SKELETAL SYSTEM 171 



II. THE ANATOMY OF AMPHIOXUS. 



Examine under a low power a young specimen of Amphi- 

 oxus which has been stained and mounted whole. Examine 

 also a series of transverse sections through various parts of 

 the body of an adult specimen. 



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 in 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 cells are 

 comparatively unaltered. 



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



curved bar, resembling 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 

 posterior ends, is a longitudinal series of connective- 

 tissue compartments filled with a gelatinous sub- 

 stance. The compartments are more than twice as 

 numerous as the segments of the body, and their 

 function is uncertain. A similar structure extends 

 along the ventral fin between the anus and the atrial 

 pore. 



