SECTION THROUGH PHAEYNX 187 



nerve-cells growing across it from side to side. 

 The greater part of the cord has a finely punc- 

 tate appearance, caused by the cut ends of the 

 nerve-fibres. 



b. The nerves wiU only be recognised if the section 

 happens to pass through their roots of origin, 

 i. The dorsal nerves arise by large single roots from 

 the side$ of the cord, about midway between 

 the dorsal and ventro-lateral angles, 

 ii. The ventral nerves arise by very ^inute and 

 multiple rootlets from the ventro-lateral 

 angles of the cord. 



B. Transverse Section through the Anterior Part of the 

 Pharynx. 



1. General characters. The section is triangular in shape, 



the apex being formed by the dorsal fin, the base by 

 the ventral surface, and the basal angles by the meta- 

 pleural folds. The pharynx, perforated by the giU- 

 slits, and surrounded by the atrial cavity, lies below 

 the notochord. 



2. The skin of the ventral surface is thrown into very 



strong folds, to allow for the distension of the atrial 

 cavity when the reproductive organs attain their full 

 size. Its general characters are the same as in A. 



3. The skeletal system is the same as in A, except that the 



branchial skeleton is present, and that there is no 

 buccal skeleton. 



4. The mnscular system. 



a. The myotomes are similar to those of A, but con- 



siderably larger. 



b. The ventral muscles form a thin sheet running 



in the atrial fold of each side from the lowest 

 myotome to the mid-ventral line, where the 

 muscles of the two sides meet in a median raphe. 

 0. Small muscular bands are present in the suspensory 

 folds of the pharynx and in the giU-arches. 



