[Note.—Partly from the wish to incorporate further evidence, and partly from the wish to present the evidence, old and new, in a more effective order, I decide here to recast the foregoing exposition.
Significant traits of development are exhibited in common by two groups otherwise unallied—certain of the Platyhelminthes and certain of the lower Annulosa. Of the Platyhelminthes the ordinary type is an unsegmented creature: a Planarian or a Trematode exemplifying it. Among the free forms, as in some Planarians, there occurs transverse fission, and prompt separation of the segments; while among some other free forms, as the Microstomida, the two segments first produced, themselves become segmented while still adherent, and this process is repeated until a string is formed. Another group of the Platyhelminthes, the Cestoid Entozoa, exhibit analogous processes. There are unsegmented forms, as the Caryophyllæus, and there are forms in which the segments, now few now many, adhere together in chains; the terminal members of which, however, eventually separate, and having before separation approached the trematode structure, become independent individuals which grow, creep about, and continue the race. In both of these types the condition under which the gemmiparously-produced members remain connected, is that they shall be able to feed individually: in the one case by lateral mouths, in the other case by absorption through the integument. It is further observable that in both cases separation of the component individuals occurs at sexual maturity, when advantage in nutrition has ceased to be the dominant need and dispersion of the species has taken its place in degree of importance. Among Annelids, higher though they are in type, we find parallelisms. Usually in its first stage an annelid is unsegmented, but as fast as it elongates lines of segmentation indent its surface. This segmentation proceeds in various ways, and the segments exhibit various degrees of dependence. In some low types, spontaneous fission goes on to the extent of producing single segments, each of which has such vitality that it buds out anterior and posterior parts at its two ends. Thus alike in the simple form which exists before segmentation and in the form exhibited by a detached segment, we have a unit analogous to each of the units which are joined together in certain free Turbellaria and in the Cestoids: the difference being that in the Annelids the sexually mature units do not individually disunite. But though there does not take place separation of single completed segments, there takes place separation of groups of segments, which are either sexually mature at the time they drop off or presently become so. And the groups of segments which have become sexually mature before they drop off, have simultaneously acquired swimming organs and developed eyes, enabling them to spread and diffuse the species. Sundry biologists recognize a parallelism between that detachment of developed segments which goes on in the cestoid Entozoa, and that which goes on in the Scyphomedusæ. The successively detached members of the strobila are sexually matured or maturing individuals which, as medusæ, are fitted for swimming about, multiplying, and reaching other habitats; while each detached proglottis of the cestoid is, by the nature of its medium, limited to creeping about. Clearly this fissiparous process in such Annelids as the Syllidæ, which has similarly been compared to the strobilization of the Scyphomedusæ, differs simply in the respect that single segments are not adapted for locomotion, and it therefore profits the species to separate in groups. All these facts and analogies point to the conclusion that the remote ancestor of the Annelids was an unsegmented creature homologous with each of the segments of an existing Annelid.
This conclusion is supported by other kinds of evidence here to be added. The larvæ of Annelids are very various; but amid their differences there is a recognizable type. “The Trochophore is the typical larval form of the Annelid stem”: a trochophore being a curious spheroidal ciliated structure suggestive of cœlenterate affinities. And this unsegmented larva, representing the remote ancestor from which the many Annelid types diverged, is similar to the larvæ of the Rotifera and the Mollusca: a trochophore is common to all these great classes. Moreover since, among the Rhizota (a sub-class of the Rotiferæ), there is a species, Trochosphæra, solitary and free-swimming, resembling in form and structure a trochophore, though it is not a larva but an adult, we get further evidence that there was a primitive creature of this general character, of which the trochophores of Mollusca, Rotifera, and Annelida are divergent modifications, and which was unsegmented: the implication being that the segmentation of the Annelida was superinduced. That this segmentation resulted from gemmation is implied by what are called polytrochal larvæ. These “sometimes appear as a stage succeeding other larval types. Thus those of Arenicola marina arise from larvæ which at first were monotrochal, later became telotrochal, and finally, by the appearance of new ciliated rings between those already present, assumed the stage of polytrochal larvæ.... This condition warrants the assumption that the segmented forms are to be looked upon as the younger, the unsegmented, on the other hand, as the phylogenetically older.” (Korschelt and Heider, i, 278.) And that the above-described rings of cilia mark off segments is shown by the case of Ophryotrocha puerilis, which “remains, as it were, in a larval condition, since the segments retain their ciliation throughout life.” (Ib., 277.) Yet one more significant fact must be named. In early stages of development each segment of an archiannelidan has cœlomic spaces separate from those of neighbouring segments, but in the adult the septa “generally break down either partially or completely, so that the peri-visceral cavity becomes a continuous space from end to end of the animal.” (Sedgwick, Text Book, 449.) While this fact is congruous with the hypothesis here maintained, it is incongruous with the hypothesis that the annelid was originally an elongated creature which afterwards became segmented; since in that case the implication would be that the cœlomic septa, not arising from recapitulation of an ancestral structure, but originated by the process of segmentation, were first superfluously formed and then destroyed.
