398 E. W. MACBRIDE. 
nervous tissue. The collar cavities were probably prolonged 
into tentacles with which nervous tissue was associated. 
If this supposition is correct, the group Protoceelomata was 
a pelagic cosmopolitan one, and it isin 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 : 
Protocclomata 
Dipleurula 
Hemichordata (Tornaria). 
Fixed ancestor Balanoglossus, Cephalodiscus, 
of Echinoderms. 
Crinoids. Protochordata. 
Asterids. Ascidians. 
Protoechinids, Ophiurids. | Amphioxus. 
Echinids. Holothurids. Vertebrata. 
