136 ^VILLIAM BATBSON. 



two or three hours. As will be shown when the internal deve- 

 lopment is described, the appearance of this groove occurs at 

 the time of the delamination of the dorsal nerve-cord in this 

 region of the body. 



Stages G and H. — Simultaneously with the disappearance 

 of this neural groove, two pores are to be seen perforating the 

 skin behind the collar, being placed in a dorso-lateral position, 

 one on each side of the middle line. Tliese pores in which long 

 cilia may be seen working, constitute the first pair of gill slits 

 (fig. 15, ff. s.). In this stage the collar has become much 

 shorter, and the cilia of the baud are so densely crowded as to 

 give an appearance of a thickened ring in preserved specimens. 

 After the formation of this pair of gill slits no further external 

 change of importance is to be noted in any of the oldest larvae 

 which I found. The proboscis becomes longer and more 

 muscular, and the body increases somewhat in size ; the region 

 of the body behind the ring of cilia especially elongates. The 

 ring of cilia may, moreover, in a profile view, be seen to lie no 

 longer in a truly transverse direction, but rather obliquely with 

 its dorsal limb anterior to the ventral portion (fig. 17). The 

 body is usually somewhat bent upon the ventral surface while 

 the animal swims. This feature is usually exaggerated after 

 death, and in consequence from the dorsal side the whole length 

 of the animal is not seen (fig. 16). 



The mouthis formed in Stage F as a minute pore placed on 

 the ventral surface in the groove which divides the proboscis 

 from the collar; from its position and small size it is not 

 visible when the animal is examined as a whole. The forma- 

 tion of the anus occurs rather later by a perforation of the 

 skin at the posterior end ; it also is best seen in sections. In 

 position it is approximately coincident with the point at which 

 the blastopore finally closed. 



I have not hitherto been able to obtain any older larvse whose 

 external features differ essentially from Stage H. Judging 

 from the fact that at this stage a considerable increase in size 

 takes place, and from the commencement of several important 

 internal structures at this time, it seems probable that the 



