20 Evolution and Distribution of Fishes 



shows a fundamental structural basis, in nearly every detail, 

 on which could continuously and gradually be evolved or 

 built up those characters which we associate with euchor- 

 date or vertebrate organisms. For whether one compares 

 the ectodermal or skin derivatives, the brain and nervous 

 systems, the alimentary canal with associated gland tissue, 

 the proboscis sheath — or rhynchocoelom — as forerunner of 

 the notochord of vertebrates, the proboscis as sub- 

 sequently modified in part into oral structures and in part 

 into the pituitary body of vertebrates, the blood vascular 

 system, the excretory system, the origin and fate of the 

 reproductive elements, and finally the development of the 

 embryo, all exhibit homological details and continuity that 

 are approached by no other group of organisms. 



(b) Under the second heading, the writer would draw 

 attention to the facts and arguments already adduced by him 

 which showed that a freshwater environment was the prim- 

 eval ecological arena in which evolved all of the important 

 animal groups. But on first sight it might seem vain to 

 advocate a freshwater origin for Nemerteans in light of 

 Burger's synopsizing statement ((5: 481) thus: "The 

 Nemerteans are almost exclusively inhabitants of the sea. 

 We ought so to express it, because out of 406 assured 

 species, which are at present known, only 7 live in fresh- 

 water, 8 on land, and i as a parasite in a freshwater snail." 

 But it should be said that the Rhabdocoela, which are 

 evidently progenitors of the Nemertinea, include to-day a 

 majority of freshwater species, and may well have included 

 a preponderating majority in former ages, when all the cir- 

 cumstances of the case are considered. Further, the writer 

 has shown that alike amongst sponges, coelenterates, poly- 

 zoans, worms, crustaceans and other groups, the simplest 

 known types are still freshwater inhabitants, even though 

 they may now be few in numbers there. So though the fresh- 

 water and land nemerteans are now poor in genera and 

 species, compared with marine ones, this need not cause us 

 to consider that such has always been true in past geologic 

 times. It rather seems to be true that from a more primi- 

 tive freshwater rhabdocoel ancestry two main evolutionary 

 lines of nemertean specialization started. One which in part 



