II 



in stnu'tiuv ; that is, their parts are synniiotrieally ar- 

 ranged on either side of a longitudinal axis. Two end- 

 edves^ has gained the mastery over the sphere. There 

 are here an (interior and a j/ostcrior pole, and these 

 usnally correspond to the head and tail of the animal, 

 although the lower mollusoa are headless. 



The great stem of the Veijtebijata divides, primari- 

 ly, into four lateral hranehes, with a fifth central stem, 

 destined to produce the most important vertebrate 

 forms. The primary whorl, composed of these lateral 

 ))ranches, includes fish-like animals which are entirely 

 distinct from the true fishes. The lowest in u'rade of 

 these generalized vertel)rates is the Lancelet, or Ani- 

 jiltio.riis. forming the division Piiaryxgobranchii, or 

 Leptocakdia. This is the animal which durino- its em- 

 Itrvological develo))ment recalls the ( 'Oelenterata and 

 lower Mollusca, and reaches maturity with no resem- 

 blance to any but an embryonic vertebrate. It is with- 

 ont vertebra, possessing only the noto-chord which is 

 transitory in other vertebrates. Xo fossil Leptocardia 

 are known. It is reasonable, but not necessary, to 

 conclude that it must have come into existence early 

 in time. In many other instances, forms low in struc- 

 ture have ap])eared at ('()m|)aratively late geologic 

 eras. 



The Marsipobkanciiii, or the Lamprey P]els, are the 

 next in order. Although no fossil forms are known, 

 they, in some respects, jipproximate the earliest of the 

 Silurian vertebrates. 



The Elasmobranchii (Sharks. Rays, and Chimoeras), 

 unlike the Leptocardia and Marsipobranchii, came ear- 

 ly into existence, occurring in the Upper Silurian 



