INTRODUCTION. XXXV 
mostly at fixst out of the reach of the stronger currents. 
As the vitality of the animal diminishes, the arterial 
current is observed to lessen in force, until it is propelled 
only by jerks, coexistent with every pulsation of the 
heart. 
RESPIRATION. 
The organs of respiration in the Isopoda are homo- 
logically distinct from those of the Amphipoda. We have 
already stated that Professor Wagner has shown, in the 
genus Porcellio, and M. Kowalewsky in Jdotea, that the 
blood in the Jsopoda runs in arterial channels. We are 

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Fic. 12. 
not aware that any of the Amphipoda have been put to the 
same test as the two genera named in the Jsopoda; and 
certainly, to microscopic observation, the structure of the 
c2 
