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, l.jn. Longitudinal muscle, in. Mouth. 

 oes. CEsophagus. p.b. Peripharyngeal band. 

 py. Pyloric gland, st. Stomach. /. Coronarv 

 tentacles. v.n. Visceral nerve (cordon ganglion- 

 naire visceral). 



