POST-EMBRYONIC DEVELOPMENT OF CULEX. 181 
other insects and generally regarded as universal. (See 
for instance Claus’s Text-book of Zoology, where a figure 
of Acridium is given and spoken of as if the arrangement 
there shown were universally true of other insects.) 
The heart consists of three layers. ‘The innermost, the 
endocardium, is a single layer of exceedingly thin flat cells, 
easily recognised by their large nuclei of which four pairs 
are seen in each abdominal segment; similar but smaller 
nuclei are seen, one in each of the two flaps of each valve. 
The middle layer consists of fibres encircling the heart, 
what Weismann (op. cit., pl. xu. fig. 50) mistook in 
Musca for the striations of the single hollow striped 
muscle-cell of which he believed the heart to consist. The 
outer layer also consists of fibres, but their main direction 
is longitudinal, and they curve outwards on the dorsal 
surface to be continuous with the fibres of the ale. 
The ale as seen from above, are triangular sheets runn- 
ing from the sides of the heart outwards: there are four 
pairs in each segment. In transverse sections they are 
seen to be double, the fibres of the upper sheet being con- 
tinuous with those of the outer layer of the heart. The 
fibres of the lower sheet are in part directly continuous 
immediately beneath the heart, with those of the ale of the 
other side, while in part they are attached to the heart 
itself. ‘he two sheets extend from the heart outwards 
and downwards, and after uniting are attached to the 
‘‘yeritoneal”’ investment of the stomach and of the main 
tracheal trunks. They are certainly not attached to the 
body-wall, and the tracheal trunks certainly he aove them. 
I believe the fibres attaching them are muscular. 
In the cavity between the dorsal and the ventral lamine ~ 
of the alzw are the large “ pericardial cells” which Kowa- 
levsky (8) regards as excretory. ‘They are arranged in 
masses at the sides of the heart, four pairs of masses to 
