GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 339 



sory locomotive stnictures, which transmit to the surrounding or sup- 

 porting medium the thrusts of the primary and essential locomotive 

 organs, which are the myomeres. 



If in turn we inquire into the nature and origin of the myomeres we 

 enter upon some of the master problems of vertebrate morpholog}', espe- 

 cially the origin of the mesench3'me, mesoderm and ccelom, the early 

 metamerism of the chordates and the phyletic relations of the Chordata 

 to other phyla of Metazoa. AYhile no one would claim that these greater 

 problems are fully settled, yet the modern studies of Sedgwick, Lankes- 

 ter, Goodrich, Patten, Castle, Kingsley, Willey, Delage and Herouard 

 (1898) and many others, afford a strong evidential basis for some such 

 synthetic concept of the earlier evolution of the chordates as may now be 

 outlined.''' 



The pre-Silurian and perhaps pre-Cambrian ancestors of the Chordata 

 were, I believe, related neither to the Arthropoda, Annelida or any other 

 phylum exhibiting metamerism: the elaborate resemblances discerned by 

 Patten and others between limuloid and chordate structures being re- 

 garded as the homoplastic results of similar locomotive adaptations on 

 the part of independent phyla having in common chiefly the following 

 characters: (a) anteroposterior motion, (h) a metameric repetition of 

 mesodermal tissue, (c) a subsequent independent process of cephalogen- 

 esis or concentration of neuromeres. 



Possibly these pre-Silurian chordates may have traced back their origin 

 to the stem of the echinoderms, or they may have been coelenterates of 

 some sort, as suggested by Sedgwick and by Masterman. Possibly they 

 are pictured in a general way by the early larvae of Balanoglossus and 

 echinoderms. At any rate they were at first more radiate than bilateral 

 in plan. They had a more or less ciliated epidermis, the cilia being the 

 first locomotive organs of all known ph5da. Their primitive gut or 

 archenteron may have borne several diverticula, more or less similar to 

 the five "archimeres'' of Balanoglossus, which were destined to give rise 

 to the myocoelomic pouches of chordates (Lankester). These myocoe- 

 lomic pouches are thought by some to have surrounded the gonads, which 

 were derived from the archenteron, and a primary segmentation of the 

 gonads, corresponding to that of the myotomes, was assumed ; but Kings- 

 ley states (1912, p. 319) that in the existing vertebrates no metamerism 

 of the gonads exists. At any rate the segmental myocoelomic pouches 

 very early came into functional relations with the gonads, through the 

 formation of segmental nephridial ducts. 



' A preliminary report on this subject was read before the New York Academy of 

 Sciences. Nov. 13, 1011. Abstract in Science N. S.. vol. XXXfV. p. SDl*. lUll. 



