THE PUPAL STAGE OF CULEX. 17 



longitudinal direction on the heart, and forming its outermost layer. 

 The heart is further bound by fibrous strands to the dorsal body 

 wall. 



Graber (10) appears to believe that the septum is invariably 

 attached to the outer wall of the abdomen, dividing the cavity of the 

 abdomen into two cavities, a small dorsal "pericardium" containing 

 only the heart and pericardial cells, and a large ventral cavity con- 

 taining all the other organs of the abdomen. His figure of Acridium 

 is reproduced in the most popular text-book (Claus), and his view 

 that this arrangement is universal, and that the " septum " serves as 

 a pump driving blood from the large abdominal cavity to the pericar- 

 dium^ is reproduced in other text-books, in such form as to lead to the 

 belief that the arrangement is the same in all insects. Whatever may 

 be the case in other insects, this view is certainly not applicable to 

 Culex. Here the " septum " does not extend to the body wall, and 

 if a " pericardium," in Graber's sense of the term, exist at all, the 

 extensor muscles and the main tracheal trunks lie in it, and the 

 septum cannot, judging from its anatomical relations, have the 

 function ascribed to it. 



The heart itself is a more or less cylindrical tube, about 0-06 mm. 

 in diameter. Its hinder end at the anterior limit of the eighth 

 segment is open, but I am unable to give an account of any valvular 

 apparatus which may be present here. There is no sharp division 

 into chambers either by constrictions or by valves. In the first 

 segment a pair of valved ostia opens backwards ; in segments III to 

 VII paired ostia are present, their margins being turned in and 

 directed fonvards to form the valves. I have not detected any 

 aperture or valve in the second segment. The ostia are small paired 

 slits in the sides of the heart, and between the two laminse of the 

 alse, putting the cavity of the heart in communication with the 

 " pericardial " cavity. The infolded margins of the slits serve as 

 valves in two ways ; firstly, they prevent the blood from flowing out 

 through the ostia ; and, secondly, they prevent the blood within the 

 heart from flowing backwards during systole. 



Of the histology of the heart I would speak with the greatest 

 caution. Graber [op. cit.) has made the subject his own, and has 

 applied very special methods to the investigation. My object has 

 been rather to record the anatomical structure and the development 



