62 C. HERBERT HURST. 



of the pupa, and I simply note histological results incidentally, 

 referring those who wish to learn the histological structure of insect 

 hearts to the classical work just mentioned. 



Without the application of special methods, I have recognised 

 three layers in the wall of the heart. 



The inmost layer, or endocardium, is an exceedingly thin layer 

 of flat cells. Their nuclei are conspicuous objects, occurring with 

 striking regularity in pairs, four pairs to each segment of the 

 abdomen, and a similar but smaller nucleus is to be seen in each 

 flap of each valve, from which I conclude that this endocardium 

 extends also to the valves. Whether the other layers also extend 

 into the valves or not, I cannot say with certainty. 



The middle layer consists of encircling fibres, slightly oblique in 

 direction, and probably muscular. 



The outer layer is also fibrous, its fibres being on the whole longi- 

 tudinal in direction, but they curve outwards to be continuous with 

 the fibres of the dorsal laminse of the alse. 



Between the laminse of the alse cordis, that is in the pericardial 

 cavity, are large ovoid masses of brown cells, the "pericardial cells!' 

 Of these masses there are two pairs near the anterior, and two near 

 the posterior end of each segment of the abdomen ; but the number 

 increases towards the end of the pupal stage, and still further in the 

 imago, by the division of some of them into two or more masses. 

 The protoplasm of these cells is extraordinarily spongy, and contains 

 numerous granules, which stain deeply with borax carmine. The 

 nuclei vary in number from three or four to ten in each mass, 

 though the boundaries of so manj r cells cannot be made out. The 

 cells appear to be undergoing division very slowly. The excretory 

 function of these cells has recently been shown by Kowalevsky (12). 



The aorta runs from the ventral border of the anterior end of the 

 heart forwards above the stomach and oesophagus to the head, where 

 it ends, the end being open. In transverse sections of the thorax, 

 the aorta is seen as a laterally compressed tube. I have not seen 

 any branches given off" from it. 



The Respiratory System. 



Culex, as already mentioned, breathes air in all three states — 

 larva, pupa, and imago — and also breathes it directly, but the air is 



