X INTRODUCTION, 



valves ; each receiving blood from a pericardial cham- 

 ber by a pair of lateral valve-guarded orifices ; current 

 always forwards; intrinsic muscles produce contrac- 

 tion of chambers ; dilation by lateral muscles attached 

 to the walls of the body; from anterior chamber one 

 vessel runs into thorax, and divides into three branches, 

 which cannot be traced very far. The blood thus 

 poured into the head and thorax flows backwards 

 along the venter and sides of the body, and then to 

 the pericardial chamber. This circulation takes place 

 in the perivisceral cavity. The blood-corpuscles in 

 transparent larva: can be seen penetrating into all the 

 interstices of tissues, and every minutest branch of 

 the tracheal system is invested by a sheath prolonged 

 from the perivisceral cavity, in which blood circulates 

 on its way to the heart. (22, yo). 



Respiratory system.— T\\^ respiratory organs consist 

 of a system of air tubes, tracliece, branching to all 

 parts of the body, and opening externally by means 

 ot lateral apertures, spiracles, or stigmata. Take 

 notes of the following points,— The minute anatomy 

 of the tracheae ; the variations in number and position 

 of spiracles; structure of spiracles ; structure of bran- 

 chia: ; and other modifications of respiratory organs, 

 found in aquatic insects. 



Nervous system.— T\iQ typical form of the nervous 

 system of insects, as seen in larvae, consists of a series 

 of double ganglia, a pair for each segment, united by 

 two longitudinal cords. These ganglia, except the 

 cephalic, lie on the floor of the body cavity. The 

 cephalic ganglion is placed above the alimentary canal 

 and is proabably composed of several ganglia coalesced. 

 From the prothoraek ganglia, the longitudinal cords 



