. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 246 THE Fig. 115. — Diagrammatic transverse sec- lion through hinder region of proboscis of Balanoglossus. (From a drawing kindly lent by Professor T. H. MORGAN.) D. Dorsal. V. Ventral, bc'^. Proboscis- ca\ity, almost filled up by mesenchymatous and muscular tissue.* proliferated from the original ccelomic epithelial layer (indicated bv the black line below the ectoderm). Pulsating vesicle, h. Heart, ck. Noto- chord. nj. Integumentary ner\-e-plexus. The genital organs, testes or ovaries,
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 246 THE Fig. 115. — Diagrammatic transverse sec- lion through hinder region of proboscis of Balanoglossus. (From a drawing kindly lent by Professor T. H. MORGAN.) D. Dorsal. V. Ventral, bc'^. Proboscis- ca\ity, almost filled up by mesenchymatous and muscular tissue.* proliferated from the original ccelomic epithelial layer (indicated bv the black line below the ectoderm). Pulsating vesicle, h. Heart, ck. Noto- chord. nj. Integumentary ner\-e-plexus. The genital organs, testes or ovaries, accord- ing to the sex of the individual, occur as a paired metameric series of pouch-like bodies or gonadic sacs which ex- tend backwards far be- yond the region of the gill-slits. The gonadic sacs are suspended in the body-cavity by solid cords attached to the dorsal integument, which be- come perforated in the spawning season to ad- mit of the expulsion of the reproductive elements. Metanicrisnt. Although there is no muscular metamerism in Balano- glossus, yet we have seen that other organs (gill-slits and gonads) are arranged metamerically. And in point of fact, among those Invertebrates which are not included under the phylum of the Articulata, if there is one pecu- liarity of organisation more sporadic in its occurrence than another, it is metamerism. It may affect the most differ- ent organs of the bodv either collectivelv or individually, and nothing is more patent than the fact that the meta- meric repetition of parts has arisen independently over and over again in different groups of animals.^ * This tissue is not represented in Figs. 114 and 116, although it is present throughout the 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 Willey, Arthur, 1867-. New York, London, Macmillan
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