J 20 NEMERTINEA chap, v 



If, however, the recent work of Biirger on the excretory system 

 is to be relied upon, the existence of flame cells would be a strong 

 reason for classing them among the Platyhelminthes. 



Hubrecht l has instituted an interesting comparison between 

 Nemertines and Vertebrates. He compares the median dorsal 

 nerve of Nemertines to the spinal cord of Vertebrates ; the lateral 

 nerve-cords to the nerve of the Vertebrate lateral line ; and the 

 lateral swellings which constitute the brain in Nemertines to the 

 lateral ganglia of the cephalic region in Vertebrates. This view is 

 strengthened by the existence of transverse nerves connecting the 

 lateral and dorsal nerves of Nemertines, since these may be com- 

 pared with the spinal nerves of Vertebrates. He suggests that both 

 Nemertines and Vertebrates may have arisen from a vermiform 

 animal possessing a nervous layer in the form of a plexus of nerve- 

 fibres, the nerve tissue having become concentrated along three 

 lines to form a median dorsal and two lateral nerve trunks ; the 

 former being specially developed in the Vertebrata and the latter 

 in the Nemertines. Hubrecht further suggests that the noto- 

 chord of Vertebrates may be a survival of the proboscis sheath of 

 Nemertines, while the proboscis of the latter may be represented 

 by the invagination to form the pituitary body in Vertebrates. 



Certain authors 2 have suggested that indications exist of a 

 relationship between Nemertines and Balanoglossus. 



The features which are supposed to indicate this are the 

 elongated vermiform shape showing no external signs of segmenta- 

 tion ; the ciliated smooth skin and the possession of unicellular 

 mucous glands; and the protrusible proboscis, which may be 

 comparable to the non- retractile proboscis of Balanoglossus, a 

 comparison which is strengthened by the fact that in some 

 Nemertines a sheath of nerve-fibres exists in the wall of the 

 proboscis corresponding to the nerve plexus in the proboscis of 

 Balanoglossus. In both cases an ectodermic nerve plexus exists 

 with local thickenings along definite lines, although these lines 

 are not the same in the two cases. Both possess a straight 

 alimentary canal, ending in a terminal anus and thrown out into 

 paired lateral caeca, between which are the paired metamerically- 

 arranged generative sacs. 



1 Quart. J. Micr. Sci. vol. xxiii. 1883, p. 349 ; Ibid. vol. xxvii. 1887, p. 605. 



2 Cf. Willey, Amphioxus and the Ancestry of the Vertebrates, Macmillan, 1894. 



