. Elementary text-book of zoology. AMPHIOXUS 297 break up, the tail is resorbed, and the trunk rotates through nearly 180° upon its papillae. In this way the active, sensi- tive, highly - organised " tad- pole" is reduced to a quiescent, sedentary, vegetative ascidian. In Chapter VI. it is ex- plained that ontogeny may in many cases be interpreted as a repetition of phylogeny. This principle applied to the case in hand leads us to the conclusion that the ascidians are descended from active, free-swimming, highly - organ- ised Chordata which have degenerated on the adoption of a seden
. Elementary text-book of zoology. AMPHIOXUS 297 break up, the tail is resorbed, and the trunk rotates through nearly 180° upon its papillae. In this way the active, sensi- tive, highly - organised " tad- pole" is reduced to a quiescent, sedentary, vegetative ascidian. In Chapter VI. it is ex- plained that ontogeny may in many cases be interpreted as a repetition of phylogeny. This principle applied to the case in hand leads us to the conclusion that the ascidians are descended from active, free-swimming, highly - organ- ised Chordata which have degenerated on the adoption of a sedentary habit. Fig. 211.—Transverse Sec- tion THROUGH THE TAIL OF AN Ascidian Larva. (After Seeliger.) Median Fin. Notochord. ,Nerve Tube. ,Muscle. Caudal Hypoblast, II.—AMPHIOXUS. Phylum Sub-Phylum Class Chordata (p. 403). Atriozoa (p. 404). Cephalochorda (p. 405). Fig. 212.—Lateral View of Amphioxus Lanceolatus x f. (Adnat.) Myomere Muscles.^ Dorsal Tail. Anal Fin. Atriopore. Metapleural Fold. Amphioxus lanceolatus (the Lancelet) is a small marine organism about one to one-and-a-half inches in length. It is of elongated, fish-like shape, tapering at each. 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 Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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