Figs. 160–161.
The reasons for believing one of these creatures to be an aggregate of the third order, are greatly strengthened when we turn from the adult structure to the mode of development. Among the Dorsibranchiata and Tubicolæ, the embryo leaves the egg in the shape of a ciliated gemmule, not much more differentiated than that of a polype. As shown in Fig. [162], it is a nearly globular mass; and its interior consists of untransformed cells. The first appreciable change is an elongation and a simultaneous commencement of segmentation. The segments multiply by a modified gemmation, which takes place from the hinder end of the penultimate segment. And considerable progress in marking out these divisions is made before the internal organization begins. Figs. [163, 164, 165], represent some of these early stages. In annelids of other orders, the embryo assumes the segmented form while still in the egg. But it does this in just the same manner as before. Indeed, the essential identity of the two modes of development is shown by the fact that the segmentation within the egg is only partially carried out: in all these types the segments continue to increase in number for some time after hatching. Now this process is as like that by which compound animals in general are formed, as the different conditions of the case permit. When new individuals are budded-out laterally, their unfolding is not hindered—there is nothing to disguise either the process or the product. But gemmæ produced one from another in the same straight line, and remaining connected, restrict one another’s developments; and that the resulting segments are so many gemmiparously-produced individuals, is necessarily less obvious.
Figs. 162–165.
§ 206. Evidence remains which adds very greatly to the weight of that already assigned. Thus far we have studied only the individual segmented animal; considering what may be inferred from its mode of evolution and final organization. We have now to study segmented animals in general. Comparison of different groups of them and of kinds within each group, will disclose various phases of progressive integration of the nature to be anticipated.
Figs. 166–169.
Among the simpler Platyhelminthes, as in some kinds of Planaria, transverse fission occurs. A portion of a Planaria separated by spontaneous constriction, becomes an independent individual. Sir J. G. Dalyell found that in some cases numerous fragments artificially separated, grew into perfect animals.[21] In these creatures which thus remind us of the lowest Hydrozoa in their powers of agamogenetic multiplication, the individuals produced one from another do not continue connected. As the young ones laterally budded-off by the Hydra separate when complete, so do the young ones longitudinally budded-off by the Planaria. Fig. [166] indicates this. But there are allied types which show us a more or less persistent union of homologous parts, or individuals, similarly arising by longitudinal gemmation.[22] The cestoid Entozoa furnish illustrations. Without dwelling on the fact that each segment of a Tænia, like each separate Planaria, is an independent hermaphrodite; and without specifying the sundry common structural traits which add probability to the suspicion that there is some kinship between the individuals of the one order and the segments of the other; it will suffice to point out that the two types are so far allied as to demand their union under the same sub-class title. And recognizing this kinship, we see significance in the fact that in the one case the longitudinally-produced gemmæ separate as complete individuals, and in the other continue united as segments in smaller or larger numbers and for shorter or longer periods. In Tænia echinococcus, represented in Fig. [167], we have a species in which the number of segments thus united does not exceed four. In Echinobothrium typus there are eight or ten; and in cestoids generally they are numerous.[23] A considerable hiatus occurs between this phase of integration and the next higher phase which we meet with; but it is not greater than the hiatus between the types of the Platyhelminthes and the Chætopoda, which present the two phases. Though it is doubtful whether separation of single segments occurs among the Annelida,[24] yet very often we find strings of segments, arising by repeated longitudinal budding, which after reaching certain lengths undergo spontaneous fission: in some cases doing this so as to form two or more similar strings of segments constituting independent individuals; and in other cases doing it so that the segments spontaneously separated are but a small part of the string. Thus a Syllis, Fig. [168], after reaching a certain length, begins to transform itself into two individuals: one of the posterior segments develops into an imperfect head, and simultaneously narrows its connexion with the preceding segments, from which it eventually separates. Still more remarkable is the extent to which this process is carried in certain kindred types; which exhibit to us several individuals thus being simultaneously formed out of groups of segments. Fig. [169], copied (omitting the appendages) from one contained in a memoir by M. Milne-Edwards, represents six worms of different ages in course of development: the terminal one being the eldest, the one having the greatest number of segments, and the one that will first detach itself; and the successively anterior ones, with their successively smaller numbers of segments, being successively less advanced towards fitness for separation and independence. Here among groups of segments we see repeated what in the previous cases occurs with single segments. And then in other annelids we find that the string of segments arising by gemmation from a single germ becomes a permanently united whole: the tendency to any more complete fission than that which marks out the segments, being lost; or, in other words, the integration having become relatively complete. Leaving out of sight the question of alliance among the types above grouped together, that which it here concerns us to notice is, that longitudinal gemmation does go on; that it is displayed in that primitive form in which the gemmæ separate as soon as produced; that we have types in which such gemmæ hang together in groups of four, or in groups of eight and ten, from which however the gemmæ successively separate as individuals; that among higher types we have long strings of similarly-formed gemmæ which do not become individually independent, but separate into organized groups; and that from these we advance to forms in which all the gemmæ remain parts of a single individual. One other significant fact must be added. There are cases in which annelids multiply by lateral gemmation.[25] That the longitudinally-produced gemmæ which compose an annelid, should thus have, one of them or several of them, the power of laterally budding-off gemmæ, from which other annelids arise, gives further support to the hypothesis that, primordially, the segments were independent individuals. And it suggests this belief the more strongly because, in certain types of Cœlenterata, we see that longitudinal and lateral gemmation do occur together, where the longitudinally-united gemmæ are demonstrably independent individuals.
§ 207. Though it seems next to impossible that we shall ever be able to find a type such as that which is here supposed to be the unit of composition of the annulose type, since we must assume such a type to have been long since extinct, yet the foregoing evidence goes far towards showing that an annulose animal is an aggregate of the third order. This repetition of segments, sometimes numbering several hundreds, like one another in all their organs even down to those of reproduction, while it is otherwise unaccountable, is fully accounted for if these segments are homologous with the separate individuals of some lower type. The gemmation by which these segments are produced, is as similar as the conditions allow, to the gemmation by which compound animals in general are produced. As among plants, and as among demonstrably-compound animals, we see that the only thing required for the formation of a permanent chain of gemmiparously-produced individuals, is that by remaining associated such individuals will have advantages greater than are to be gained by separation. Further, comparisons of the annuloid and lower annulose forms, disclose a number of those transitional phases of integration which the hypothesis leads us to expect. And, lastly, the differences among these united individuals or successive segments, are not greater than the differences in their positions and functions explain—not greater than such differences are known to produce among other united individuals: witness sundry compound Hydrozoa.
Indirect evidence of much weight has still to be given. Thus far we have considered only the less developed Annulosa. The more integrated and more differentiated types of the class remain. If in them we find a carrying further of the processes by which the lower types are here supposed to have been evolved, we shall have additional reason for believing them to have been so evolved. If we find that in these superior orders, the individualities of the united segments are much less pronounced than in the inferior, we shall have grounds for suspecting that in the inferior the individualities of the segments are less pronounced than in those lost forms which initiated the annulose sub-kingdom.