EXTERNAL FORM. 



19 



body to the anterior wall of 

 the brain. This is known 

 as Kollikcrs olfactory pit, 

 after its discoverer. The 

 cells which line its walls 

 carry long vibratile cilia, and 

 it possibly subserves in some 

 degree an olfactory func- 

 tion. In the larva the cavity 

 of the brain opens into the 

 base of the olfactory pit by 

 a pore known as the neiiro- 

 pore, which we shall consider 

 later. In the adult this 

 pore becomes closed, but 

 the base of the olfactory pit 

 appears to remain connected 

 with the roof of the brain 

 by a solid stalk. The olfac- 

 tory pit, like the anal open- 

 ing, lies asymmetrically on 

 the left side of the body 

 (Fig. 5). It is forced to one 

 side in the course of the 

 development consequent on 

 the formation of the fin-like 

 expansion of the integument 

 in this region, which, as we 

 have seen, is nothing more 

 than the cephalic continua- 

 tion of the dorsal fin. 



The mouth of Amphioxus 

 would seem to be well 



Fig. 5. — Transverse secrion through 

 region of olfactory pit. (After Lan- 



KESTER.) 



Tile oifactory pit is seen as an ecto- 

 dermic involution on the left side in con- 

 tact with the wall of b, the cerebral vesicle. 

 ch. Notochord. f. Lymph-space of ce- 

 phalic portion of dorsal fin. r.k. and Lk 

 Right and left portions of oral hood. 

 ?ny. Muscles of first myotome; outside of 

 the muscles is the myocQelic lymph-space 

 of first myotome; inside of the muscles 

 is the apex of the mvocoelic lymph-space 

 of the second myotome. ;/. Cranial 

 nerve {second pair). 



N.B. — The dotted shading represents 

 the thickened gelatinous connective tissue 

 of the head-region in which irregular 

 lymph-spaces occur. 



