This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(92). The reproduction of these animalcules is effected in various ways; and not unfrequently the same individual would appear to propagate by two or three different modes of increase.
(93). The first is by external gemmules or buds, which sprout like minute gelatinous tubercles from the surface of the body, and, gradually attaining the shape of their parent, develope the cilia characteristic of their species, and soon become independent beings, although they do not attain to their full growth until some time after their separation.
(94). The most usual mode of propagation, however, is by spontaneous fissure, or division of the body of an adult animalcule into two or more portions, each of which is perfect in all its parts. This singular kind of generation, by which the old animalcule literally becomes converted into two or more young ones, is accomplished in various ways, which will require separate notice.
(95). In the oval forms of the Infusoria, the line of separation generally divides the body transversely into two equal portions, by a process the different stages of which are represented in fig. 20,1,2,3. The body of an animalcule about to divide in this manner becomes at first slightly-elongated, and a line more transparent than the rest of its body is seen to cross its middle portion: a constriction becomes gradually apparent at each extremity of the line of division; this soon grows more decided, and at length the two parts are only united by a narrow isthmus (fig. 20, 3), which, getting thinner and thinner, allows a slight effort on the part of either of the now nearly distinct portions to tear itself from the other half and complete the separation.
(96). In some elongated species (fig. 20, 4) the fissure is effected in a longitudinal direction, the separation gradually proceeding from the posterior to the anterior extremity of the body (fig. 20, 6); yet even in these the division is occasionally transverse, the newly-formed creature appearing truncated at one end (fig. 20, 5) for some time after the completion of the process.
(97). The above mode of generation, as exhibited in Vorticella, a group of which is seen at fig. 20,9,10,11,12,13, is very curious; and from the different forms assumed by the young during the progress of their development much confusion has occurred, each stage of growth having been described as the permanent appearance of a distinct species. This beautiful animalcule seems to be propagated in several ways. Sometimes this is effected by external gemmules, which appear like minute points, scarcely more than 1/1000th of a line in diameter, upon the pedicles of the adult Convallariae; these in time become pedunculated, and, although still very small, exhibit the cilia upon the margins of the delicate cups; in this state they were called by Schrank Vorticellce mona-dicce (fig. 21.) The Vorticellae generally, however, multiply by fissure, the bell-shaped cup at the extremity of their highly irritable pedicles separating longitudinally into two; but the progress of this division requires our particular notice, as the unpractised observer might be considerably puzzled on witnessing some of the phenomena attending it.
(98). The adult animalcule (seen with its pedicle fully extended, fig. 20,9), when it is alarmed, shrinks by throwing its stem into spiral folds (10.) In the latter figure, the bell or body of the animalcule is seen to have extended considerably in breadth, preparatory to its becoming divided into two distinct creatures. At 11, the commencement of its division is depicted, - the separation gradually extending from the base, or ciliated extremity, to the point where the body is attached to its stem. When the division has extended thus far (12), the newly-formed portion is seen with surprise to have become furnished with cilia at both ends, and when finally detached (13), only at the opposite extremity to that on which they originally existed; it then becomes freed from its pedicle, and thus losing the great characteristic of its species, swims about at large, exhibiting forms represented at 14,15,16,17, all of which have been described as distinct species by different writers; at last it puts forth a new stem, and, assuming the adult form, becomes fixed by its pedicle to some foreign body.
(99). This fissiparous mode of reproduction is amazingly productive, and indeed far surpasses in fertility any other with which we are acquainted, not excepting the most prolific insects or even fishes. Thus a Paramecium, if well supplied with food, has been observed to divide every twenty-four hours, so that in a fortnight, allowing the product of each division to multiply at the same rate, 16,384 animalcules would be produced from the same stock; and in four weeks the astonishing number of 268,435,456 new beings would result from a continued repetition of the process: we therefore feel but little surprise that, with such powers of increase, these minute creatures soon become diffused in countless myriads through the waters adapted to their habits.
If the organization of these animalcules were as simple as it was supposed to be a few years ago, when they were thought to be mere specks of living jelly, imbibing nourishment at every point of their surface, which became diffused through all parts of the homogeneous texture of their bodies, their spontaneous division would be perfectly intelligible, and every step of the process easily understood; but, setting aside the conformation of their digestive apparatus, there are many circumstances attending the operation indicative of a power of developing new organs in the construction of every fresh individual, which must be looked upon as a very interesting feature in their history. Thus a new oral orifice, surrounded with cilia, must be formed upon the posterior segment of each divided animalcule. In Nassula elegans the curious dental apparatus, complex as its structure seems to be, must be developed upon the new part of the body preparatory to every separation; and accordingly a new mouth or dental cylinder is actually seen to sprout from the hinder half of the creature before its transverse fissure is complete.