CHAPTER IX 



INVERTEBRATES OF MORE OR LESS UNCERTAIN 

 SYSTEMATIC POSITION 





There are a number of groups of animals whose relationships 

 are so difficult to determine that authorities do not agree as 

 regards their position in the animal series. Most of these groups 

 contain only a few marine species which are of very little 

 economic importance. A few groups like the Rottfera and 

 Bryozoa include fresh-water species which are quite common. 



i. Mesozoa 



The term Mesozoa (Gr. mesos, middle; zoon, animal) has been 

 employed by a number of zoologists to include three families of 



parasites of obscure systematic position, 



(i) theDiCYEMiD^:, 

 (2) the Orthonec- 

 tid,e, and (3) the 

 Heterocyemid^e. 

 They have been 

 regarded as inter- 

 mediate between 

 the Protozoa and 

 Metazoa, hence 

 the name Mesozoa. 

 It is probable, how- 

 ever, that they are 

 degenerate Meta- 

 zoa closely allied to 

 the flatworms. 



%r\ 



Fig. 114. — A Meso 

 zoon, Dicyema para- 

 doxum. (From Parker 

 and Haswell, after Kol- 

 liker.) 



Fig. 115. — A Meso- 

 zoon, Rhopalura giardii, 

 male. (From Sedgwick, 

 after v. Beneden.) 



176 



