172 THE HARMONIES OF NATURE. 



creation. We wonder at the immense variety of animals which 

 people the abounding land or the teeming waters ; but our 

 admiration at this exuberance of life rises to a still higher 

 point, when we consider that this visible world of animated 

 beings encloses another world of equally sensitive creatures, which 

 hidden from the eye lead a parasitic existence in their intes- 

 tinal recesses. Worms of the most various forms and sizes are 

 found within the bodies of all vertebrated animals, and even in 

 many molluscs and insects; and though above 1,400 species have 

 already been described, yet there can be no doubt that a far 

 greater number still remains undiscovered, as comparatively but 

 few quadrupeds, birds, or fishes have hitherto been accurately 

 dissected ; and as far as our observation reaches, every genus or 

 even every species of the vertebrate animals has been found to 

 possess its own peculiar parasitical worms, each of whom again 

 selects some favourite organ for his abode. 



If size alone were a criterion of classification, the Rotifera 



would have to be ranked 

 among the Infusoria, as 

 they are scarcely discern- 

 ible by the naked eye ; 

 but a more complicated 

 organisation separates 

 them widely from these 

 lowest members of the 

 animal kingdom, and as- 

 signs them a place among 

 the worms. Their most 

 striking external charac- 

 ter are the rotatory or- 

 gans or ciliary wheels 

 with which their head is 

 surmounted, and whose 



Ptygura melicerta. 



1. partially expanded, 2. completely expanded, the cilia Vibratory motions, whll'l- 

 in action causing currents indicated by the arrows, 3, . , -, , . 



contracted. ing the water about in 



swift circles or eddies, engulf in a fatal vortex the micro- 

 scopic animals or plants on which they feed, or enable them 

 to swim from place to place. The great transparency of these 

 curious little animals permits their general structure to be 

 easily recognised. The mouth lies between the wheels, and 



