14 ANATOMY OF AMPHIOXUS. 



About two-thirds of the way from the front end of the 

 body there is a comparatively large aperture in the mid- 

 ventral line. It is the excurrent orifice of a spacious 

 cavity which surrounds to a large extent the internal 

 organs, including above all the pharynx, and is known as 

 the atrial chamber, or simply atrium, while its opening to 

 the exterior is the atriopore. 



The anus or outlet of the digestive tract occurs near the 

 posterior end of the body ; it does not lie in the mid- 

 ventral line, but high up on the left side. At its first 

 appearance in the young embryo, the anus does lie ap- 

 proximately in the mid-ventral line (cf. Fig. 64, p. 117), 

 but as soon as the caudal fin begins to develop, it is pushed 

 on to one side, always the left, and so attains its final 

 position. A similar displacement of the cloacal aperture 

 occurs in the Dipnoan fish Protopterus, where, however, 

 the direction of displacement is not constant, the aperture 

 lying now to the right, now to the left, of the middle line. 

 Again, in the tadpoles of certain Batrachians the cloacal 

 aperture is displaced to the right of the middle line.* (Cf. 

 Fig. 8.) The fact of the displacement of openings by the 



* The asymmetrical position of the cloacal aperture of certain Batrachian 

 tadpoles has been systematically worked out by BOULENGER. In tadpoles 

 of the genera Rana and Hyla, the cloacal aperture is dextral, while in the 

 Toads and Pelobatoids it is median. (See G. A. BOULENGER, A Synopsis 

 of the Tadpoles of the European Batrachians. Proc. Zool. Soc. London, 

 1891. pp. 593-627. Plates 45-47.) 



In Rana the cloacal aperture may occasionally occur in a median position 

 as a variation. (WiLLEY, Note on the position of the cloacal aperture in 

 certain Batrachian tadpoles. Transactions New York Acad. of Sciences, Vol. 

 XII. 1893. pp. 242-245.) My attention to the previous literature on this 

 subject was kindly drawn by Mr. G. A. Boulenger. 



Since writing the above my attention has been called to the following 

 paper by Professor BURT G. WILDER, Lateral Position of the Vent in Am- 

 phioxns [Branchiostoma] and in the Larva: of Rana Pipiens [Catesbiana]. 

 Proc. Amer. Assoc. Adv. Sc. XXII. 1873. pp. 275-300. 



