INTRODUCTION. 



XXXV 



mostly at first 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 Idotea, that the 

 blood in the Isopoda runs in arterial channels. We are 



FIG. 12. 



not aware that any of the Amphipoda have been put to the 

 same test as the two genera named in the Isopoda ; and 

 certainly, to microscopic observation, the structure of the 



