26 



PP.OCEEUINOS OF THE 



point to a remote common ancestor in which the supportino 

 notochord was not yet formed, the nervous system was superficial 

 and more diffuse, and the segmental ion less perfect. 



We have seen that the study of the Vertebrates leads us back 

 step by step to a simple undifferentiated ancestor, in which the 

 complex sense-organs, the highly developed brain, tlie chambered 

 heart, and other structures so characteristic of this phvhira had 

 not yet appeared. Now, the same conclusion is reached on 

 studym- such other groups as the Mollusca and Arthropoda. 

 Here also we are led back along an irreversible series of forms to 

 a simpler generalized ancestor. Tlie Vertebrates, Molluscs and 

 Arthropods, have diverged along fundamentally different lines of 

 aifferentiation. 



Just as the organisation of the Vertebrata is governed by 

 the appearance of a dorsal nervous system, a notochord gUl-slits 

 a mesoblastic skeleton, etc., so the whole organisation of the 

 Arthropoda is dominated by the secretion of a complete superficial 

 chitmous skeleton, and the accompanying development of jointed 

 appendages serving for feeding and progression. Similarlv, the 

 Molluscan organisation has been throughout influenced by the 

 secretion of a calcareous dorsal shell, and the development of a 

 soft body capable of distention by the blood-vascular system. Of 

 all the systems of organs the nervous system mav be considered 

 as the most important, and it is just in the study "of this system 

 that we can most easily trace the divergence in structure of the 

 three groups. 



Owing to adaptation to similar environment or function certain 

 striking resemblances may occur between animals of widely 

 separated origin; this is especially the case with sense-orf-ans 

 adapted to receive definite stimuli. Thus, a Cepha]o[)od Moliusc 

 ha^ a large brain enclosed in a cartilaginous skull, with paired 

 orbits containing large eyes remarkably hke those of the Craniate 

 Vertebrate. But the resemblance is due to convergence; these 

 complex organs were not present in more primitive Mollusca. and 

 have been acquired within the Molluscan phylum. Examined 

 carefully they are found to differ as fundamentally in every detail 

 from those of the Vertebrate as does the whole organisation of 

 the Mollusc differ from that of the Vertebrate in general. 



Eesemblances between the Arthropod and the Vertebrate are 

 not so striking; when they do occur they can be shown to be of 

 the same nature. Here also the various organs which acquire 

 some likeness to each other in the two groups are found to differ 

 as fundamentally in detail as they do in origin. What the two 

 groups really have in common is only that which they have both 

 inherited from a ^ery early undifferentiated ancestral stock. 



