XVIII] 



NERVOUS SYSTEM 



397 



of which has been already explained (Fig. 186). The walls of 

 the stomodaeum are known collectively as the oral hood. The 

 position of the primary mouth is still marked by a projecting lip, 

 the velum, which is produced into ten or twelve delicate tentacles. 

 These form a filter to prevent coarse material from reaching the 

 alimentary canal. 



The nervous system is a simple tube with thick walls and very 

 narrow cavity. It is almost as extended as the notochord, and lies 

 above it. It does not however quite reach the front end of the 

 body. Its extreme front tip is called the cerebral vesicle ; it has 

 a wide cavity with thin walls, so that the total diameter is not 



FIG. 193. Median vertical section through the cerebral vesicle of Amphioxus. 



After Kupffer. 



c.v. Cavity of cerebral vesicle. e. Eye-spot. g.c. Dorsal group of ganglion- 

 cells, inf. Infundibulum. l.o. Olfactory lobe. tp. Tuberculum 

 posterius. 



increased. There is a pit reaching down to it from the external 

 skin, possibly a rudimentary olfactory organ (Fig. 193), and in the 

 wall of the vesicle itself is a mass of pigmented cells forming an 

 eye-spot. In the young larval Amphioxus this part of the nerve- 

 tube remains for a considerable time as an uncovered medullary 

 plate, and one is inclined to imagine that it corresponds to the 

 sensitive nervous surface of the proboscis in the Hemichorda, 

 since in the larvae of these animals there is a sensitive plate 

 with two eye-spots at the apex of the prae-oral lobe. It must be 

 remarked however that pigment cells in each of which a nerve-cell 



