If, however, the recent work of Bürger 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[[153]] 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 compared 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 notochord 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[[154]] 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 segmentation; 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.

NEMATHELMINTHES & CHAETOGNATHA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge.

CHAPTER VI

NEMATHELMINTHES