Various lines of evidence thus converge to the conclusion that an annulose animal is an aggregate of the third order.
In June, 1865, when No. 14 of my serial containing the foregoing chapter was issued, I supposed myself to be alone in holding this belief respecting the annulose type, and long continued to suppose so. Over thirty years later, however, in M. Edmond Perrier’s work, La Philosophie Zoologique avant Darwin, I found mention of a lecture delivered by M. Lacaze-Duthiers at the École Normale Supérieure in Paris, and reported in the Revue des Cours Scientifiques for January 28, 1865, in which he enunciated a like belief. Judging, however, by the account of this lecture which M. Perrier gives (he was present), it appears that M. Lacaze-Duthiers simply contended that this view of the annulose structure as arising by union of once-independent units, is suggested by certain à priori considerations. There is no indication that he assigned any of the classes of facts above given, which go to show that it has thus arisen.
For further facts and arguments concerning the genesis of the annulose type, see [Appendix D 2.]]
CHAPTER V.
THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CONTINUED.
§ 208. Insects, Arachnids, Crustaceans, and Myriapods, are all members of that higher division of the Annulosa[26] called Articulata or now more generally Arthropoda. Though in these creatures the formation of segments may be interpreted as a disguised gemmation; and though, in some of them, the number of segments increases by this modified budding after leaving the egg, as it does among the Annelids; yet the process is not nearly so dominant: the segments are usually much less numerous than we find them in the types last considered. In most cases, too, the segments are in a greater degree differentiated one from another, at the same time that they are severally more differentiated within themselves. Nor is there any instance of spontaneous fission taking place in the series of segments composing an articulate animal. On the contrary, the integration, always great enough permanently to unite the segments, is frequently carried so far as to hide very completely the individualities of some or many of them; and occasionally, as among the Acari, the consolidation, or the arrest of segmentation, is so decided as to leave scarcely a trace of the articulate structure: the type being in these cases indicated chiefly by the presence of those characteristically-formed limbs, which give the alternative name Arthropoda to all the higher Annulosa. Omitting the parasitic orders, which, as in other cases, are aberrant members of their sub-kingdom, comparisons between the different orders prove that the higher are strongly distinguished from the lower, by the much greater degree in which the individuality of the tertiary aggregate dominates over the individualities of those secondary aggregates called segments or “somites,” of which it is composed. The successive Figs. [170–176], representing (without their limbs) a Julus, a Scolopendra, an isopodous Crustacean, and four kinds of decapodous Crustaceans, ending with a Crab, will convey at a glance an idea of the way in which that greater size and heterogeneity reached by the higher types, is accompanied by an integration which, in the extreme cases, nearly obliterates all traces of composite structure. In the Crab the posterior segments, usually folded underneath the shell, alone preserve their primitive distinctness. So completely confluent are the rest, that it seems absurd to say that a Crab’s carapace is composed of as many segments as there are pairs of limbs, foot-jaws, and antennæ attached to it; and were it not that during early stages of the Crab’s development the segmentation is faintly marked, the assertion might be considered illegitimate.
