IS 14.] On the Uses of the Dorsal Vessel 347 



than that which has for its object to determine the influence which 

 the heart exercises on their organization, and the changes which 

 that organization undergoes when the heart, or organ of circula- 

 tion, comes to he wanting; but, to determine with precision this 

 double influence of the presence or absence of the heart, we must 

 attend to those animals in which we see the transition from those 

 which have a heart to those which have only vestiges of it; and 

 likewise to the corresponding transition from those which breathe by 

 circumscribed organs to those in which the trachea? distribute air 

 over every part; Articulated animals being the only ones that offer 

 these two svstems of organization, to them alone we have directed 

 our researches. When we study in a general manner the organiza- 

 tion of articulated animals, we soon perceive how many modifica- 

 tions their heart undergoes before it is replaced by the organ known 

 in insects by the name of the dorsal vessel, which has nothing in 

 common with a heart but its position. Thus in the anelides the 

 heart is in some respects double; at least we do not perceive a single 

 organ of circulation, but only a spelling in the two principal 

 vessels, which, situated the one below the other, produce the circu- 

 lation. The elongated form of these vessels shews that the heart will 

 speedily undergo greater modifications. If these modifications are 

 not yet very sensible in the crustaceous decapodes in which the 

 heart is rounded, they become very apparent in the crustaceous 

 braehiopode.s, in which that organ assumes an elongated figure little 

 different from the dorsal vessel in insects. In short, as the organi- 

 zation simplifies, the heart assumes a greater analogy with the 

 dorsal vessel, both in its form and position, as is observable in the 

 organ of circulation of the arachnides. The heart is reduced to a 

 simple longitudinal vessel, which scarcely exhibits any sensible 

 swelling. The first, of the articulated animals, the arachnides, 

 exhibit respiratory organs, which receive air by stigmatiform open- 

 ings placed on their external face. These organs become more 

 numerous, though they are circumscribed. This ought to be the 

 case, because the blood in these animals, having a true circulation, 

 is contained in particular vessels which carry it to the respiratory 

 organs, where it receives air. 



The case is very different in insects. The respiratory organs, 

 instead of being circumscribed, arc formed of elastic tracheae, 

 which distribute air over every part. In them there is no heart 

 whatever, ami the blood is no longer contained in particular canals. 

 ifnple vessel without vascular ramifications occupies the place of 

 a heart, of which it may be considered as a vestige, or, if the idea 

 i-> preferred, which it replaces ; but its uses are no longer the same, 

 and have nothing analogous. 



It is the functions of this vessel that we must determine. This is 

 one of the objects to which we have directed our chief attention. 

 W < hat varied all the experiments respecting it which it was pos- 

 sible 10 make. This has enabled us to throw some fight on one of 

 the most difficult parts of the anatomy of articulated animals, 



