. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 258 THE PROTOCHORDATA. place from the tip backwards by the in-roIHng of its walls. According to the graphic description of Hubrecht, it is retracted "in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the ; When at rest within the body the proboscis lies freely within a hollow cylinder, the wall of which is thick and muscular, and constitutes t\i& proboscis-sheath (Fig. 123).. Fig. 123. — Diagrammatic transverse
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 258 THE PROTOCHORDATA. place from the tip backwards by the in-roIHng of its walls. According to the graphic description of Hubrecht, it is retracted "in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the ; When at rest within the body the proboscis lies freely within a hollow cylinder, the wall of which is thick and muscular, and constitutes t\i& proboscis-sheath (Fig. 123).. Fig. 123. — Diagrammatic transverse section through the middle of the body of a N'emertine. (After LANG, Text-book of Comp. A?iat.) bjn. Basement-membrane, Circular muscles. , Dorsal or " medullary " nerve, Dorsal blood-vessel, g. Gonads, int. Intestine, Longitudinal muscles, Lateral nerves, Lateral blood-vessel, p. Proboscis. Pro- boscis-sheath. Sometimes beneath the ectodermal epithelium of the Nemertine proboscis there is a continuous sheath of nerve- fibres, comparable to the nervous plexus in the proboscis of Balanoglossus. Partly, therefore, on account of its structure, and partly on account of its topographical relations when extruded, we are led to suppose that a certain homology exists. 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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