No Nematoneurous class presents an example of generation by spontaneous fision or gem mation, but these modes of reproduction are common in the Acrite division.
The planariae among the sterelmintha are capable of indefinite multiplication by simple division; and the medusze are stated to pro duce, not ova, but ciliated locomotive gem mules or internal buds. The various examples of these plant-like modes of generation which the polypi and polygastrica present are fa miliar to most persons, and will be especially treated of under their respective articles.
The fissiparous and gemmiparous modes of reproduction are not, however, the exclusive modes by which the Acrite classes are perpetu ated. Most of the sterelmintha are propagated by means of ova : in the cystica and cestoi dea, the generative organs consist of ovaries alone, or are cryptandrous ; in the tremato . da, a fecundating gland is superadded to the ovary ; while in the acanthocephala the sexes are separate, so that thus early in the animal kingdom, we find typified all the different modes of generation by which the race is con tinued in the higher classes of animals.
The different conditions of the important organic systems which are thus seen to obtain in the great group of animals called Radiata and Zoophyta fully justify a partition of the group corresponding with those differ ences. For the lower organized division we retain the name proposed by Macleay, but ex tend its application to the acaleph ; and thus constituted it may be characterized as follows.
Sub-kingdom ACRITA.—Gelatinous polymor phous animals, without distinct nervous fibre, or visceral cavities.
Alimentary canal excavated in the parenchyma of the body, generally without an anus.
Sanguilerous system composed qf reticulate canals without proper tunics.
Generation in most fissiparous or gemmi parous ; in some oviparous.t The Acrita have been termed Protozoa, as being on the first step of animal organization. They are analogous to the ova or germs of the higher classes, and have, therefore, been termed by Carus Oozoa; and as the changes of the embryo succeed each other with a rapidity proportionate to the proximity of the ovum to the commencement of its development, so also we find that in each class of Acrita there are genera which advance into close approximation with some one or other of the classes belong ing to the higher divisions of the animal king dom. It results, therefore, from this tendency to ascend in the scale of organization that there is greater difficulty in assigning constant or gene ral organic characters to the Acrita than to any of the higher divisions of animals. Even in the nervous system, we find as we are led step by step from the hydra to the actinic in the class Polypi, that the nervous globules begin to manifest the filamentary arrangement about the oral orifice in the last named genus. That, again, in tracing the successive complica tion of the sterelmintha from the hydatid to the echinorhynchus we also come to perceive traces of longitudinal nervous filaments in the latter highly organized genus of parenchymatous worms. In the acalephae the examples of the ag
gregate form of the nervous system would seem to be more numerous and distinct. Ehrenberg has detected what he considers as a nervous sys tem in a gelatinous medusa; and Dr. Grant has recently described a nervous collar giving off simple filaments in the more highly or ganized beroe, which, in its distinct intestine and anal outlet, recedes too far from the medu sidce to be placed in a natural arrangement in the same class. Many of the polygastrica are endowed with simple visual organs or ocelli, in the form of red or yellow spots ; similar organs of a dark colour are exhibited by the planari, and Nordmann also describes them in some internal parasitic trematoda. Ehren berg has recently discovered coloured ocelli in a medusa, and he ascribes a sense of taste to the polygastrica.
The indications, however, of the special senses in the Acrita are feeble and obscure, and in the least doubtful instances the organs are evidently of the simplest and most elementary nature.
For the most part all the different systems seem blended together, and the homogeneous granular parenchyma possesses many functions in common.
Where a distinct organ is eliminated it is often repeated indefinitely in the same individual. Thus in the polypi the nutritious tubes of one individual are generally supplied by numerous mouths, and it has, consequently, the semblance of a composite animal; the polygastrica derive their name from an analogous multiplication of the digestive organ itself. Among the sterel mintha we find instances where the generative system is the subject of a similar repetition, each joint of the tnize being the seat of a separate ovary, though all are nourished by continua tions of one simple system of nutritious tubes. The calcareous and siliceous sponges, again, which, in eliminating the first sketch of an in ternal earthy skeleton, seem to lose the few characteristics of animal life which they before possessed, are limited to the repetition of a simple spiculum.
The formative energies of the Acrita being thus expended on a few simple operations, and not concentrated on the perfect development of any single organ, it is not surprising that the different classes should exhibit the greatest diversity of external figure! But it has been well observed that Nature, so far from forgetting order, has, at the commencement of her work, in these imperfect animals given us a sketch of the different forms which she intended after wards to adopt for the whole animal kingdom. Thus in the soft, sluggish sterelmintha we have the outline of the mollusca ; in the fleshy living mass which surrounds the earthy hollow axis of the polypi natantes, she has sketched a verte brated animal ; and in the crustaceous covering of the living mass, and the structure more or less articulated of the polypi vaginati we trace the form of the annulose or articulate classes.