SKELETAL SYSTEM 175 



length, mounted whole as microscopical preparations ; while 

 the details of structure should be confirmed by microscopical 

 examination of transverse sections through various regions of 

 the body. 



Examine under a low power a young specimen of Amphi- 

 oxus jnhich has been stained and mounted whole. 



Examine also a series of transverse sections through 

 various parts of the body of an adult specimen. 



The main featv/res in the anatomy of Amphioxus may 

 readily be determined by dissection of an adult specimen 

 which has been macerated in 20 per cent, nitric acid for a 

 day. 



A. The Skeletal System. 



The skeleton of Amphioxus is extremely simple. It con- 

 tains neither bone nor cartilage ; but remains throughout life- 

 in a condition corresponding to a very early stage of develop- 

 ment in higher vertebrates. 



1. The notochord is a median elastic rod which traverses 



the body from end to end, lying rather nearer the 

 dorsal than the ventral surface, and between the 

 central nervous system and the alimentary canal. It 

 is ia contact with the integument at the two ends of 

 the body ; and it serves to slightly stiffen the animal. 

 It consists of cells in which, at an early period,, 

 large vacuoles appear : these increase in size, and 

 ultimately almost completely fill the cells, render- 

 ing their outlines difficult to trace. On the dorsal 

 and ventral surfaces of the notochord, the ceUs are 

 comparatively unaltered. 



2. The buccal skeleton. At each side of the mouth is a 



curved bar, resembhng the notochord in structure. 

 Each bar is divided transversely into about a dozen 

 short segments, and from these filaments arise which 

 support the buccal tentacles. 



3. The skeleton of the median fins. Along the whole length 



of the dorsal fin, except at its extreme anterior and. 



