ROTIFERS. 



447 



The order Rhizota takes its name from its members being fixed when adult, and 

 usually inhabiting a gelatinous tube. The foot is not retractile and ends in an 

 adhesive disc or cup. In the flower-animalcules ( Floscularia), which may be found 

 everywhere in fresh 

 water adhering to weeds, 

 the edges of the wheel- 

 disc are produced into 

 distinct bristle - bearing 

 lobes ; but in the allied 

 Melicerta there is no such 

 production of the disc. 



The last order, 

 Scirtopoda, comprises 

 only the two genera 

 Pedalion and Hexar- 

 thra, each of which is 

 represented by a single 

 species. The two re- 

 semble each other and 

 differ from all other 

 rotifers in possessing 

 three pairs of limbs, 

 ending in a fan-shaped 

 tuft of setae. The body 

 is conical with a broad 

 square -.cut head, fur- 

 nished with two wreaths 

 of cilia and a pair of 

 conspicuous eyes. In 

 Hexarthra, which bears 

 a strong superficial re- 

 semblance to the Naup- 

 lius larva of some 

 crustaceans, the three 

 pairs of limbs spring 

 from the ventral surface, 

 the first pair being considerably the largest and the third the shortest ; but in 

 Pedalion they are arranged round the body in pairs, one limb projecting from the 

 middle of the back, another from the ventral middle line and two from each side, 

 the ventral limb being the largest of the six. By means of these appendages 

 the creatures are able to project themselves through the water in a series of 

 jerks. Pedalion has been discovered in various parts of England, and Hexarthra 

 in brackish water in Egypt. The male of Pedalion is a veritable dwarf as 

 compared with the female, the body and limbs being greatly reduced in size, and 

 the latter merely represented by three stumps, each of which terminates in a pair 

 of long bristles. 



flower-animalcule (magnified 200 times). 



