186 ARTHUR WILLEY. 



which are added after this period never communicate with the 

 intestine (Hatschek). 



II. The Period of Early Larval Development during 

 which fresh gill- slits appear one after the other — i. e. meta- 

 merically — slightly to the right side of the median line (sub- 

 sequently passing well up to the right side), to the number of 

 twelve to fifteen. Towards the close of this period the longi- 

 tudinal metapleural folds appear, and the closure of the atrium 

 commences behind by the fusion of the small subatrial ridges 

 which are developed on the inner faces of the raetapleura. 



III. The Period of Later Larval Development during 

 which the second row of gill-slits is formed on the right side ; 

 the first or primary row of slits passes across to the left side, 

 the mouth assumes an anterior median and vertical position, 

 the prseoral cirri appear, and the endostyle is developed from 

 its pre-existing rudiment. 



IV. The Adolescent Period when the young Amphioxus, 

 having now attained most of the essential features of the adult 

 structure, has definitely ceased to lead a pelagic life, and has 

 taken up its abode in the sand, where its further growth in size 

 and maturity is accomplished. 



Of the above four main periods in the development of 

 Amphioxus, the first has been studied by Kowalewsky (2) and 

 more thoroughly by Hatschek (3) ; the second has been treated 

 by Professor Lankester and myself (8), and the third forms the 

 subject of the present paper. The fourth entails the considera- 

 tion of the adult anatomy, for which Professor Lankester^s 

 paper (5) may be consulted. 



The only observations recorded as to the course of events in 

 the third period were made by Kowalewsky (1), whose all too 

 brief account was considered so extraordinary that Balfour^ was 

 " tempted to suppose that his observations were made on patho- 

 logical specimens." 



Kowalewsky confined himself wholly to the question of the 

 origin of the second row of gill-slits, and with reference to 

 that he said that six disc-like thickenings appear together on 

 > 'Comparative Embryology,' vol. ii, 1S85. 



