58 INTRODUCTION. 



on the contrary, one is found in an entire family, 

 comprehending nearly three hundred species, which 

 have been collected in our Museums and examined 

 by Naturalists, and not one of which has more than 

 three opercular bones instead of four : and this cha- 

 racter is constant, whatever may be the variations 

 of the other parts, which, without entering into 

 particulars, are as great and unexpected here, as 

 those which are found in the other families of the 



LYMPHATIC HEARTS. 



A lymphatic heart having, in the year 1831, been 

 discovered by that able physician and philosopher, 

 Dr. Marshall Hall, in an Eel, though at the time 

 he was not aware either of its nature or function, 

 and many of these organs being now known to occur 

 in the class of reptiles, and their existence being 

 moreover anticipated even in birds and mammals, 

 we shall here say a few words regarding them. 



In all living beings, besides the very necessary 

 process of the ingestion and absorption of aliment, it 

 is now very generally understood that there is a di- 

 rectly contrary, or. at all events, a very distinct 

 operation going forward, whereby the effete mat- 

 ter of the system is unceasingly withdrawn and 

 discharged from the body by a process which is 

 designated absorption, and interstitial absorption, 

 and which is unremittingly operating in every part 

 and tissue of the living frame. In invertebrate 



