LATEE LARVAL DEVELOPMENT OP AMPHIOXUS. 



191 



gradual change in the shape and level of the anterior half of the 

 mouth, show also the progressive increase in the number of the 

 elements of the buccal cirri — which, as they grow in number, 

 grow also in size, and soon becoming irregular in their contour, 

 show signs of growing out into tentacle-like processes. 



I append a list of half a dozen observations on different larvse 

 at this stage, to show the nature and extent of the variations in 

 the condition of the secondary slits. In all cases the first slit 

 or thickening was above and between the third and fourth 

 primary slits. 



In the second row of the above table the numbers are 

 inclusive. 



It is worth noting here that the first primary slit is distinctly 

 smaller than the second and following slits. If reference be 

 made to the figure of the larva with three gill-slits given in the 

 paper previously quoted (8) it will be found that the first slit 

 is larger if anything than those which follow it. At a certain 

 stage its growth is arrested, and later still, in a most remarkable 

 way, it not only becomes relatively but actually smaller in size. 



Stage III.— Figs. 4 and 25. 



In a larva of this stage the closure of the atrium has extended 

 forwards so as to leave only a small portion unclosed in front, so 

 that in this and all subsequent stages the gill-slits are entirely 

 seen through the transparent wall of the atrium. 



In fig. 4 there are twelve primary slits ; the usual number 

 at this stage is, however, thirteen. The secondary slits are 

 seven in number; the first is circular in outline, and has just 

 opened ; while the seventh is only present as a thickening not 

 yet perforated, and is also circular. The other secondary slits 



VOL. XXXII, PART II. — NEW SEE. N 



