my loving regard, that I might feast myself upon the waving grace of those pink and white feathers; and I thought of the poetical passage in which Edward Forbes expressed his emotions about these Crinoidea which "raise up a vision of an early world, a world the potentates of which were not men, but animals-of seas on whose tranquil surfaces myriads of convoluted Nautili sported, and in whose depths millions of Lily-stars waved wilfully on their slender stems. Now, the Lily-stars and Nautili are almost gone; a few lovely stragglers of those once abounding tribes remain to evidence the wondrous forms and structures of their comrades. Other beings, not less wonderful, and scarcely less graceful, have replaced them while the seas in which they flourished have become lands whereon man in his columned cathedrals and mazy palaces emulates the beauty and symmetry of their fluted stems and chambered cells."* The delight of getting new animals is like the delight of childhood in any novelty, an impulse that moves the soul through the intricate paths of knowledge,-knowledge, which is but broken wonder; and this delight the naturalist has constantly awaiting him. Satiety is not possible, for Nature is inexhaustible. Knowledge unfolds vista after vista, for ever stretching illimitably distant, the horizon moving as we move. New facts connect themselves with new forms; the most casual observation often becomes a spark of inextinguishable thought running along trains of inflammable suggestion. To this intent the naturalist should alwaps have pencil and note-book on his working-table in which to record every new fact, no matter how trifling it may seem at the moment; the time will come when that and other facts will be the keys to unlock many a casket. Not that Observation alone is, as many imagine, the potent instrument of Zoology. Lists of details crowd books and journals, yet these are in themselves no better than the observations of Chaldean shepherds, History of British Star-fishes, p. 2. which produced no Astronomy in centuries of watching. They find their place in science, only as the architectural mind disposes them in due co-ordination. What should we think of a chemist who, on mere inspection of substances, unaided by re-agents, and his balance, hoped to further Chemistry? What would lists of such observations avail? And in the far more complex science of Biology, how shall cursory inspec tion, superficial observation, avail! We must follow the Methods which have led to certainty in the exact sciences. We must render the complex facts of Life as simple as we can, by processes of elimination. Experiment must go hand in hand with Observation, controlling it, and assuring us that we have correctly observed. Much has been done, and is daily done, in this way, yet still men too easily content themselves with observation, or, what is equally fallacious, with anatomical deduction, declaring an organ to have such or such a function, merely because it resembles an organ known to have the function :† when in most of these cases, direct experiment would show the error of the conclusion. In former papers I have illustrated this point, and have again to do so apropos of the digestive power of the Sea Anemones. In my note-book is pencilled this brief query, "Do the Actinia digest at all?" a doubt which, in its naked simplicity, might rouse contempt in the mind of any zoologist accidentally reading it. What! here is an animal notoriously carnivorous, and you ask whether it can digest? Have not you yourself repeatedly fed these animals with limpets and cooked beef? are they not greedy of such food? It is perfectly true. Nevertheless a doubt occurred to me whether they did really digest, in any proper sense of the term; and I made a note of the doubt, as of a point to be investigated immediately on my arrival at the coast. Experiment should settle the doubt. Before narrating the experiments, it will be On this point, see the luminous Leçons de Physiologie Expérimentale of Claude Bernard, vol. ii. needful to settle with the reader a few generalities on the subject of digestion; since, in point of fact, the interest of the question falls mainly on the general subject, and only with a secondary importance on the digestive powers of the Anemones. What are we to understand by Digestion ? At first the question seems so easy; yet the closer it is investigated, the remoter seems the possibility of answering it. Let us make a clearance by first discriminating Digestion-as a special function of the intestinal canal from Assimilation, which is the general property possessed by all living tissues. For an animal to grow, and to repair the waste which the action of life incessantly produces, it must assimilate, which, as the word implies, means to separate from the external medium such substances as are like to its own substance, or can be converted into them by the vital chemistry, rejecting all such as are unlike, or not convertible. Very simple organisms find assimilable food in the element they live in, and the process of separation is easy: they have no stomach, not even a mouth, much less