206 AMPHIOXUS. 



12. The nervous system. 



Remove the nerve-cord along its entire length : stain it ; 

 mount on a slide in glycerin, and examine with the micro- 



The dorsal roots of the nerves and their 

 alternate arrangement are well seen (compare 

 pp. 200, 201). 



13. The structure of the notochord. 



Remove a piece of the notochord : stain it ; mount in 

 glycerin, and examine with the microscope. 



The notochord breaks up very readily into 

 transverse discs. 



III. EXAMINATION OF TRANSVERSE SECTIONS. 



Many points in the anatomy of Amphioxus are determined 

 far more easily by an examination of transverse sections of 

 the animal than by dissection. For this purpose a specimen 

 should be cut by means of a microtome, and the sections mounted 

 on slides in their proper sequence. It is only necessary to 

 examine in detail a few of the more typical ones. 



A. Transverse Section passing through the Buccal Cavity. 



1. General characters. The dorsal surface is indicated by 



the laterally compressed dorsal fin : the ventral 

 surface by the mouth-opening, which is bordered at 

 the sides by the thickened lips ; within the buccal 

 cavity are usually seen sections of the tentacles. 

 About the middle of the section is the notochord, 

 and above this is the spinal cord. At the sides are 

 the myotomes, separated from each other by con- 

 nective-tissue septa. 



2. The skin consists of two layers. 



a. The epidermis is a single layer of short columnar 



cells, with a thick cuticle on their outer ends. 



b. The dermis is a very thin layer of connective tissue 



underlying and supporting the epidermis. 



