PHYLUM ARTHROPODA 



643 



dorsal vessel or heart (Fig. 522) extends through the abdomen— and 

 sometimes thorax— immediately below the terga. Its cavity is 

 divided internally into a series usually of eight chambers by 

 a system of valves. In its walls are a series of slits or ostia, by 

 which a communication is effected between the internal cavity and 

 a surrounding loericardial sinus. In front the heart gives origin to 

 a main vessel, or aorta («), by means of which the blood is 

 conveyed throughout the body to enter a system of sinuses in free 

 communication with the general body-cavity, from the various 

 parts of which it finds its way back to the pericardial sinus. 



The nervous system (Figs. 520 and 523) is on the same general 

 plan as in the Crustacea. There is a double su^n-a-oisophageal 



Fig. 521. — Thorax and anterior abdoiniiial segmentd 

 of a larval Ephemerid with tracheal gills. 

 HF, hind wings ; (ri, (?-2, tr'^, tracheal gills ; tl, 

 longitudinal tracheal trunks ; VF, fore wings. 

 (From Lang's CoitLiiarative Anatomy.) 



Fig. bt-l. — Heart of Cockchafer 

 (JHelOlontlia). a aorta; 

 m, rii, alary muscles. (From 

 Gegenbaur.) 



ganglion or train, a suh-CBSopheigcal ganglion, also double, and a series 

 oi thoracic a,nd abdominal pairs of ganglia, which are closely united 

 together in the middle line. The brain is relatively large in the 

 higher Insects, and is divided into several lobes. It gives off nerves 

 to the antennae, the ocelli and the labrum, and on each side arises 

 a large lobe — the optic ganglion — on which the compound eye 

 rests. A pair of cesopheigeal connectives pass backwards on either 

 side of the mouth from the brain to the sub-oesophageal ganglia. 

 These connectives are very short, and, as a consequence, the brain 

 and sub-oesophageal ganglia are closely approximated. From the 

 latter there originate nerves to the appendages of the mouth — the 

 mandibles and the two pairs of maxillae. There are sometimes three 

 pairs of thoracic, and as many as eight of abdominal ganglia in 

 the adult insect; but in many cases there is a greater or less 



