VASCULAR SYSTEM OF LARVA 14:3 



On the outer side of the tracheal trunk a hii-ge branch arises; this, 

 the visceral branch (v.tr.), bends ventrally to the lateral trunk, aii<l 

 thus becoming internal to it enters the coinolutcd visceral mass 

 with its fellow of the other side. 



The visceral branches extend anteriorlj^ as far as the seventh 

 segment. In the twelfth and thirteenth segments the lateral 

 tracheal trunk has a double appearance. A dorsal aod a ventral 

 branch arise in most of the segments, the dorsal branch chiefly 

 supplies the fat-body, and the ventral branch supplies the viscera ; 

 both give off branches to the muscular body-wall. The anterior 

 commissure (a.com.) is situated in the fourth segment. It crosses 

 the oesophagus immediately behind the pharyngeal mass. On the 

 internal side of tlie portion of the lateral tracheal trunk, that is 

 anterior to the commissure, a branch arises, and iiuining ventral 

 to the pharyngeal mass it supplies the anterior end of the larva 

 and the oral lobes. A branch that supplies the muscles of this 

 region is given off external to the origin of the anterior commissure. 

 Internal to the origin of the commissure two tracheae arise ; the 

 anterior branch enters and supplies the pharyngeal mass, and the 

 posterior branch (tr'.) enters the ganglion ventral to the cerebral 

 lobes. In the fifth segment another internal tracheal branch (tr".) 

 enters the gajiglion. These tracheae which supply the ganglion 

 appear to run chiefly in the peripheral regions, where they divide 

 into a number of branches, the fate of some of these being inter- 

 esting. These branches are extremely fine, and they arise, as I 

 have previously mentioned, in association with a number of the 

 segmental nerves with which they run to the body-wall. 



The Vascular System and Body Cavity. 



The relations and structure of the vascular system of the larva 

 are on the whole similar to those of the fly ; there are, however, a 

 number of modifications. 



The dorsal vessel, which includes the so-called " heart," is a 

 simple muscular tube lying on the dorsal side immediately beneath 

 the skin, and extending from the posterior tracheal commissure to 

 the level of the cerebral lobes of the compound ganglion in the 

 fifth segment. Its wall is composed of fine striated muscle-fibres 

 arranged transversely and longitudinally, but chiefly in the latter 



