166 M. Agassiz o?i the Relations bettveen Animals 



their mode of existence within other animals shows that they do 

 not even reach that degree of independence which might allow 

 them a free existence. 



Among the Annelides, again, there will arise similar difficulties 

 respecting the relative position of the branchiate types of that 

 group which are provided with external appendages, performing 

 simultaneously the functions of respiratory and locomotive or- 

 gans, and those families which are deprived of external appen- 

 dages, or which have stiff bristles upon their joints, independent 

 of their aerial respiratory organs. Indeed at present the position 

 of earth worms and leeches among the Annelides has not been 

 the subject of any direct investigation as regards their relative 

 position and rank. But if I were allowed to be guided by the 

 impressions I have received from the study and comparison of 

 the larvfe of insects, I should be inclined to consider the Anne- 

 lides with external gills as inferior to those which have no such 

 appendages, and place the lumbricine Annelides highest in the 

 class. So that the Helminths should be placed lowest in the class 

 of worms ; next the Branchiate Annelides with external branchise ; 

 next those having internal branchise, and highest those with aerial 

 respiratory sacs. 



The second class in the type of Articulata is that of the Crus- 

 tacea, the natural circumscription of which can hardly be in any 

 degree a matter of doubt, for these animals, with their distinct 

 articulations and aquatic mode of respiration, external appen- 

 dages and particular mode of combination of the rings of their 

 body, wherever they are combined to subdivide the body into 

 distinct regions, are so peculiar as to determine well the natural 

 limits of this class, to which we refer also the Cirripeda, notwith- 

 standing their transformations, also the Lerneean parasites, though 

 they may assume in their parasitic mode of existence so extrava- 

 gant forms, and an appearance so entirely different from that of 

 common Crustacea. In this class, again, the parasitic vermiform 

 types rank lowest ; next follow the Entomostraca, and highest the 

 Malacostraca, in most of which the anterior rings are combined 

 into a distinct region, assuming a peculiar appearance differing 

 widely from the posterior free moveable rings. The circumstance 

 that among Crustacea the organization reaches a point where the 

 anterior part of the body assumes so peculiar an appearance, leaves 

 no doubt as to the relative position of the Crustacea among the 

 Articulata ; they rank higher than worms ; though they must be 

 placed below the insects, notwithstanding their perfect circulation 

 and their otherwise highly developed structure ; for, in every re- 

 spect, insects considered as a whole class, are more highly orga- 

 nized, their higher types assuming a division of the body into 

 three distinct regions ; — undergoing also far more extensive me- 



