94 INTRODUCTION. 



thors "with a different practical application, considera- 

 ble confusion prevails in this part of tlie classification. 

 Many of them being imperfectly known, their places 

 must remain uncertain and subject to change, until 

 more accurate knowledge of their structure shall be 

 obtained; and it wUl not be until all of them have 

 been investigated, that any system can be proposed, 

 which, so far as these sub-divisions are concerned, wiU 

 have any chance of permanence. The following ar- 

 rangement, used in the sense of the authors whose 

 names are appended, is therefore to be considered as 

 merely a provisional one, to be varied from time to 

 time, and possibly to be entirely abandoned, by sub- 

 stituting another founded upon a different organ or 

 organs, and expressing, perhaps, more correctly, their 

 natural relations. Recent observations render it cer- 

 tain, indeed, that much error prevails relative to the 

 mode of respiration, and to the organs by which it is 

 performed in tliis class. The number of orders might 

 be, even now, reduced by combining two or more 

 together. 



ORDERS. 



1. Pneumobranchiata. Lamarck, and Gray. Organs of res- 

 piration consisting of branchial vessels spread like a net- 

 work over the internal parietes of a cavity in the anterior 

 part of the body, communicating with the air by a small 

 orifice on the right side. Respiring air. 