glands secreting solvent fluids. Very complex organisms, on the contrary, do not, in the air they breathe, or on the earth they tread, find the variety of substances necessary to build up their bodies; the substances have to be sought, captured, and when found, are not found in an assimilable condition, but in a condition requiring great changes, mechanical and chemical, before the substances are able to enter into the construction of the tissues. polype is carried further: no sooner does one of the filaments seize a prey than it retracts; all the others round it bend their points over the captive, and gradually enclose it; they then retract, and bring the food in contact with the body of the animal. The point of contact is next seen to yield inwards, retracting as the filaments had retracted, and, as it deepens, the food sinks into the substance of the body, the edges of the cavity closing over it. In the centre of the body the soluble parts are dissolved, the body having resumed its original appearance. This done, the insoluble parts make their way out, much as they made their way in; and thus the whole process of ingestion and egestion is accomplished. We need not pause to trace the episodes of the complex story of digestion in the higher animals, episodes of mastication, insalivation, chymification, chemical transformations aiding mechanical actions every one is familiar with the general facts. Let us only note that even milk, which contains all the substances needed for the nourishment of the child, contains them in a condition perfectly useless, as far as the direct and immediate nourishment of the child is concerned; until the milk has undergone the digestive process, namely, a succession of chemical decompositions and recompositions, it is no more competent to nourish the muscles, bones, and nerves of the child, than so much chalk and water, which is delusively sold as milk in. virtuous cities. The mutton chop, too, which we justly reckon such exAn example will make this plain: cellent food, is only" food potential;" Let us first consider the process in it must undergo a very curious series the Actinophrys, a microscopic ani- of changes before it can be converted mal carefully studied by Kölliker.* into blood. Nor is the business It is a mere mass of jelly-like sub- finished there. We are erroneously stance, very contractile, without the accustomed to consider blood as the slightest trace of organs, without final stage of food, previous to its even a distinct envelope separable assimilation. Physiologists trace the from the mass. The outer layer is story of digestion up to this point, formed into long tentacular fila- and there leave it; as story-writers ments, which, like the tentacles leave their heroes married, thereby of a polype, seize hold of young indicating that nothing more remains animalcules, or even minute crus- to be said. But just as marriage is taceans. The resemblance to the the beginning of a new act in the * Siebold ů. Kölliker's Zeitschrift für Wissenschaftliche Zoologie, i. 198. drama, and the act in which all life more fluid is secreted from the blood culminates, so is this blood-formation and poured into this canal during a but the commencement of a new single day than would make up the series of changes, and these the most whole mass of fluid circulating in important. I think it can be shown the blood vessels at any given that the blood itself is not more im- period.t mediately and directly assimilable than the mutton chop from which it was formed. In its passage through the walls of its vessels, it undergoes specific changes, fitting it for assimilation; without such changes it is not assimilable; blood, as blood, nourishes no tissue, but lies on it like any other foreign substance which must be got rid of by reabsorption into the veins as we see when a vessel is ruptured, and the blood gets deposited in the parenchyma. Blood is, in fact, as Bergmann and Leuckart well express it, "a depôt of assimilable and secretory substances; and its purpose in the economy is that of a regulating apparatus, which is necessitated by the fluctuations in the procuring of food.* Remember, also, that before Assimilation can take place, the food must be rendered soluble. Solubility is a primary condition, but not the only one. Many soluble substances have to undergo chemical changes, both of decomposition and allotropism, before they form parts of the living body. If albumen or sugar be injected into the veins, they will not be assimilated, but cast out unaltered in the excretions; whereas, if injected into the alimentary canal, or into the portal vein, which would carry them through the laboratory of the liver, they are entirely assimilated. Thus we see that solubility and transformation are the two digestive effects, to produce which, two agencies are needful, the mechanical and chemical. From these two points all other questions expansively radiate, to them they all converge. A single fact strikingly impresses the mind with a sense of the extent to which chemical agency reaches, namely, that in the course of fourand-twenty hours a sixth part