CHAPTER 1 



HEMICHORDATA 



CHORDATA AND VERTEBRATA HEMICHORDATA ENTEROPNEUSTA 



EXTERNAL CHARACTERS AND HABITS STRUCTURE GENERA 



DEVELOPMENT PTEROBRANCHIA CEPHALODISCUS AND 



RHABDOPLEURA PHORONIDEA PHORON1S 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 CHOKDATA. 

 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- 



