THE CCELOM AND DORSAL VESSEL. 89 



through the floating branches of a rich tracheal arborescence. 

 Thus the respirator}^ function appears to be localized in the 

 last segment, and there are few tracheae in other regions of 

 the body.' 



The paucity of tracheae in the young larva of the Blow-fly 

 was observed by Weismann, but in the adult larva they are 



Fig. ig. — Sections through the pericardial sinus, with the dorsal vessel in situ : 

 I, near the anterior, and 2, near the posterior extremity of the ventricle ; / s, 

 pericardial sinus ; f b, cells of fat body ; a vt, aire musculares ; is, ventral valve ; 

 ?/i, dorsal recti muscles ; /, intestine ; / c, cells of the pericardial septum ; 

 tr, trachea. 



abundant, a fact which does not, however, render the presence 

 of numerous tracheae in the pericardial sinus less interesting. 



The Pericardial Septum (Figs. 18 and 19) forms the floor of 

 the pericardial sinus. It consists of a double row of large 

 ovoid cells, 



Weismann [2] states that there are thirteen on each side of 



