CHAPTER XVII 



PHYLUM CHORDATA 



SUB-PHYLUM HEMICHORDA 



Under the title Hemichorda are included a number of 

 interesting types which seem to have affinities with Verte- 

 brates. These affinities are clearest in certain worm-like 

 animals with distinct gill-clefts, e.g. Balanoglossus and 

 Ptychodera, w^hich form the class Enteropneusta. Perhaps 

 allied to these are two peculiar types — Rhahdopleura and 

 Cephalodiscus, which may be united in the class Ptero- 

 branchia. Very doubtfully allied is Phoronis (see Fig. 153). 



It will be useful here to take a general survey of the 

 Chordate phylum. 



First, there may be grouped together as tentative and 

 primitive Chordates — the transitional Hemichorda or 

 Enteropneusts, the mostly degenerate Urochorda or Tuni- 

 cates, and the pioneering Cephalochorda or Lancelets. 



Second, there are the peculiar and primitive jawless and 

 limbless Cyclostomes or Round Mouths. 



Third, there is the very successful class of Fishes, 

 including the Dipnoi which have lungs as well as gills. 



Fourth, there is the dwindling class of Amphibians. 



Fifth, at a much higher level, with foetal membranes and 

 no branchial respiration, is the heterogeneous class of 

 Reptiles. 



Sixth and seventh come Birds and Mammals. 



General Characters of Enteropneusta 



The worm-like body has three regions — a pre-oral '"pro- 

 boscis,'' a " collar " around and behind the mouth, and a 

 trunk, the anterior part of which bears gill-slits. A dorsal 



and in part tubular nerve-cord arises from the ectoderm along 



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