182 AMPHIOXUS. 



of the anus, at the junction of the ventral and 

 lateral fins, and about one-third the length of the 

 animal from its hinder end. Through it the water 

 escapes which has been taken in at the mouth with 

 the food and which serves for respiration. 



B. The Skeletal System. 



The skeleton of Amphioxus is extremely simple, and con- 

 tains neither bone nor cartilage. 



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 stage, 

 large vacuoles appear : these vacuoles increase in size 

 and ultimately almost completely fill the cells, ren- 

 dering their outlines difficult to trace. On the dorsal 

 and ventral surfaces of the notochord, the cells are 

 comparatively unaltered. 



In higher vertebrates also a notochord is formed 

 in early stages of development, but it is afterwards 

 replaced more or less completely by the vertebral 

 column. 



.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 a number of 

 short segments, and from these filaments arise which 

 support the buccal cirri, which vary from twenty to 

 forty in number, becoming more numerous as age 

 advances. 



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

 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 



