. Senescence and rejuvenescence. Age; Reproduction. REJUVENESCENCE IN EMBRYO AND LARVA 421 The larval form of the annehds, moUusks, Crustacea, and some other invertebrate groups represents chiefly the head and anterior regions of the body, and metamorphosis consists, not only in changes in the parts already formed, but in the addition of new segments from a growing region just in front of the posterior end. The fully developed larva of the anneHd Nereis, for example, consists of the head and the first three segments, as indicated in Fig. 197, and during the transformation of this free-swimming
. Senescence and rejuvenescence. Age; Reproduction. REJUVENESCENCE IN EMBRYO AND LARVA 421 The larval form of the annehds, moUusks, Crustacea, and some other invertebrate groups represents chiefly the head and anterior regions of the body, and metamorphosis consists, not only in changes in the parts already formed, but in the addition of new segments from a growing region just in front of the posterior end. The fully developed larva of the anneHd Nereis, for example, consists of the head and the first three segments, as indicated in Fig. 197, and during the transformation of this free-swimming form into the worm new segments are added successively at the posterior end. In this and. Fig. 197.—^Trochophore larva of Nereis. Af ter E. B. Wilson,' 9 2 in other related species the axial gradient, which is so clearly marked during prelarval stages, becomes less and less distinct in the larva, imtil, as metamorphosis approaches, the growing region at the posterior end shows the highest metaboHc rate of any part of the body. These changes enable us to gain some insight into the process of formation of new segments. The head-region under- goes rejuvenescence and begins senescence before other parts, so that in the larval stage its metabolic rate begins to decrease before that of the more posterior regions. But even before its metaboHc. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Child, Charles Manning, 1869-1954. Chicago, Ill. , The University of Chicago Press
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