k 



LATER LAKVAL DEVELOPMENT OE AMPHIOXUS. 231 



DESCRIPTION OF PLATES XIII— XV, 



Illustrating Mr. Arthur Willey^s paper on " The Later Larval 

 Development of Amphioxus/^ 



All the figures, without exception, were drawn from the living animal, and 

 each figure is from a separate larva. The drawings are made to scale. Erom 

 Stage III onwards all the slits are seen through the transparent atrial or 

 body-wall — i. e. they open into the atrium, and not directly to the exterior. 



N.B. — The long cilia on the surface of the body are not represented in the 

 figures, nor in those of the memoir by Professor Lankester and myself (8). 

 Their character and position are well shown by Hatschek in his memoir in 

 Claus's 'Arbeiten,' 1881. 



EiG. 1. — Stage I. Eight side. Fourteen primary slits— notice small size of 

 the first. Six secondary thickenings occurring in a continuous ridge above 

 the primary slits, the first of them being above and between the third and 

 fourth primary slits. The praeoral pit of course opens on the left side, and is 

 only seen by transparency on this side. 



Fig. 2. — Stage I. Eight side. Twelve primary slits. Five secondary 

 thickenings, the last being much smaller than the others, and the first being 

 situated between the fourth and fifth primary slits. The second as well as 

 the first secondary thickening has in this case been slightly retarded ; thus 

 showing that the retardation of the first secondary slit, which has been stated 

 in the text to be usual, is not specially important. Of course, the general 

 retardation of development which affects the whole series of secondary slits 

 is extremely important. Notice the endostyle and club-shaped gland. 



Fig. 3. — Stage II. Right side. Fourteen primary slits, the fourteenth 

 appearing in side view merely as a pit in the floor of the pharynx ; this shows 

 that it is on the way to close up. Seven secondary slits, one having been 

 added behind ; the first still lies between the third and fourth primary slits ; 

 a new one is subsequently formed in front, so that what appears to be the first 

 slit or thickening at this stage eventually becomes the second secondary slit. 

 In this larva the second to the sixth of the secondary thickenings have just 

 been perforated. The appearance of a seventh thickening behind is rather 

 early to happen at this stage. 



Fig. 4. — Stage III. Right side. Twelve primary slits ; the first of them is 

 getting smaller. The fin-chambers seem to form about this stage. There is 

 a small anterior portion of the atrium still unclosed. The right wall of the 

 future velum is now seen from this side, owing to the great extent to which 



