244 THE PROTOCHORDATA. 



openings on the dorsal side of the body, placing the anterior 

 portion of the digestive tract in communication with the 

 outer world. These are the gill-slits, and they are arranged 

 strictly in consecutive or metameric pairs to the number of 

 upwards of fifty in the adult. In their structure, and more 

 especially in the possession of tongue-bars, they bear a 

 remarkable resemblance to the gill-slits of Amphioxus. 

 This is particularly striking in young individuals. As the 

 adult form is approached in the development, the bulk of 

 the gill-slits sinks below the surface, only opening at the 

 latter by small slit-like pores, and thus their true character 

 is obscured in a superficial view. 



Projecting into the interior of the proboscis is a rod-like 

 structure which arises as an outgrowth from the alimentary 

 canal dorsal to the mouth. The lumen of this endodermic 

 diverticulum becomes narrowed down and, in fact, partially 

 obliterated, while the cells constituting its walls give rise 

 to a spongy vacuolar tissue which strongly resembles the 

 notochordal tissue of Amphioxus and the higher Verte- 

 brates. On account of its dorsal position above the mouth, 

 its endodermic origin, and the vacuolisation of its cells, this 

 structure was identified by BATESON in 1885 as tne ' noto- 



cJwrd. 



Nervous System and Gonads. 



The nervous system of Balanoglossus presents many 

 features of the utmost interest and suggestiveness. It 

 consists essentially of an ectodermal net work of nerve-fibres 

 forming the inner layer of the skin (ectoderm) all over the 

 body. In this primitive nervous sheath, which envelops 

 the whole body, there are certain definite local thickenings. 

 Two of these thickenings occur respectively along the 

 whole length of the dorsal and ventral middle lines in the 

 trunk-region, thus producing the dorsal and ventral median 



