AMPHIOXUS 1 69 



I. EXTEENAL CHAEACTEES. 



Take an adult specimen preserved in spirit, place it in a 

 small dish in weak spirit or water, and examine with a pocket 

 lens or dissecting microscope. 



a. The shape. Amphioxus is elongated, pointed at 



both ends, and compressed from side to side, 

 especially towards the posterior end. There is 

 no obvious head, and no trace of limbs. 



b. The fins. A low dorsal fin runs along the mid- 



dorsal line from end .to end of the animal, be- 

 coming more prominent at the hinder end as 

 the upper lobe of the caudal fin. 



The ventral surface bears a median fin in its 

 posterior third, but in front of this is flattened, 

 so that the body is triangular in transverse sec- 

 tion. The sides of this flattened surface are 

 bordered by the lateral fins, or metapleural folds. 



c. The myotomes. The sides of the body are marked 



by a series of > shaped lines, formed by septa of 

 connective tissue which divide the great lateral 

 muscles of the body into segments or myotomes. 



d. The mouth is a large oval aperture on the ventral 



surface of the anterior end of the body, and is 

 fringed on each side by a series of ciliated 

 tentacles or cirri. 



e. The anus is on the ventral surface, not far from the 



hinder end of the body. It is placed a little to 

 the left of the median plane, at the base of the 

 caudal fin. 



f. The atrial pore is a median aperture on the ventral 



surface, bordered by prominent lips. It is in 

 front 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 which is taken in at the mouth for 

 respiration passes out.. 



