SECTION VIII. ROTIFERA, POLYZOA AND 

 BRACHIOPODA 



1, THE ROTIFERA 



A GROUP of Metazoa of microscopic size, the Rotifera r Wheel 

 Animalcules (phylum Trochelminth.es)> which are of exceedingly 

 common occurrence in fresh water, and are also found, though much 

 less abundantly, in the sea are readily mistaken on a superficial exami- 

 nation for Infusoria, on account not only of their minuteness and the 

 frequent general resemblance in shape to certain members of that class, 

 but also of the presence of cilia as organs of locomotion. A more 

 careful examination, however, shows that these minute creatures are 

 relatively highly organised multicellular animals, and reveal certain 

 general features of resemblance between them and the Trochosphere, 

 which is the characteristic larval form in a phylum the Anmilata to 

 be subsequently dealt with (Section IX.). 



The majority of the Rotifera are free-swimming. The cilia, by means 

 of which the swimming movements are effected, are confined to the 

 anterior or oral extremity of the body, and are borne on a very 

 characteristic organ termed the trochal disc (Fig. 94, tr.d.). This in its 

 simplest form is a disc with a prominent rim, fringed with strong cilia, 

 which surrounds the oral end. The mode of movement of the cilia is 

 such as to cause the trochal disc to assume the appearance of a rapidly 

 rotating wheel, and it is from this appearance that the name Rotifera 

 or Wheel-bearers is derived. Sometimes, however, the form of the 

 trochal disc is less simple, the disc with its circlet of cilia becoming 

 divided into lobes, or drawn out into long processes. Sometimes 



