20 



pulsatory dorsal vessel by its alternate contraction and dilatation; 

 certain portions appear also in many cases to enter the sides of the 

 vessels, just in advance of the valves, which aid the pulsating action 

 by allowing a free passage to the fluids only in a forward direction ; 

 in the larval state, the vascular walls of the pseudocardium are often 

 so slight as scarcely to be perceptible;. but they are distinct in the 

 perfect stage, and as the vessel contracts toward the head of the 

 larva its nature becomes more apparent. More than a century ago, 

 Lyonet showed that the dorsal vessel of Cossus has only a single out- 

 let for the passage of the fluids, namely, where it terminates with a 

 flaring opening at the extreme front of the head., just above the base 

 of the (psopliagus, into a cavity closed by membrane. The direct 

 passage uf the fluids thence has never been observed, but they must 

 in some way enter at once the peritrachean passages, since they 

 are next found circulating in them all over the body. Other writers 

 have desi^ribed the anterior extremity as branching, but in like man- 

 ner have not traced the passage of the fluids beyond the main trunk. 



Mr. Scudder observed that the only large tracheal vessel connect- 

 ing the two sides of the body, lay in the first segment behind the 

 head; and that, according to the recent and little known studies of 

 Barthelemy,^ this transverse tracheal conduit appears at the earliest 

 formation of the system in the embryo, before any regular distribu- 

 tion of fluids over the body can be observed ; from the middle line of 

 this transverse trachea lialf a dozen or more prominent branches 

 pass directly forward, and some at least of their branches penetrate 

 the cavity into which the dorsal vessel empties its contents; it is 

 therefore highly probable that through them the fluids enter the 

 peritracheal system ; for by this channel the fluids would pass in the 

 easiest possible manner to every part of the body, from the moment 

 that the pulsations of the embryo commence. This, therefore, is the 

 principal point toward which it is desirable tliatr future investigation 

 should be directed ; and especially the structure of the tracheal 

 threads which enter this cavity should be minutely studied. 



The tracheae, as shown by Dufour, Blanchard, Agassiz, Williams, 

 Kunckel and others, consist of two entirely distinct parts ; but 

 though these authors do not wholly agree in their explanation of the 

 structure of the trachea?, there appears to be lit,tle doubt that it is 

 substantially as follows: first, a main stem and its branches, in which, 



1 R«5cherches d'Auatomie et de Physiologic g<5ii6rales sur la classe de Lepidop- 

 teres. 4°. Toulouse, 186-1. 



