EXTERNAL CHARACTERS. 181 



II. Directions for the dissection of Amphioxus ; by which 



the more sahent points in its anatomy may be 

 determined. 



III. Descriptions of transverse sections taken through 



different regions, and examined as microscopical 

 objects. 



I. THE ANATOMY OF AMPHIOXUS. 

 A. External Characters. 



1. 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. 



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



line from end to end of the animal, becoming 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 section. 

 The sides of this flattened surface are bordered by 

 the lateral fins, or metapleural folds. 



3. The myotomes. The sides of the body are marked by 



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

 nective tissue which divide the great lateral muscles 

 of the body into segments or myotomes. 



4. The ' mouth ' or opening of the oral vestibule 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. 



5. 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. 



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



surface, bordered by prominent lips. It is in front 



