ROTIFERA. 



119 



a delicate transparent envelope of considerable consistency, often 

 terminating at the upper extremity in wavy indentations or tooth- 

 like processes, as in Brachionus urceolaris* (Jig. 49, c, c ). This 

 harder integument is essentially analogous to the cell of a Bryo- 

 zoon, but in this case is so constructed as to allow the animals to 

 move at large in the element they inhabit, instead of being per- 

 manently fixed to the same locality. Continuous with the free 

 margin of the shell is a delicate membrane connecting it with the 

 bases of the cilia-bearing lobes around the mouth, so as to allow 

 those organs, when not in use, to be retracted within the cell by a 

 mechanism resembling that provided in Bowerbankia for the re- 

 traction of the tentacula. 



To the posterior extremity of the body is generally appended a 

 pair of forceps composed of two moveable pieces (Jigs. 50 and 

 51), used as anchors or instruments of prehension; and by 

 means of these the little creatures fix themselves to the confervse 

 or aquatic plants amongst which they are usually found. In 

 Brachionus urceolaris the prehensile forceps (fig. 49, o p,) is at- 

 tached to the extremity of 

 a long flexible tail in which 

 the muscular fibres des- 

 tined for its motions are 

 distinctly visible. 



(156.) The cilia, whose 

 action produces the ap- 

 pearance of wheels turn- 

 ing upon the anterior part 

 of the body, are variously 

 disposed, and from their 

 arrangement Ehrenberg 

 has derived the characters 

 whereon he bases the di- 

 vision of the class into 

 orders. The peculiar 

 movements excited by the 

 vibration of these organs, 

 was long a puzzle to the 

 earlier microscopic observers, who, imagining them to be really 

 wheels turning round with great velocity, were utterly unable to 



* The engravings of the Rotifera are all copied from Ehrenberg's papers. Abhand- 

 lungen der Koniglichen Akademie der Wissenchaften zu Berlin, for 1833. 



