THE LAEVA OE TIPULA MAXIMA. 131 



Wall of "heart " (see PI. 26. figs. 20 a & 20 b).— Sections of the dorsal vessel show the 

 following layers : — 



1. A middle layer of circular striated muscle (a). This appears to form a continuous 

 layer throughout the contractile region, though it is generally described as interrupted 

 at intervals. 



2. An elastic fibrous layer (b) on the outer and inner sides of the muscle-layer. Very 

 large spindle-shaped nuclei (n.) occur at intervals in these layers. 



3. The surface ("cuticle ") of these last layers (<?) appears to be distinctly differentiated 

 from the inner portions, taking stains more deeply. Owing to contraction of the 

 dorsal vessel these outer layers generally appear raised into irregular ridges. On the 

 outside of the heart, and running in a longitudinal direction, a number of deeply staining 

 fibrils (/.) occur. These are branches of the tracheal system, but it is difficult to deter- 

 mine whether they possess any lumen or not ; they certainly appear to be solid. In 

 transverse sections these longitudinal tracheal threads appear to be supported at the 

 summits of outstanding projections of the outer (" cuticle ") layer, giving the appearance 

 of short pegs capped by a slight enlargement and deeply stained. Upon these the 

 pericardial cells {p.c.) generally rest, so that they appear to be supported by short props 

 outstanding from the wall of the heart. 



Kowalewsky describes the pericardial cells as being attached to the heart by short 

 muscular props. They do not, however, appear in this case to be muscular. 



The surface view of the wall of the heart shows fine cross-striations running obliquely 

 round the vessel. Two series of striations cross in opposite directions. These striatums 

 are not continuous throughout the length of the vessel, but occur interrupted at regular 

 intervals. This, perhaps, has some meaning with reference to the observation made 

 above that the muscle-layer is generally described as interrupted at regular intervals. 



The large binucleated pericardial cells (mentioned above, but described in detail below) 

 are irregularly arranged as a loose tissue on the outer surface of the heart, while a few 

 occur also on the inner side (PI. 26. fig. 20 b, p.c.'). They generally appear to be more 

 or less in longitudinal rows, and bound together partly by the tracheal threads. 



Valves. 



1. The intersegmental [interventricular) valves (PI. 26. fig. 15, v.i.) occur as paired 

 lateral thickenings of the inner fibrous layer of the heart, and project inwards and 

 slightly forwards, and are of such size that during the contraction of the vessel they 

 meet in the mid line. In structure each appears to be of fibrous substance — an enlarge- 

 ment of the inner fibrous intirna — with a pair of large nuclei at its base, and numerous 

 nucleated cells (which have the appearance of pericardial cells) embedded in or arranged 

 on the surface. 



There seem to be no muscles in connection with these valves, and from observation 

 of living animals they appear to work automatically, being forced slightly backwards, 

 and hence closing, during the contraction of the segment immediately in front. 



2. The ostial valves (PI. 26. figs. 15, v.o., & 16) occur immediately posterior to the 



