PHYLUM ARTHROPODA 



249 



The blood-vascular system is, in comparison with the 

 other systems of organs, not very highly developed, the need 

 of an elaborate system of vessels being greatly diminished 

 by the thorough way in which all the organs are supplied 

 with oxygen by means of the 

 trachea?. The blood is colour- 

 less, or faintly yellowish or green- 

 ish. A contractile tubular heart 

 divided internally into a row of 

 eight chambers by a system of 

 valves extends through the abdo- 

 men on the dorsal aspect. 



The nervous system (Fig. 148) 

 is on the same general plan as in 

 the Crustacea. There is a double 

 supra - oesophageal ganglion or 

 brain (or), a sub-cesophageal 

 ganglion (in/), also double, and 

 a series of thoracic and abdomi- 

 nal pairs of ganglia, which are 

 closely united together in the 

 middle line. The brain is rela- 

 tively large in the higher insects, 

 and is divided into several lobes.. 

 It gives off nerves to the antenna? 

 and ocelli and the labrum, and 

 on each side it gives off a large 

 lobe, the optic ganglion, on which 

 the compound eye rests. A pair 

 of oesophageal connectives (conn) 

 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-cesophageal 



Fig. 148. — Periplaneta. 

 view of the nervous 

 abd. 6, sixth abdominal g 

 ant, antennary nerve ; by, brain ; 

 conn, oesophageal connective ; 

 iiif, sub-cesophageal ganglion ; 

 opt, optic nerve : thoy. /, thoy. 2, 

 thor. 3, first, second, and third 

 thoracic ganglia. (After Miall 

 and Denny.) 



