140

INFUSORIA.-PORIFERA OR SPONGES.

the mouth, becomes most conspicuous. The alimentary par ticles introduced into the mouth commonly have to pass down a short canal before they enter the general cavity of the body; and within this cavity a number of minute particles are commonly aggregated into a sort of little pellet, as may be well seen when Infusoria are fed with carmine or indigo. One after another of these pellets being thus introduced into the interior, which is occupied by a soft sarcode, each seems to push onwards its predecessors; and a kind of circulation is thus occasioned in the contents of the cavity. The pellets that first entered make their way out after a time (their nutritive materials having been yielded up), generally by a distinct anal orifice, sometimes, however, by any part of the surface indifferently, and sometimes by the mouth.

135. The multiplication of Infusoria ordinarily takes place by spontaneous fission, precisely after the manner of the multiplication of ordinary cells (§ 33). This process, under favourable circumstances, may be performed with such rapidity, that, according to the computation of Ehrenberg, no fewer than 268 millions might be produced in a month by the repeated subdivision of a single Paramecium. Sometimes, instead of undergoing subdivision into two equal parts, the Animalcule puts forth a bud, which gradually increases, and then detaches itself from the parent stock. Whether anything equivalent to the sexual generation of higher animals. occurs among Infusoria, is yet uncertain; but recent researches afford a probability in the affirmative.

136. In the tribe of PORIFERA, or Sponges, we seem to have the connecting link between Protozoa and Zoophytes. For their animality does not lie so much in the individual particles, as in those aggregations which begin to shadow. forth that distinction into organs which is carried out more completely among Zoophytes: and there is a large section of the last-named group, in which the polypes are connected together, not by a regular stony or horny stem, but by a sponge-like mass; while the extension of the fabric is provided for by the budding out of this spongy portion of it, the orifices of whose canals after a time become furnished with polype-mouths. The true Sponge (fig. 81) consists of a fleshy substance, composed of an aggregation of particles of sarcode, supported upon a skeleton which usually consists of a net

PORIFERA OR SPONGES.

141

work of horny fibres, strengthened by spicules of mineral matter, sometimes calcareous, but more commonly siliceous. The entire mass is traversed by a great number of canals, which may be said to commence in the small pores upon its surface, and which discharge themselves into the wide canals that terminate in the large orifices, or vents, that usually project more or less from the surface

of the Sponge. Through this system of canals, there is continually taking place, during the living state of the animal, a circulation of water, which is drawn in from without through the minute pores, then passes into the large canals, and is ejected in a constant stream from the vents. The immediate cause of this movement seems to lie in the vibration of cilia so extremely minute that their existence can only be detected by the most careful microscopic examination. Its purpose is evidently to convey to the animal the nutriment which it requires, and to carry off the matter which it has to reject. No distinct indications of sensation, or of power of locomotion, have been seen in the Sponge but changes in the form of its projecting vents may be seen to take place from time to time, if it be watched sufficiently long.

137. The reproduction of the Sponge is commonly effected by the budding forth of little particles of sarcode, from the layer which lines the larger canals; these become furnished with cilia, and, when detached and carried out by the current that issues from the vents, swim freely about for some time; so as, before fixing themselves and beginning to develope into Sponges, to spread the race through distant localities. But it appears that Sponges are also reproduced by a true sexual process; "sperm-cells" and "germ-cells" being produced (as in the Hydra, § 123) in different parts of the organism, and a true embryo taking its origin in the action of the contents of the former upon those of the latter.

142

NATURE AND SOURCES OF ANIMAL FOOD.

138. We thus conclude our general survey of the Animal Kingdom; which, it is hoped, will be found to answer the purpose for which it was designed,—that of giving such an amount of preparatory knowledge respecting the principal types of animal structure, as may enable even the beginner to comprehend what will hereafter be stated of their physiological actions. It has not been attempted to observe any proportion in the notice of these several types; the higher forms having been slightly passed over, because the details of their vital phenomena will constitute the principal subject of the following pages; whilst some among the lower have been more fully treated, because the ordinary reader cannot be expected to have even that outline-acquaintance with their nature and actions, which he can scarcely help possessing in the case of animals that are familiar to him.

CHAPTER III.

NATURE AND SOURCES OF ANIMAL FOOD.

