Phylogeny of Animals 543 



The Cephalochordata or group of the lancelets finally claims 

 attention. These still more clearly than the last, and in all 

 stages of their life-history, proclaim evident chordate char- 

 acters. But the elongate notochord, the simplified spinal 

 cord, the simple saccular liver, and other structural details, 

 are all correlated with the burrowing habits, and indicate 

 simplification by degradation. 



In summary then we would conclude that fresh- water nemer- 

 teans, derived from a rhabdocoel turbellarian ancestry, became 

 in increasing number marine, in part remained fresh-water, in 

 part assumed life on land but amid moist surroundings. From 

 the second of the three the Protochordata evolved, by conver- 

 sion of the nemertean proboscis-tube, into a notochord, by 

 modification of the proboscis into highly sensitive accessory 

 pharyngeal parts, and by resulting development, specialization, 

 and condensation of the brain-masses along the anterior dorsal 

 region, where nerve action and reaction were centered round 

 the proboscis, mouth, and dorsal sense organs. 



Migration seaward of protochordate members originated the 

 urochord and later cephalochord series, while the main proto- 

 chordal stem gradually evolved into a protocyclostome and 

 later a paracyclostome and cyclostome series. Some of these 

 remained in fresh water like species of lamprey that are now 

 world-wide therein, others became sea migrants but homing 

 species in spawning and early larval years, as is true of the 

 sea lamprey, still others like Bdellostoma and Myxine became 

 littoral or deep sea genera. From the first we would trace 

 the origin of the Protoamphibia and later the Apoda, that 

 tended increasingly to pass from a fresh-water to a moist land 

 environment, at least during part of their life history. The 

 last in turn evolved the urodele Amphibia, from which, by 

 transformation changes already indicated, the marsupial mam- 

 mals evolved. 



