. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 228 THE ASCIDIANS. surface-view, but they are situated at the inner or con- cave sides of the two thiclcenings. On either side of the latter can be seen the ordinary cavity of the pharynx proceeding to- wards the oesophagus. At a later stage the openings of the two first-formed stigmata become distinctly visi- ble (Fig. 105 C). Mean- while a change of axis is taking place in the body of the young Ascidian. During the extraor- dinary change of axis which we are about to describe the probos- ciform praeoral lo
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 228 THE ASCIDIANS. surface-view, but they are situated at the inner or con- cave sides of the two thiclcenings. On either side of the latter can be seen the ordinary cavity of the pharynx proceeding to- wards the oesophagus. At a later stage the openings of the two first-formed stigmata become distinctly visi- ble (Fig. 105 C). Mean- while a change of axis is taking place in the body of the young Ascidian. During the extraor- dinary change of axis which we are about to describe the probos- ciform praeoral lobe (snout, organ of fixa- tion) remains station- ary, and the rest of the body actually rotates through an angle of 90 degrees, using the or- gan of fixation as a pivot about which it turns. In Fig. 105 C the rotation which takes place very gradu- ally is only half performed ; while in Fig. 107 it is complete. The method of growth by which this rotation takes place. Fig. 107. — Young Ciona iniestinaUs after the completion of the change of axis ; from the left side. (After WlLLEY.) /, IV. Primary stigmata, a. Anus, situated immediately below the left atrial aperture, efid. Endostyle. f. Organ of fixation. _^. Ganglion. hy. Hypophysis, inf. Intestine. Lat. Left atrial aperture, Longitudinal muscle, in. Mouth. oes. CEsophagus. Peripharyngeal band. py. Pyloric gland, st. Stomach. /. Coronarv tentacles. Visceral nerve (cordon ganglion- naire visceral).. 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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