PHYLUM CHORDATA 



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1. THE ADELOCHORDA 



Of somewhat doubtful relationships both to 

 one another and to the other Chordata are cer- 

 tain remarkable marine animals which have 

 been grouped together under the name of 

 Hemichorda or Adelochorda. These are 

 Balanoglossus and its allies, which occur in 

 shallow water on the coasts of most of the 

 warmer parts of the world, and two are deep- 

 sea animals, Rhabdopleura and Cephalodiscus. 



Balanoglossus (Fig. 195) is a soft-bodied, 

 cylindrical, worm-like animal, the surface of 

 which is uniformly ciliated. It is divisible into 

 three regions : in front there is a large, club- 

 shaped, hollow organ — the proboscis ; imme- 

 diately behind the proboscis and encircling its 

 base is a prominent fold — the collar; the 

 third region or trunk is long and nearly cylin- 

 drical, but somewhat depressed. 



Balanoglossus lives in the sea, burrowing 

 in sand or mud by means of its proboscis. It 

 occurs as far north as Salem, Mass., between 

 tide-marks. Numerous glands in the integu- 

 ment secrete a viscid matter to which grains 

 of sand adhere in such a way as to form a 

 fragile temporary tube. The proboscis (Fig. 

 195, pr, Fig. 196, prob) has muscular walls; 

 its cavity opens on the exterior usually by a 

 single minute aperture — the proboscis pore 

 (Fig. 196, prob. po) — rarely by two. The 

 collar (Fig. 195, co) is also muscular, and 

 contains one cavity or two (right and left) 

 separated from one another by dorsal and 

 ventral mesenteries, and completely cut off 

 from the proboscis. The collar cavity com- 

 municates with the exterior by a pair of collar 

 pores — -ciliated tubes leading into the first 

 gill-slit or first gill-pouch. 



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