234 ZOOLOGY sect, iv 



ventral surface is flattened, and is covered with fine vibratile cilia, while on the 

 dorsal surface and the sides are regularly arranged rows of straight sette (non- 

 motile cilia). The internal cavity (enteron) is found to contain sand, plant 



Fig. 181. — Salinella, transverse section. (After Frenzol.) 



fragments, and Bacteria ; its surface is beset with long cilia. Multiplication is 

 said to take place by transverse fission ; and a process of conjugation followed 

 by encystation has also been observed. 



Trichoplax and Treptoplax, which have been supposed to be Mesozoa, appear 

 to be merely special modifications of developmental phases of Hydrozoa. 



