3S6 BALANOGLOSSUS. 



(3) Proboscis-pores opening into body-cavity of pre- 



oral lobe. 



(4) Collar-pores opening into collar-cavity, overhung 



by an operculum developed from the collar, 

 (s) The presence of gill-slits. 



(6) " Existence of a notochord as a diverticulum of 



the alimentary canal, growing forwards into the 

 proboscis-stalk." 



(7) " Dorsal central nervous system, most richly de- 



veloped in the collar, but extending on to the 

 proboscis." 



These affinities, and the last three characteristics in par- 

 ticular, perhaps justify the position of Cephalodiscus beside 

 Balanoglossus, as a second genus of Hemichordata. 



Appendix to Hemichordata. 



Rhabdopleura. 



A unique genus found at considerable deptlis in the North Sea. It 

 forms branching colonies attached to foreign objects. The head of each 

 individual bears a pair of lateral arms with ciliated tentacles. Between 

 the bases of the arms is a ciliated mobile disc overhanging the mouth. 

 This disc seems to secrete a tube which surrounds the animal, and it 

 also helps the animal to move within its lube. The arms and the 

 tentacles have a skeletal support, and there is an axial skeleton in the 

 body. The gut has a U-shaped curvature, as in Polyzoa. Neither 

 nervous system nor nephridia are known. The affinities oi Rhabdopleura 

 are uncertain, perhaps it is related to Polyzoa, perhaps to Cephalodiscus. 



