SECTION VIII. — ROTIFERA, POLYZOA 

 AND BRACHIOPODA 



1. THE ROTIFERA 



A group of Metazoa of microscopic size, the Rotifera or Wheel 

 Animalcules, 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 examination for Infusoria on account 

 not only of their minuteness and the general resemblance in shape of 

 many of them to certain members of that class, but of the presence of 

 cilia as organs of locomotion. A more careful examination, however, 

 shows that these minute creatures are relatively highly organised multi- 

 cellular animals, and reveals certain general features of resemblance 

 between them and the Trochosphere, which is the characteristic larval 

 form in a phylum to be subsequently dealt with, the Annulata (Section 

 IX.). 



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

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

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

 characteristic organ termed the trochal disc (Fig. ioo, tr. d). This is in 

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

 and 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 to this circumstance that the name Rotifera or 

 Wheel-bearer is applied to the group. Sometimes, however, the form 

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

 ing divided into lobes, or drawn out into long processes. In some 

 forms ciliated prominences are present within the circlet of cilia, and 

 in others there is a second circlet internal to the first. 



The body is usually distinguishable into the trunk and the tail (/). 

 The latter, which is situated at the extremity of the body most remote 



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