For if{171} the annulose animals have been formed by aggregation, we ought to find this process much less perfect in the oldest form.

But a complete development, such as already obtains in the lobster, &c., was reached by the Eurypterida and Trilobites of the palaeozoic strata; and annelids, probably formed mainly like those of the present day, abounded during the deposition of the oldest fossiliferous rocks.
[Illustration: TRILOBITE.] Thirdly, and lastly, as regards such serial homology as is exemplified by the backbone of man, there are also several objections to Mr.Spencer's mechanical explanation.
On the theory of evolution most in favour, the first Vertebrata were aquatic.

Now, as natation is generally effected by repeated and vigorous lateral flexions of the body, we ought to find the segmentation much more complete laterally than on the dorsal and ventral aspects of the spinal column.

Nevertheless, in those species which, taken together, constitute a series of more and more distinctly segmented forms, the segmentation gradually increases _all round_ the central part of the spinal column.
Mr.Spencer[173] thinks it probable that the sturgeon has retained the notochordal (that is, the primitive, unsegmented) structure because it{172} is sluggish.

But Dr.Guenther informs me that the sluggishness of the common tope (_Galeus vulgaris_) is much like that of the sturgeon, and yet the bodies of its vertebrae are distinct and well-ossified.