448 E. RAY LANKESTER AND ARTHUR WILLEY. 



ridgeSj Avhicli are subsequently concerned in the formation of 

 the atrium, have appeared (see Fig. 6). In this stage the larva 

 rests habitually on one side at the bottom of the vessel in which 

 it is kept, and does not bury itself in sand or mud. 



At the time of the completion of the atrium, which occurs at 

 the close of the larval period, some remarkable changes in the 

 relative position of parts of the body in the anterior region 

 take place, by which the mouth becomes median, and the gill- 

 slits are arranged in two series, a right and a left. The larva 

 emerges from this critical phase in its development as a 

 symmetrical animal, but the details of the process of "^symme- 

 trisation" — the strongly marked character of which justifies the 

 use of an otherwise undesirable term — are still rather obscure. 

 The larva, now really a young Amphioxus, with atrium and 

 paired gill-slits, ceases to lead a pelagic life, and takes to the 

 sand, where it passes the rest of its life. In this condition it 

 does not rest on one side on the sand, but buries itself upright 

 tail downwards with the oral hood alone projecting from the 

 sand (Willey obs.). Hence in the adult condition there are not 

 one-sided relations of the Amphioxus to its environment. 



Spawning occurs at least from April to September inclusive. 

 The best month, however, in which to obtain the embryos is 

 June, while all the larval stages, up to the passage into the 

 adult form, are to be found during July and August. 



Previous View as to the Formation of the Atriu 



m. 



The hitherto accepted method of formation of the atrial 

 chamber of Amphioxus is that described by Kowalevsky,^ 

 and more fully by Rolph.- 



Kowalevsky says that after a certain number of gill-slits 

 have been formed, two longitudinal folds appear on oppo- 

 site sides of the body, which grow round and meet, and 

 finally fuse together in the median ventral line, leaving a wide 

 aperture at one end — the atriopore. His figures, two of which 



' ' Archiv fiir Mikrosk. Anat.,' vol. xiii, 1S77. 

 - 'Morpliol. Jalirbucb,' vol. ii, 1S70. 



