Animals and the Elements in which they Live. 209 



locomotive appendages may be, and in which the foremost 

 joints hardly ever assume a peculiar structure vi^ith the ap- 

 pearance of a head. This class for which the name of 

 worms is best retained, will contain the Helminths and An- 

 nelides, exclusive, however, of the vermiform parasitic 

 Crustacea, which embryology has taught us to refer unhesi- 

 tatingly to the class of Crustacea. The extraordinary di- 

 versity which exists among these animals renders it rather 

 difficult to subdivide them into natural groups, and to assign 

 to these a natural succession agreeing with the gradation 

 of their structure, as there are so many the development of 

 which is as yet very imperfectly known, and others which 

 undergo so complicated metamorphoses as to leave great 

 doubt respecting their natural relations to each other. How- 

 eveP'i there can be no doubt that Helminths rank lower than 

 Annelides, for their structure indicates plainly their infe- 

 riority, and their mode of existence within other animals 

 shews that they do not even reach that degree of independ- 

 ence which might allow them a free existence. 



Among Annelides again, there will arise similar diffi- 

 culties respecting the relative position of the branchiate 

 types of that group, which are provided with external ap- 

 pendages performing simultaneously the functions of respir- 

 atory and locomotive organs, and those families which are 

 deprived of external appendages, or which have stiff bristles 

 upon their joints, independent of their aerial respiratory 

 organs. Indeed, at present the position of earth worms and 

 leeches among Annelides, has not been the subject of any 

 direct investigation, as regards their relative position and 

 rank. But if I were allowed to be guided by the impressions 

 I have received from the study and comparison of the larvae 

 of insects, I should be inclined to consider the Annelides with 

 Qxternal gills, as inferior to those which have no such appen- 

 dages, and place the lumbricine Annelides highest in the 

 class. So that Helminthus should be placed lowest in the 

 class of worms ; next the branchiate Annelides with external 

 branchiae ; next those having internal branchiae ; and highest 

 those with aerial respiratory sacks. 



The second class in the type of Articulata is that of Crus- 



VOL. XLIX. NO. XCVIII. — OCTOBER 1850. 



