Classification 435 



INVERTEBRATE GROUPS OF UNCERTAIN POSITION 



Certain groups of invertebrates have not been assigned a definite 

 relation to other groups. Opinion differs so widely as to their affinities 

 that they may well be kept out of our regular classification for the 

 present. 



Mesozoa. Parasites apparently intermediate between the Protozoa 

 and Metazoa. Not improbably degenerate relatives of the flatworms. 



Nemertinea. Terrestrial, fresh water, and marine animals resem- 

 bling flatworms but with a proboscis, blood vascular system, and ali- 

 mentary canal with two openings. 



Nematomorpha. Long thread-like animals with the body cavity 

 lined with epithelium, a pharyngeal nerve ring and a single ventral nerve 

 cord. 



Acanthocephala. Parasitic worms with spiny proboscis, a complex 

 reproductive system and no alimentary canal. 



Chaetognatha. Marine invertebrates with a distinct coelom, alimen- 

 tary canal, nervous system and two eyes. 



Rotifera. Invertebrates with a head provided with cilia, usually a 

 cylindrical or conical body often with a shell-like covering and a bifur- 

 cated tail or foot provided with a cement gland. 



Bryozoa. Mostly colonial invertebrates resembling hydroids in 

 form, with distinct coelom, and with digestive tract bent in the form of 

 a letter U. 



Phoronidea. A single genus of worm-like animals having tentacles 

 and living in membranous tubes in the sand. 



Brachiopoda. Marine tentacles animals with a calcareous shell, 

 composed of two unequal shell-parts (commonly called valves), a dorsal 

 and a ventral. 



Gephyrea. Worm-like animals of doubtful affinities. 