of the whole weight of the body is poured into the alimentary canal, under the form of various secretions. Much Bat The reader's attention has been so fully directed to this twofold agency of Digestion, and especially to its chemical agency, that a clear view may be taken of the question which must arise as to what, in the abstract, is the purpose of Digestion. In the abstract we may declare it to be the preparation of the food, rendering it fitted for Assimilation. if we descend from heights of abstraction, and approach concrete questions, we soon find this answer including several processes-such as the prehension and mastication of food, its absorption and circulation, its aeration in the blood, and finally, its transudation through the walls of the capillaries-none of which can, without great impropriety, be called digestive. We must be more specific. No man would confound mastication with digestion, or circulation with digestion; and we must therefore limit the term digestion to specific meaning; mastication is the special function of the jaws, circulation of the vessels, respiration of the lungs, and digestion of the alimen tary canal. But even this is too vague for our purpose; we must affix a still more specific character to Digestion; and this may be expressed in the following formula: That, and that only, is a specifically diges tive act which takes place in an alimentary canal, by means of secretions capable of chemically modifying the food, so as to prepare it for Assimilation. some The preparation of food we have seen to be both mechanical and chemical, but I select the latter as the specific characteristic of the digestive process, in order to prevent confusion. Claude Bernard says: "We can conceive an animal without any digestive apparatus, mechanical or chemical, because living in an element which furnishes nutritive material directly; we can also conceive * Vergleichende, Anatomie und Physiologie, p. 164. plex city, not possible in a group of cottages. In the same way we should not expect to find digestion, respiration, sensation, or any other complex result, in animals so simple as a Sea Anemone. Nor could the notion ever have gained currency, had there been the proper precision in our zoological language, and had not the "fallacy of observation" misled us. Now to the experiments. The first point to be settled was this: Have the polypes anything of the nature of a solvent fluid secreted by their stomachs? "It is obvious," says Dr. Carpenter, the latest writer on this subject, "that a powerfully solvent fluid is secreted from the walls of the gastric cavity; for the soft parts of the food which is drawn into it are gradually dissolved, and this without the assistance of any mechanical trituration." Obvious, indeed, the fact seems, until it is interrogated a little more closely, and then we find, 1st, that no solvent fluid is secreted; 2d, that the food the digestive act reduced to a simple mechanical apparatus which has to press out certain alimentary juices capable of nourishing the tissues without undergoing chemical modifications; but usually the digestive act is composed of two orders of phenomena, physical and chemical."* This is a brief and luminous classification as regards the whole animal series, and it well expresses the ascending complexity of that series; but inasmuch as special functions only make their appearance at certain stages of that ascending series, inasmuch as the simpler animals have not the special functions of more complex animals, we must deny unto the two first classes of M. Bernard's series, any such special function as Digestion, and confine it to the third class. We do not, except in loose latitude of phrase, speak of the legs of an animalcule, meaning its organs of progression; because a leg is a specific organ of progression, unform in its elements throughout the series of animals possessing is not dissolved; but only the juices legs; nor should we, otherwise than in easy speech, talk of the digestion of a polype, meaning thereby its nutrition. The purpose of a leg, progression, is fulfilled by the cilia which move the animalcule; the purpose of digestion, preparation of food, is performed by the cavity of the polype; but the specific organs, named legs and alimentary canal, and the specific functions of those organs, walking and digestion, are in both cases absent. If the reader has followed me thus far, he will have understood that, when I doubted whether the Actinia digested, there was no doubt entertained of their power of preparing food, but only of their power of chemically digesting it. I doubted, in short, whether they should not be separated from the more complex animals which digest, and whether they should not rank in M. Bernard's second class. We do not call a hut or group of cottages a city. We do not speak of its commerce, its government, its literature; these are social functions, developed in a com a pressed out. My first experiment was to test the presence or absence of a secretion, which was accomplished thus: Tying a narrow strip of litmus-paper round a small piece of recently caught fish, and