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CLASS II. ANNELIDA. 



IT is now universally acknowledged among 

 naturalists, that the progress of development in 

 the forms and structure of animals cannot be re- 

 presented by an unbroken linear series, though, 

 from the nature of a book, we are compelled so to 

 treat of them in our descriptive works of zoology. 

 The great Class of Worms is an example in point. 

 At the foot of its scale it manifestly merges into 

 the TURBELLARIA ; and, indeed, the distinction 

 between some of the Leeches and the Planarise is 

 almost too delicate to be traced, while the Nemer- 

 tina occupy a confessedly disputable border terri- 

 tory. But the TURBELLARIA lead us into the 

 INFUSORIA, the simplest forms of animal being ; and 

 thus there is a well-trodden highway from those 

 minute and almost structureless creatures, to the 

 active, highly-endowed Nereis and Aphrodita. 

 There is, however, another pathway by which we 

 may leave the inferior forms of the ANNELIDA, 

 passing into the domains of the ECHINODERMATA 

 by those very worm-like creatures the Sipunculidce, 

 which are no less debateable than are the Nemer- 

 tina. Thus we might descend to the Polypes, 

 and through them to the PROTOZOA by another 

 route. The same thing might be predicated of 

 the higher extremity of the scale, and it would be 

 easy to show that the ANNELIDA have manifest links 

 of connexion with the ROTIFERA, with the MYRIA- 

 PODA and Insects, with the MOLLUSOA (through 



