144 



DEVELOPMENT OF AMPHIOXUS. 



In Fig. 8 1, a larva corresponding in age approximately 

 to that of Fig. 74 is represented as seen from the left side. 



As noted above, the posterior primary slits bend nor- 

 mally under the pharynx at this stage, and some of them 

 extend as much on one side of the body as on the other, 

 being continued across the ventral side of the pharynx. 

 The great feature of this larva is the relatively prodigious 

 mouth, through which the upper portions of the first four 

 primary slits can be seen. 



From this side we look into the depths of the praeoral 

 pit, having only seen it by transparency in the preceding 







Fig. 81. Anterior portion of larva, with thirteen gill-slits, from the left side. 

 (After WILLEY.) 



olf. Olfactory pit, communicating with neuropore. x. " Nephridium " of Hat- 

 schek. n.t. Spinal cord. ch. Notochord. p.p. Prseoral pit. ex. External open- 

 ing of club-shaped gland, ci. Rudiment of buccal cirri, p.b. Peripharyngeal band. 

 m. Mouth, /.ji 3 . Thirteenth primary slit. 



figures. It is continued backwards into a ciliated groove, 

 which abuts on the dorsal margin of the mouth. Prob- 

 ably most of the food which enters the mouth passes 

 along this groove. 



Below the pointed anterior extremity of the mouth is to 

 be seen the external aperture of the club-shaped gland, 

 and a short distance behind this is a round, refringent 

 body, which has become differentiated from the gelatinous 



