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B. W. MACBEIDE. 



nervous tissue. The collar cavities were probably prolonged 

 into tentacles with which nervous tissue was associated. 



If this supposition is correct, the group Protocoelomata was 

 a pelagic cosmopolitan one, and it is in accordance with what we 

 know of wide ranging groups that some of its members should 

 adopt changed habits and modified structure. The Echino- 

 dermata, then, represent the earliest offshoot which took to a 

 sessile life and acquired radial symmetry. A little later the 

 Hemichordata branched off, a burrowing life being adopted 

 and consequent degeneracy resulting. The main stem, how- 

 ever, remained pelagic and gave rise to the Chordata. The 

 Ascidians were the next offshoot, and then came Amphioxus. 

 We see, therefore, that the track of the great Chordata 

 phylum through past ages is traced by examining those 

 of its members who at very different periods of its history, 

 and at different stages in its evolution, have forsaken their 

 high vocation, and taken to a sessile or burrowing life, with 

 the inevitable consequence — degeneracy. 



The following diagram may represent these relationships a 

 little more clearly : 



Protocoelomata 



Dipleurula 



Fixed ancestor 

 of Echinoderms. 



Criuoids 



Hemichordata (Tornaria). 



Balanoglossus, Cephalodiscus. 



Protochordata. 



Protoecbinids. Ophiurids. 



Ascidians. 



Amphioxus. 



Echinids. Holothurids. Verlebrata. 