fastening it to a thread, I gave it to an Anthea cereus who greedily swallowed it; another thin slice of the same fish was folded longitudinally over similar bit of litmus-paper, and given to a Crassicornis. If any acid secretion were present, the paper would redden; if not, the blue colour would remain. On the following morning the ejected morsels were examined, but not a trace of acid reaction was visible. Repeating the experiment several times under varying conditions, I came to the conclusion that no acid fluid was present in the digestive process of the Actinia. There still remained a doubt. Solvent secretions are either acid or alkaline. It was necessary to make similar experiments with an alkaline reagent. This was done, and with similar results. It is worth noting that M. Hollard equally failed in detecting * Leçons de Physiol. Expérimentale, ii. 490. an acid or alkaline reaction,* which is a confirmation of my experiments. The Actinia do not effect their preparation of nutriment by chemical means; and in our strict sense of the term, they cannot be said to digest. I was anxious to see how far mechanical means were employed, and for this, Reaumur's admirable experiment was a guide. In his day it was supposed that digestion was a purely mechanical operation, the food being ground into a pulp in the stomach. He took hollow silver balls, perforated with holes, and filling them with meat, caused them to be swallowed by a dog. When they had remained a suitable period in the animal's stomach, they were withdrawn by the thread attached to them. If the digestive process were mechanical, the meat would be protected from all grinding action, by the silver covering; if chemical, the meat would be digested; and digested (or rather chymified) it proved to be; showing that a solvent fluid had penetrated the holes and dissolved the meat. I took a piece of quill, of about half an inch in length, open at both ends, and having six good openings cut in the sides, thus affording ample means for any solvent fluid to exert its action on the roast-beef enclosed in the quill. On examination of the ejected quills, I found no appreciable difference between the contained meat, and similar pieces of meat left in the water during the same period; in one of them which had the meat protruding somewhat from each end of the quill, there was a maceration of the protruded ends, which looked like a digestive effect, but on submitting it to the microscope, I found the musclefibres not at all disintegrated, the striæ being as perfect as in any other part, and the maceration obviously of a purely mechanical nature. A similar appearance is presented by meat, after its ejection by the Actinia: it is pulpy, colourless, but the muscles are not disintegrated. I dare not pause now to touch upon the many topics which are suggested by the conclusions to which these investigations led me. It will be enough just to note here the progressive complication of the digestive function in the progressive complexity of the animal series. Starting from the simple cell which draws its nutriment from the plasma surrounding it, by a simple process of endosmosis, we first arrive at the mouthless Actinophrys, or Amaba, which, folding its own substance over the food, presses out such nutriment as it can; we then reach the Infusory with a mouth, but without stomach of any kind;† and the Polype, which has a portion of its integument folded in, serving both for mouth and stomach, but not anatomically differing from the external integument, nor physiologically differing in its action from that of the Amaba's gelatinous substance; we then ascend to the Annelids having a real intestine, lying free in the general cavity, but only moderately, when at all, farnished with secretory apparatus; and so on till at length we reach the Mammalia, with their marvellously complex digestive apparatus. Corresponding with this increasing complexity of the organs is the increasing complexity of the food which the animals digest, from simple gases up to meat. If all were not so marvellous in Nature, would not the marvellous fact that food at all exists, arrest us? Food is what the organism can separate from the world around it, con * "Il est remarquable, et je m'en suis souvent assuré, que les papiers réactifs plongés dans cet organe, et dans la cavité inférieure, soit au moment de la digestion, soit chez l'animal à jeûn, ne donnent aucun indice d'acidité ni d'alcalanité."-"Etudes Zoologiques sur le genre Actinia."-Revue et Magazin de Zoologie, No. 4. 1854. Nobody now believes in Ehrenberg's Polygastrica, or many-stomached animal eules. Trembley turned a Hydra inside out, and found the outside perform the function of a stomach. This has been held as proof that a mucus membrane is only a reflection of the skin. But from what has been advanced in this paper the reader may suspect that, inasmuch as the polype has no mucus membrane whatever, the socalled stomach not being anatomically distinguishable from the external skin, and the process of digestion being wholly mechanical, the current opinion is not proved by Trembley's experiment. |