94 INTRODUCTION 
thors with a different practical application, considera- 
ble confusion prevails in this part of the 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 will not be until all of them have 
been investigated, that any system can be proposed, 
which, so far as these sub-divisions are concerned, will 
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. Kecent 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 this 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. 
