Note 44 — Chapter XIV · THE GENERAL SHAPES OF ANIMALS.

Criticisms on the above passage have shown the need for naming sundry complications. These complications chiefly, if not wholly, arise from changes in modes of life—changes from the locomotive to the stationary, and from the stationary to the locomotive. Referring to my statement that (ignoring the spherical) the radial type is the lowest and must be taken as antecedent to the bilateral type, it is alleged that all existing “radial animals above Protozoa are probably derived from free-swimming, bilaterally-symmetrical animals.” If this is intended to include the planulæ of the hydroid polyps, then it seems rather a straining of the evidence. These locomotive embryos, described as severally having the structure of a gastrula with a closed mouth, can be said to show bilateralness only because the first two tentacles make their appearance on opposite sides of the mouth—a bilateralness which lasts only till two other tentacles make their appearance in a plane at right angles, so giving the radial structure. I think the criticism applies only to cases furnished by Echinoderms. The larvæ of these creatures have bilaterally-symmetrical structures, which they retain as long as they swim about and which such of them as fix themselves lose by becoming similarly related to conditions all round: the radial structure being retained by those types which, becoming subsequently detached, move about miscellaneously. But, as happens in some of the Sea-urchins and still more among the Holothurians, the structure is again made bilaterally-symmetrical by a locomotive life pursued with one end foremost. Should it be contended that the conditions and the forms are reciprocally influential—that either may initiate the other, it still remains unquestionable that ordinarily the conditions are the antecedents, as is so abundantly shown by plants.