THE LARVAL PERIOD. 75 



which the larva takes to living in the sand varies greatly in 

 different individuals. At the close of the larval period the 

 larva measures about 3*5 mm. in length. 



1. The Gill-slits. 



It has already been mentioned, in the general account of the 

 development of Amphioxus, that the gill-slits of the two sides are 

 not formed simultaneously ; those of the left side, which may be 

 termed primary slits, appearing before those of the right side, or 

 secondary slits. The primary slits, of which there are as a rule 

 fourteen, are formed, not on the left side, but in the mid-ventral 

 wall of the pharynx, and, after their formation, shift upwards so 

 as actually to be for a time on the right side of the pharynx. 

 The secondary slits, usually eight in number, are formed at a 

 later stage, along the right side of the pharynx, dorsal to the 

 primary slits. Towards the close of the larval period, as the 

 mouth assumes its median position, the primary slits shift across 

 to their permanent position on the left side : at the same time, 

 by an actual diminution in number, through disappearance of 

 the slits at the two ends of the series, the primary slits become 

 reduced to eight, and the critical stage is reached, at which the 

 primary and secondary slits are equal in number, and symme- 

 trically arranged along the left and right sides of the pharynx 

 respectively. 



a. The primary gill-slits, or the gill-slits of the adult left 

 side of the pharynx, are formed in succession from before back- 

 wards. Like the first gill-slit, the development of which has 

 already been described, each of the succeeding primary gill-slits 

 lies at first in the mid-ventral wall, but, with the exception of 

 the hindermost two or three, shifts almost at once to the right 

 side of the pharynx. The full number of primary gill-slits is 

 as a rule fourteen, but varies in different specimens from twelve 

 to fifteen. The slits are at first metarnerically arranged, corre- 

 sponding, when they are fourteen in number, to the somites 

 from the second to the fifteenth inclusive ; this metameric 

 arrangement is, however, entirely lost in the later stages of 

 development. 



The condition with three fully developed primary gill-slits, 

 and a fourth slit in the act of forming, is shown in Fig. 36 ; and 

 the stage in which all fourteen primary gill-slits are present, in 



