CE AP TER 
HEMICHORDATA 
CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA 
—EXTERNAL CHARACTERS AND HABITS—STRUCTURE—-GENERA 
— DEVELOPMENT — PTEROBRANCHIA—CEPHALODISCUS AND 
RHABDOPLEURA— PHORONIDEA PHORONIS AND ACTINO- 
TROCHA——AFFINITIES OF THE HEMICHORDATA. 
THE Hemichordata, a marine group which includes the worm-like 
Balanoglossus, owe much of their interest to the fact that they 
are believed by many zoologists to be related to the lower Verte- 
brates. This view is one of a number of mutually exclusive 
hypotheses, which seek to derive Vertebrate animals from various 
Invertebrate ancestors. It is supported by many striking re- 
semblances between Balanoglossus and the lowest forms which 
are by common consent regarded as belonging to the Vertebrate 
alliance; but it must be distinctly understood that Balanoglossus 
is at most the much modified modern representative of extinct 
forms which were also the ancestors of Vertebrates. 
The axis of the backbone of all Vertebrates is formed by an 
elastic rod known as the “ notochord” (Figs. 72, 115), which lasts 
throughout life in some of the lowest forms, but in the higher 
forms appears only in the embryo. The universal occurrence 
of this structure has been regarded as the most important 
characteristic of the Vertebrata and their allies, which are 
accordingly grouped together in the Phylum CHORDATA. 
The members of this Phylum are further distinguished from 
other animals by several important features. Of these one of 
the most important appears to be the existence of lateral out- 
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