Figs. 131–139.
Thus in the animal kingdom, as in the vegetal kingdom, there exists a class of minute forms having this peculiarity, that no one of them is separable into a number of visible components homologous with one another—no one of them can be resolved into minor individualities. Its proximate units are those physiological units of which we conclude every organism consists. The aggregate is an aggregate of the first order.
§ 201. Among plants are found types indicating a transition from aggregates of the first order to aggregates of the second order; and among animals we find analogous types. But the stages of progressing integration are not here so distinct. The reason probably is, that the simplest animals, having individualities much less marked than those of the simplest plants, do not afford us the same facilities for observation. In proportion as the limits of the minor individualities are indefinite, the formation of major individualities out of them, naturally leaves less conspicuous traces.
Figs. 140–145.
Be this as it may, however, in such types of Protozoa as the compound Radiolaria, we find that though there is reason to regard the aggregate as an aggregate of the second order, yet its divisibility into minor individualities like those just described, is less manifest. Fig. [140] representing Sphærozoum punctatum, one of the group, illustrates this. The sceptically-minded may perhaps doubt whether we can regard the “cellæform bodies” contained in it, as the morphological units of the animal. The jelly-like mass in which they are imbedded, is but indefinitely divisible into portions having each a cell or nucleus for its centre.[17] Among the Foraminifera, we find only indefinite evidence of the coalescence of aggregates of the first order, into aggregates of the second order. There are solitary Foraminifers, allied to the creature represented in Fig. [134]. Certain ideal types of combination among them, are shown in Fig. [141]. And setting out from these, we may ascend in various directions to kinds compounded to an immense variety of degrees in an immense variety of ways. In all of them, however, the separability of the major individuality into minor individualities, is very incomplete. The portion of sarcode contained in one of these calcareous chambers, gives origin to an external bud; and this presently becomes covered, like its parent, with calcareous matter: the position in which each successive chamber is so produced, determining the form of the compound shell. But the portions of sarcode thus budded out one from another, do not become distinctly individualized. Fig. [142], representing the living network which remains when the shell of an Orbitolite has been dissolved, shows the continuity that exists among the occupants of its aggregated chambers.[18] In the compound Infusoria, the component units remain quite distinct. Being, as aggregates of the first order, much more definitely organized, their union into aggregates of the second order does not destroy their original individualities. Among the Vorticellæ, of which two kinds are delineated in Figs. [144 and 145], there are various illustrations of this: the members of the community being sometimes appended to a single stem; sometimes attached by long separate stems to a common base; and sometimes massed together.
Figs. 146–147.
Thus far, these aggregates of the second order exhibit but indefinite individualities. The integration is physical; but not physiological. Though, in the Polycytharia, there is a shape that has some symmetry; and though, in the Foraminifera, the formation of successive chambers proceeds in such methodic ways as to produce quite-regular and tolerably-specific shells; yet no more in these than in the Sponges or the compound Vorticellæ, do we find such co-ordination as gives the whole a life predominating over the lives of its parts. We have not yet reached an aggregate of the second order, so individuated as to be capable of serving as a unit in still higher combinations. But in the class Cœlenterata, this advance is displayed. The common Hydra, habitually taken as the type of the lowest division of this class, has specialized parts performing mutually-subservient functions, and thus exhibiting a total life distinct from the lives of the units. Fig. [146] represents one of these creatures in its contracted state and in its expanded state; while Fig. [147] is a diagram showing the wall of this creature’s sac-like body as seen in section under the microscope: a and b being the outer and inner cellular layers; while between them is the “mesoglœa” or “structureless lamella,” the supporting or skeletal layer. But this lowly-organized tissue of the Hydra, illustrates a phase of integration in which the lives of the minor aggregates are only partially-subordinated to the life of the major aggregate formed by them. For a Hydra’s substance is separable into Amœba-like portions, capable of moving about independently. If we bear in mind how analogous are the extreme extensibility and contractility of a Hydra’s body and tentacles, to the properties displayed by the sarcode among Rhizopods; we may infer that probably the movements and other actions of a Hydra, are due to the half-independent co-operation of the Amœba-like individuals composing it.