Page:The American Cyclopædia (1879) Volume I.djvu/549

ANIMALCULES 517 cess of time discharged from the parent body, either by splitting or through some orifice. Yet another mode of reproduction has been observed by Stein and other microscopists. It has been called the encysting process ; and although it has been studied in relation to but few forms, yet the facts already established render it very probable that many, if not indeed all the in- fusoria multiply by this or some closely allied process. An infusory animal about to become encysted secretes from the surface of its body a thick glutinous substance, which, gradually hardening, forms a firm case in which the ani- mal is shut up, but not so closely as to prevent tolerably free motion. A change now takes place in the animal itself; the cilia upon its surface are retracted, and the body assumes a pretty regular circular outline ; then either the whole body, or the nucleus only, breaks up into many small fragments, each of which assumes an in- dependent life, and moves freely in the parent organism; this mother-cell now bursts and is disintegrated, while the young brood swim forth either in the form of the parent, or in some transition shape, from which, through one or more changes, they pass into the permanent type identical with the parent organization. II. ROTIFEEA OK WHEEL ANIMALCULES. These have little in common with the order of infuso- ria of which we have spoken, being both more highly organized and formed on a different plan. Even in respect to size they differ, being gene- rally much larger, some having a length of half a line, and many being within the limit of un- assisted vision. By many naturalists they are classed with the articulated animals, under the term of cilio-articulates. Their name, as we have already stated, is derived from a particu- lar and very curious arrangement of the cilia covering two lobes near the anterior extrem- ity, which when in motion have exactly the appearance of two minute wheels rotating very rapidly. But this, though a striking peculiar- ity of many rotifers, is not common to them all. In some the cilia about the head are ar- ranged in a wavy line. The rotifera maybe defined as minute worm-like animals, very transparent, without legs, having the anterior portion of the body furnished with certain re- tractile lobes, the margins of which are covered with cilia, the alimentary canal distinct and having two orifices, the mouth having a true dental apparatus, the reproduction by ova only. They are aquatic, though a few species can ex- ist in moist earth. They are found alike in salt and fresh water, but rarely in that which is rendered foul by decaying vegetable and animal matter, and which swarms with the polygastric animalcules. It is only when these have devoured the decaying matter that the rotifer appears to feed upon them. Rotifera have great tenacity of life, and are not de- stroyed by complete and long-continued desic- cation. Individuals have been kept in vacuo with sulphuric acid and chloride of lime, in- suring the utmost possible amount of dryness, for a month, and yet revived on being placed in water. The rotifera have always two in- vesting membranes, both transparent, and the inner always flexible; the outer is in many quite firm, constituting a horn-like tube, from which the head and tail of the animal pro- trude. It never contains either lime or silica, which is probably the reason why no traces of these animal forms are found in any fossilif- erous rocks. Their bodies are retractile, and many creep like worms. They swim by means of their cilia very rapidly. Near the tail is, in most forms, either a dirk-like or claw -like pro- cess, by which the animal can attach itself. The gullet is furnished at its inferior portion with a masticating apparatus consisting of two strong semicircular jaws, each furnished with from one to five teeth, which appear to contain mineral matter. The stomach is either globu- lar or tubular, and scarcely distinguishable from the intestine below. Near the anus the intestine is enlarged into a sort of cloaca with which the genital apparatus communicates. Several small glandiform bodies are observed near the alimentary canal, and some undoubt- edly communicate with its cavity. It is a curi- ous fact that, though the digestive apparatus is in most of these animals much more fully developed than any other, yet in one genus described by Mr. Dalrymple (" Philosophical Transactions," 1849, p. 839), no anal orifice was found, and indeed scarcely any intestinal canal ; so that the excrementitious food must have been ejected from the mouth, as in some of the very low polygastric forms. We now come to locomotion. Several distinct longitu- dinal bands of a highly contractile tissue pass the entire length of the animal, and certain transverse bands have probably the same power. It is, however, very doubtful whether any true muscular tissue, with the characteristics by which we identify it in the higher animals, exists in these animalcules. The same remark applies to the nervous system. The function is certainly performed ; but whether the cords and masses which Ehrenberg describes as nerves and ganglia really have that character, is at least uncertain. Two red spots near the head are supposed on pretty strong evidence to be eyes, or at least rudimentary forms of the or- gan of vision. There is no proper circulatory apparatus, but water is very freely admitted into the body, and probably serves to aerate the tissues. It is kept in motion by cilia lining the tubes into which it is received. Reproduc- tion. All that is certainly known upon this subject is that the rotifera multiply by true ova, and never by gemming, budding, or spontaneous splitting, like the polygastrica. Until recently they were generally supposed to be hermaph- rodite, but some late observers believe them to be unisexual. Ovaries are made out with- out difficulty, and in the vast majority of indi- viduals ; but spermatozoa have been found in only a very few, perhaps only one species. If males exist as a separate sex, they are probabljr