SECTION VII 

 HEMICHORDA 



Animals included in the group ; structure and habits of Enteropneusta, their 

 development and distribution ; incipient species ; structure of Cephalodiscus and 

 Rhabdopleura. 



THERE is a widespread idea that the various big divi- 

 sions or phyla of the animal kingdom stand apart 

 from one another with sharply-defined boundaries. 

 This is rarely the case, and it is certainly not true as regards 

 Vertebrate or (more widely) Chordate animals. Although the 

 actual pedigree of Vertebrates remains quite uncertain there is 

 no doubt that the Vertebrate phylum is approached by a number 

 of different types. Some of these are included under the title 

 Hemichorda, a somewhat question-begging title that suggests 

 a position on the border-line of the Chordata. 



(i) Among these Hemichorda, those with clearest Chordate 

 affinities are included in the class Enteropneusta (literally gut- 

 breathers), represented by Balanoglossus, Ptychodera, and other 

 genera. (2) Perhaps allied to these are two peculiar types — 

 Rhabdopleura and Cephalodiscus, which may be united in the 

 class Pterobranchia. (3) Still more doubtfully in this alliance 

 is an interesting animal called Phoronis, which almost requires 

 a class for itself. 



Enteropneusts (Plate XXXVI., D) are soft, opaque, worm- 

 like animals, found beneath stones or burrowing in sand and 

 mud in almost all seas, both in shallow and deep water. 

 Uninitiated observers would call them " worms," though the 

 discoverer of the first one (Eschscholtz in 1825) called it a 

 Holothurian, but they are not like other "worms," except in 

 the most general way, e.g., in shape and burrowing habits. 

 They have no distinct segments like an earth-worm, no gills 



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