[Illustration: FIG. 61.—_Malacobdella grossa_ O. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.) A, From the dorsal surface; B, from the ventral surface.] PALAEONTOLOGY.—Nemertines are unknown in a fossil state; this is probably owing to the softness of their bodies, which would render their preservation extremely improbable. AFFINITIES.—Until recently the Nemertines were regarded as a sub-order of the Turbellaria. They were afterwards separated from the Turbellaria and placed as a distinct class of the phylum Platyhelminthes. Some zoologists have considered them to be so different in many respects from the other classes of the Platyhelminthes as to justify their being altogether separated from that phylum, and treated as a distinct group. {120}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_.