376 AyXALS XEW YORK ACADEMY OF SCIEXCEi 



transformation of minute, acraniate. radially consTrncted ccelente rates, 

 moving by the lashing of cilia, and ingesring microscopic food particles, 

 into pre-gnathostome animals with contractile myocoelomic pouches, 

 which, becoming mnscnlar, gave rise to the primary locomotive organs, 

 or myomeres, as well as to the coelom. 



Bilaterality enstiing, and pharangeal diverticula appearing, our ani- 

 mals entered the path of predatory adaptation: this led to the acquire- 

 ment of accessory locomotive structures, including the local outgrowths 

 called fins, and to the formation of a S}Tithetic or gnathostome type of 

 head, fitted for seizing and ingesting living prey. 



The acquisition of a many-layered skin, proceeding with the further 

 differentiation of the myoccelomic pouches, and of the derived mesen- 

 chyme, was followed by the development of an exoskeleton, and later of 

 an endoskeleton, the mesenchyme carrying the bone cells either to the 

 skin or to the mesench}Tiial connective tissue sheaths between the myo- 

 meres and around the dorsal axis. 



Endoskeletal supports thus evolving the Osteichthyes appeared. 



Of these the Dipnoi acquired elaborate tritoral teeth and other special- 

 ized conditions and thus removed themselves from the main ascending 

 line, while in the other direction the Actinopterygii carried to a high 

 perfection the scale-like dermal rays and other purely aquatic adaptations. 



The Crossopterygii partly paralleled the Actinopter^-gii in the evolu- 

 tion of dermal rays but were distinguished by the outgrowth of fleshy 

 lob ate, fan-like paired fins, the prerequisites for the acquirement of hands 

 and feet. The double-breathing pro-Tetrapoda reversed the direction of 

 their evolution, sacrificed dermal rays, hypural bones, caudal and dorsal 

 fins, scales, operculars, gulars and the elements connecting the shoulder- 

 girdle with the skull, largely reduced the cleithrum and emerged from 

 the water by virtue of the tractive and propulsive power of their stout 

 paired fins. 



As thus conceived the rise of the vertebrates and the origin of the 

 Tetrapoda constitute a history of successive improvements in the loco- 

 motive apparatus. 



BiBLIOGEAPHY 



Abel, O. : Grundzrige der Palseobiologie der Wirbeltiere. 8vo. Stuttgart, pp. 



1-708. 1912. [Footprints of Carboniferous Tetrapoda. pp. 66-68: origin of 



the pollex in Stegocephali. pp. 218-220.] 

 Baub, G. : The Stegocephali. A Phylogenetic Study. Anat. Anz.. Bd. XI, Nr. 



22, ss. 657-G73. 1896. 

 Bbaus, H. : Die Muskeln und Xerven der Ceratodusflosse. Semon's Zool. For- 



schnngsreisen, Erster Band : Geratodus. iii Lief. Atlas. Taf . xxi-xxix, ss. 



1.37-300. 1901. 