139. BEFORE We examine the nature of the process by which the food of animals is prepared for absorption into their bodies, it will be desirable to consider the characters of the aliment itself, and the purposes to which it is to be appropriated. The term food or aliment may be applied to all those substances which, when introduced into the living body, serve as materials for its growth, or for the repair of the losses which it is continually sustaining (§ 55). When animals are deprived of these materials, we see their bodies progressively diminishing in bulk, their strength decreases, and death at last takes place, after sufferings more or less prolonged. In warm-blooded animals, however, a yet more urgent demand for food is created by the requirements of the heat-producing process; and many substances are fitted to supply this, which cannot serve for the nourishment of the tissues.

140. The demand of the body for food is made known by a peculiar sensation, which has its seat in the stomach, namely, hunger. It is increased by mental and bodily exercise, and

NATURE AND SOURCES OF ANIMAL FOOD.

143 by everything which augments the general energy of the system; whilst, on the contrary, everything which tends to retard the operations of life, such as bodily and mental inactivity, sleep, or depression of spirits, tends also to render the demand for food less imperious. Thus, cold-blooded animals, particularly Reptiles, can sustain a very prolonged abstinence, when the general activity of their functions is kept down by a low temperature; and hybernating Mammals, which pass the winter in a state of torpidity, require no food during the continuance of their lethargy. But with this exception, warm-blooded animals require a constant supply of nutriment, not merely for the maintenance of their proper heat, but also for the repair of the waste resulting from that continuous activity which the uniform temperature of their own bodies enables them to keep up. This is the case with Man and the Mammalia generally, and still more with Birds, whose temperature is higher, and whose movements are more active and energetic. It is also more the case with young animals than with adults; since in the former the changes in the tissues, in consequence of the increase they are undergoing, take place with much more rapidity than in the latter, the bulk of whose bodies remains stationary. Hence, if children, young persons, and adults be shut up together, and deprived of food, the younger will usually perish first, and the adults will survive the longest. The Italian poet Dante has given a terrible picture of such an occurrence, in his history of the imprisonment of Count Ugolino and his children.

141. The difference in the demand for food between the young growing animal and that which has arrived at maturity, is very remarkable in the case of Insects. There are no animals more voracious than the larva or caterpillar; and there are none that can sustain abstinence, with little diminution of their activity, better than the imago or perfect insect. The larvæ of the Flesh-fly, produced from the eggs laid in carrion, are said to increase in weight 200 times in the course of 24 hours; and their voracity is so great as to have caused Linnæus to assert, that three individuals and their immediate progeny (each female giving birth to at least 20,000 young, and a few days sufficing for the production of a third generation) would devour the carcase of a horse with greater celerity than a lion. The larva of the Silk-worm

144

NATURE AND SOURCES OF ANIMAL FOOD.

weighs, when hatched, about 1-100th of a grain; previously to its first metamorphosis it increases to 95 grains, or 9,500 times its original weight. The comparative weight of the full-grown caterpillar of the Goat-moth to that of the young one just crept out of the egg, is said to be as 72,000 to 1. For this enormous increase a very constant supply of material is necessary, and many larvæ perish if left unsupplied with food for a single day. On the other hand, a black beetle (Melasoma) has been known to live seven months, pinned down to a board; and another beetle (Scarabæus) has been kept three years without food,- and this without manifesting any inconvenience or loss of activity. There are many perfect insects which never eat after their last change, but die as soon as they have performed their part in the propagation of the race.

142. The nature of the food of animals is as various as the conformation of their different tribes. It always consists, however, of substances that have previously undergone organization. There are some apparent exceptions to this, in the case of animals which seem to derive their support, in part at least, from mineral matter. Thus, the Spatangus (an animal allied to the Echinus, § 119) fills its stomach with sand; but it really derives its nourishment from the minute animals which this contains. The Earthworm and some kinds of Beetles are known to swallow earth; but only to obtain from it the remains of vegetable matter that are mixed with it. By some races of Man, too, what seems to be mineral matter is mixed with other articles of food, and is said to be nutritious; this may be beneficial, in part, by giving bulk to the aliment, and thus exciting the action of the stomach (§ 205); but it has been found, in one case at least, that the supposed earth consists of the remains of animalcules, and contains no inconsiderable portion of organic matter.

143. There are many instances in which, no obvious supplies of food being afforded, the mode of sustenance is obscure; and it has been frequently supposed that, in such cases, the animals are sustained by air and water alone. But it will always be found that, where food is taken in no other way, a supply of the microscopic forms of animal or vegetable life is introduced by ciliary action (§ 45); and it is on these, indeed, that a large proportion of the lower forms of aquatic animals depend entirely for their support.