Page:The American Cyclopædia (1879) Volume I.djvu/548

 516 ANIMALCULES complished, the undigested portion, if any such remain, is protruded toward the surface, and finally emerges from the body of the animal as it might from a mass of jelly ; the opening by which it escaped closes behind it, and the ani- mal resumes its pristine form and condition. Of the mode of reproduction in these animals, we only know they multiply by self-division, and that when portions of the mass are cut or torn away, these maintain an independent ex- istence, and soon acquire the shape and func- tions of mature animals. Of their proper sexual reproduction we know nothing, al- though all analogy leads us to suppose that this multiplication by division, whether spon- taneous or artificial, must have its limit, and a proper sexual reproduction by germ and sperm cells be interposed. Ascending in the scale, we come to those polygastric infusoria which have a proper digestive canal the en- terodela of Ehrenberg. Though the existence of an alimentary canal is made the characteristic of this group, its presence in any of the genera is by no means certain. All have beyond con- troversy a mouth into which food is taken, and many have an anal orifice from which excre- ment is discharged ; but whether there is any canal with definite walls through which the food passes, as in the higher animals, is doubted by many naturalists, and denied by not a few. Ehrenberg indeed traced the course of the ca- nal passing very nearly straight in the length of the animal's body in some genera, convoluted in others, and in a third class winding in a spi- ral around the inner surface of the body, with flask-like appendices communicating with its cavity, and making up the great mass of the body. But the disciples of Ehrenberg, work- ing with the best modern improved micro- scopes, have not been able to satisfy themselves of the existence of this so-called digestive tube. A mouth and a short, generally ciliated oesoph- agus, these animals certainly have; but the existence of an alimentary canal, beyond this short gullet, is very doubtful. The infusoria of this class differ from the amoeba and other rhizopoda, in that they have a true investing membrane or skin, which in some families can be detached as an independent membrane ; and from the internal surface of this membrane partitions are sent off", which divide the general cavity of the body into separate chambers. In these the jelly-like tissue of the animal, the sarcode of Dujardin, is lodqed ; and into these chambers the food, when it has escaped from the oasophagus, is received : H passes from one to the other till it has made the circuit of the body, not, however, with much regularity, and is in its course digested ; and all of its aliment- ary substance being absorbed, the residue is ejected either by the mouth or by an anal ori- fice. Thus is the function of digestion per- formed in the enterodela. It was stated in the definition of the infusoria, that they have no nerves or blood vessels. Nervous matter has certainly never been detected in any of the class; and although Ehrenberg supposed that two colored (generally red) spots, which are found pretty constantly near the anterior part of the body, are eyes, yet, as he was equally confident of the existence and nature of these spots in some forms which undoubtedly belong to the vegetable kingdom, it is probable that he was in error as to these. In most poly- gastric infusoria, small vessels which appear to contain a clear, nearly colorless fluid, are found, which enlarge when full, and when empty con- tract so as to be scarcely visible. Their num- ber varies from a single one to ten or twelve ; they usually occupy the same place in individ- uals of the same species, and their contents seem sometimes to be propelled from one to the other. They are probably receptacles of nutrient fluid stored up for the use of the sys- tem. Another remarkable peculiarity of the infusoria is, that in the very substance of their bodies may generally be found a solid granular- looking mass of very variable form round, oval, curved, or even in some cases branched by some called the nucleus. By Ehrenberg it was said to be a testis ; and although this opin- ion has found little favor with the more recent observers, yet that this peculiar mass has a very important connection with the reproductive function cannot be denied. When the infusoria are about to multiply by self-division, the sep- aration always begins in the nucleus. May not this be a mass of germ cells, such as we see in those insects which, after one sexual connec- tion, continue throughout a succession of gen- erations to bring forth young, till the mass of germ cells is exhausted, and a second sexual act is necessary to continue the multiplication of the species ? In none of the infusoria has any muscular or contractile tissue been found, though the very lowest form, the amcebce, possess the function in an eminent degree. Here, as ever in the animal scale, function pre- cedes organization ; and the function of muscu- lar contractility is manifested while there is as yet no appearance of muscular tissue. The stalk of the vorticella forms a notable illustra- tion of this rule, as it possesses contractility in a remarkable degree, yet no muscular tissue is to be found in it. Reproduction is effected in different ways in the different forms ; the mode which has been best studied is that by sponta- neous self-division. This is sometimes longitu- dinal, sometimes transverse. As before stated, it begins in the nucleus, and this body is often completely divided while the line of future sep- aration has scarcely begun to appear on the surface of the animal. These subdivisions are completed in so short a time, that Ehrenberg has calculated that no fewer than 268,000,000 may be produced in the space of one month from a single individual. Another mode in which new individuals are formed is by what is called conjugation. Two individuals attach themselves together, till at length their entire bodies coalesce and form one, in the interior of which a new individual is formed, and in pro-