically destined by nature to make respiration work in the very depths of the sea had to undergo modifications in relation to their faculties and their forms in the aquatic animals who, in the course of reproducing individuals of their race, went out often to expose themselves to the air and, as with many of these races, to remain there habitually.

The respiratory organ of these animals became imperceptibly accustomed to the air. This is not a supposition. For we known that all the crustaceans have gills; however, we know of crabs (cancer ruricola) who live habitually on the earth, breathing the natural air with their gills. Finally, this habit of breathing air with gills became necessary for plenty of mollusks who acquired it. The habit modified the organ itself, so that the gills of these animals did not require any more so many points of contact with the respiratory fluid and became stuck to the inner walls of the cavity which contains them.

As a result, we distinguish among the mollusks two types of gills.

Some are made up of a maze of vessels which move up onto the skin of an interior cavity which does not form any projection and which can only breathe air. We can call these air gills.

The others are the organs almost always projecting, inside or outside the animal, forming fringes or pectinate layers or thin cords, and so on, which cannot make respiration occur except with the assistance of fluid water. We can call these aquatic gills.

If differences in the habits of the animals have brought about changes in their organs, we can at this point conclude from this fact that, for the study of characteristics peculiar to certain orders of mollusks, it will be useful to distinguish those who have air gills from those whose gills cannot respire except in the water. But one way or the other, it is always a matter of gills, and it seems to us very inconvenient to state that the mollusks which breathe air possess a lung. Who is not aware how often the abuse of words and the false application of names have helped to distort objects and to throw us into error?

Is there such a great difference between the respiratory organ of the Pneumoderma, which consists of a maze or vascular threads spread over the top of an exterior skin and the vascular maze of snails which spreads across an interior skin. The Pneumoderma, however, appears to breath only water.

Besides, let us examine for a moment whether there are affinities between the respiratory organ of mollusks which breathe air and the lungs of vertebrate animals.

The characteristic property of the lung is to consist of a specific spongy mass, composed of more or less numerous cells, in which the natural air always moves, at first through the mouth of the animal, and from there through a more or less cartilaginous canal, called the trachea, which generally is divided into networks called bronchia, which end up in the cells. The cells and the bronchia are alternately filled with and emptied of air as a result of the successive swelling and collapse of the bodily cavity containing the mass of the organ. Consequently, it is a specific feature of the lung that it presents distinct and alternating inhalations and exhalations. This organ can survive only with direct contact with the air and becomes powerfully irritated by contact with water or any other material. It is thus of a different nature from the branchial cavity of certain mollusks which is always unique, which displays no distinct inhalation and exhalation and no alternating swelling and collapsing, which never has a trachea or bronchi, and in which the respiratory fluid never enters by the animal’s mouth.

A respiratory cavity which displays neither trachea nor bronchi and no alternate swelling and collapsing, in which the respiratory fluid does not enter by the mouth, and which adapts itself, sometimes to the air, sometimes to the water, would not be capable of being a lung. To mix such different objects up with the same name is not to advance science but to embarrass it.

The lung is the only respiratory organ which can give an animal the vocal faculty. After the reptiles, no animal has a lung; hence, none has a voice.

I conclude that it is not true that there are mollusks which breathe by a lung. If some breathe natural air, certain crustaceans do the same, as well as all the insects. But none of these animals has a true lung, so long as we do not give the same name to very different objects.

If the mollusks, because of their general organic structure, inferior in its improvements compared to fish, also establish, for their part, the progressive degradation which we are examining in the animal chain, the same degradation among the mollusks themselves is not so easy to determine. For among the very numerous and very diversified animals of this class, it is difficult to separate what belongs to the degradation in question and what is the product of the places where these animals live.

In truth, the two unique orders dividing the numerous class of mollusks very clearly contrast with each other in the importance of their distinct characteristics. The animals of the first of these orders (the cephalid mollusks) have a very distinct head, eyes, jaws or a proboscis, and reproduce by mating.

By contrast, all the mollusks of the second order (the acephalid mollusks) have no head, eyes, jaws, oral proboscis, and never reproduce by mating.

Now, one cannot deny that the second order of mollusks is inferior to the first with respect to an improved organic structure.

However, it is important to consider that the lack of head, eyes, and so on, in the acephalid mollusks is not uniquely the result of the general degradation in organic structure, since in the lower ranks of the animal chain, we find once more animals with a head, eyes, and so on. But it seems here that this is one of those deviations in the progressive improvements in organic structure produced by circumstances and, consequently, by causes foreign to those which gradually increase the complexity of organic structure in animals.

In considering the influence of the use of organs (the influence of an absolute absence of use and of constant use), we will see, in fact, that a head, eyes, and so on would have been particularly useless to the mollusks of the second order, because the great development in their mantle would not have permitted these organs any use whatsoever.

In conformity with that natural law which wills that all organs continually unused imperceptibly deteriorate, wither up, and finally disappear completely, we find the head, eyes, jaws, and so on, done away with in the acephalid mollusks. We will see elsewhere plenty of other examples of this.

In the invertebrate animals, since nature did not find in the interior parts points of attachment for muscular movement, she substituted, in the mollusks, the mantle with which she has furnished them. Now, this mantle in mollusks is stiffer and more compressed, the more these animals move about and the more they have only this mantle to assist them.

Thus, in the cephalid mollusks, where there is more movement than in those who do not have a head, the mantle is narrower, thicker, and stiffer. Among these cephalid mollusks, those which are bare (without shells) have in their mantle an additional breast plate even stiffer than the mantle itself, which makes the movements and the contractions of the animal (slugs) considerably easier.

But if instead of following the animal chain in the reverse order to nature’s, we move through it from the most unimproved to the most improved, then it would be easily perceived that nature, just at the point of starting the plan of the organic structure of vertebrate animals, was forced, in the mollusks, to abandon the use of a shell or horny covering for the points of attachment for muscular action and that, as nature prepared to bring these points of attachment into the interior of the animal, the mollusks were placed, in some way, at the transition point in the system of organic structure. Consequently, having nothing more than feeble means of moving around, they execute these movements only remarkably slowly.