Introduction 21 



remained freshwater or became lymnicolous evolved ulti- 

 mately as the Metanemertinea not a few of which gradually 

 became marine, another seems early to have migrated sea- 

 ward and evolved as the Protonemertinea. This in time 

 gave off two evolving but diverging lines that gave rise to 

 the Mesonemertinea and later to the Heteronemertinea. 



(c) Under the third heading it must be acknow- 

 ledged and accepted that between the simplest living Eu- 

 chordate type and the highest metanemertean there is a 

 wide gap, even if we include fossil forms like Palaeospon- 

 dyhis, Lanarkia, and others described on later pages 

 (p. 112, 121). But when we include the Ammocoetes 

 larva of Petromyzon, that of the marine Ascidians, also 

 the larva and adult state of Amphioxus the gap becomes 

 much narrower. If further one considers how long-con- 

 tinued and enormous have been the denudation, destruc- 

 tion, and rearrangement of land areas; equally the organic 

 stresses, denudations, and obliterations; also the many mil- 

 lions of years during which these processes have continued, 

 the wonder should be rather that the record is so complete 

 for such soft-bodied animals. It has likewise been ac- 

 cepted, almost without question, that Palaeospondylus and 

 Lanarkia as well as other primitive fossil fish genera were 

 marine. The writer adduces evidence later on which proves 

 that all of them remained for long ages as freshwater 

 organisms, and only slowly evolved sidelines that passed 

 into the sea, and were able to survive there alongside 

 varied competitors. 



(d) Under the last heading we need only at present 

 reiterate the view which has been generally accepted by 

 zoologists, viz. that the primitive line of fish evolution 

 early split up into at least two steadily diverging branches. 

 One of these probably continued long and wholly in fresh- 

 water surroundings, and unquestionably must once have 

 Included numerous genera which made up the group now 

 known as the Cyclostomata. For only on this hypothesis 

 can we account for the structural details of the various 

 genera that now make up the group. While some living 

 species, according to Gage, are free-swimming and In no 

 sense parasitic, the gradual perfecting of a suctorial disc, 



