290 RAMBLES OF A NATURALIST. 



gressive perfection of organisms. Like the ap- 

 paratus of the circulation for the blood, of which we 

 have already spoken, the apparatus for the lymphatic 

 circulation first shows itself to be very incomplete ; 

 and, if we were to follow it in its transformations, 

 we should find that it was not completely isolated, 

 nor definitely constituted as such, until after we have 

 passed through the lowest group of the sub-kingdom 

 of the Yertebrata, namely, the class of Fishes. 



Thus, by the presence of lateral appendages, the 

 Branchellion deviates from all the Hirudineae, while 

 by the respiratory action of these appendages, it 

 differs not only from the group in which it has been 

 placed, but also from all the neighbouring groups. 

 Finally, the identification of these respiratory organs 

 as lymphatic branchiae completes the thoroughly ex- 

 ceptional character of the animal. 



If indeed the principle of dominant characters were 

 as true as Cuvier supposed it to be, and as some 

 anatomists still assert ; if the slightest modification 

 in an apparatus destined for the accomplishment of 

 one important function really exercises over all the 

 rest of the organism the influence that is attributed 

 to it ; an anatomical examination of the digestive, 

 vascular and nervous systems ought to exhibit dis- 

 positions not less novel. Such, however, do not exist 

 in this instance. 



There can be no doubt that there are differences 

 between the Branchellion and ordinary leeches, but 

 these differences are not of a very important kind. 

 The greater number of them do not exceed in im- 

 portance those which we meet with among the most 



