In presence of statements made in works of authority, this objection greatly surprises me. I meet with the descriptive word “moniliform” applied to the intestine in some Annelids, and then in the Text Book of Claus, translated and edited by Sedgwick, it is said, concerning the alimentary canal in the Annelida:—
“This is followed by the gastric region of the gut, which occupies the greatest portion of the length of the body, and is either regularly constricted in correspondence with the segments, or possesses lateral diverticula.” (P. 365.)
And again on p. 369 it is said:—
“The intestine usually preserves the same structure in its entire length and is divided by regular constrictions into a number of divisions or chambers, which correspond to the segments and dilate again into lateral diverticula and cæca.”
The alimentary canal thus presents the segmental character as clearly as consists with fulfilment of its function. If the successive segments are co-operating units of a compound animal having but one mouth, then, necessarily, the gut cannot be completely cut into parts, each answering to a segment, for there could be, in that case, no passage for the food. If the portion of the intestine belonging to each segment has a conspicuous dilatation, or has a cæcum on each side, it exhibits the segmental character as much as the physical requirements permit. So far from being at variance with the hypothesis, its structure exhibits a verification of it.
The next objection runs as follows:—
“Then, again, the ovaries and testes do not exhibit a corresponding segmentation. When it is allowable to speak of ovary or testis at all as in Lumbricus, we find that in the case of both organs we have at most two pairs.”
It seems to me that the distribution of the generative organs in a comparatively-developed member of the Annelid type, is not the question. We have to ask what it is in undeveloped members of that type. Among them the repetition of generative parts is in some cases just what the theory implies. Thus in Claus I read:—“In the marine Chætopoda, the ova or spermatozoa originate on the body-wall from cells of the peritoneal membrane, either in the anterior segments alone or along the whole length of the body.” So that in these last cases there are, in all the segments, parts from which arise generative products. The fact that these parts are not definite ovaries and testes is irrelevant. Ovaries and testes are developed generative structures, and in the order of evolution are preceded by undeveloped ones; and the fact that these undeveloped ones are found in little-developed members of the type conforms perfectly to the hypothesis. [I may remark in passing that here is a good illustration of that process of evolution which, in the above speculation of Prof. Korschelt, is supposed to be inverted: many dispersed, similar, and indefinite parts, are integrated into a few localized and definite parts.]
In continuation the critic above quoted says:—“My position is that the repetition of segments in an Annelid is a phenomenon of the same nature as the repetition of hairs in a Mammal or of scutes in a Reptile”, and he proceeds to give instances of repetitions of organs in other types, as of the reproductive structures and excretory system in the young Dog-fish or of the ovaries in Amphioxus. These examples do not seem to me relevant. No parallelism exists between the repetition of a particular organ in an animal, and the repetition of an entire cluster of organs constituting a physiological whole. The repetitions of the ovaries in Amphioxus and of the excretory system in a young Dog-fish, occur without threatening to divide into similar parts the entire organism. But the segmental repetitions in an annulose creature implicate the structures at large, and would, if pushed a little further, result in separate creatures. The segment of a low Annelid contains alimentary, vascular, nervous, excretory, reproductive, sensory and locomotive organs—all the organs required for carrying on life, save certain organs of external relation which its position excludes. When there is shown some vertebrate animal, or proto-vertebrate animal, that is divisible into parts each of which is in great measure physiologically independent, I shall feel obliged to abandon my position.