CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 99 



there are general similarities in the external configuration 

 of the hody. Thus in Scorpions the arrangement of 

 the nervous system is not very dissimilar from that met 

 with in the Prawn and its allies. The thoracic ganglia 

 have coalesced with one another and with the anterior 

 abdominal ganglia ; thereby forming a large stellate ner- 

 vous mass which supplies the limbs and the anterior part of 

 the abdomen. The ventral cord throughout the remainder 

 of the abdomen, and its caudal prolongation, is marked at 

 intervals by a series of small ganglionic swellings. 



In Spiders proper, the nervous system attains its maxi- 

 mum amount of concentration. The bilobed brain (fig. 37, c) 

 receives nerves on each side (o), corresponding in number 

 with the ocelli which the animal may happen to possess. 



FIG. 37. Head and Nervous System of a Spider (Mygale). (Owen after Duges.) 

 c, Cerebral ganglia* (side view), receiving (o) optic nerves, and (m) nerves (sensory and 

 motor) from the powerful mandibles, m'. The cerebral ganglia are connected by 

 very short cesophageal cords with a large stellate ventral ganglion (s), from which 

 five large nerves issue on each side (p, I, I); a, mouth; b, oesophagus ; d, stomach. 



It also receives two large nerves (m), which probably con- 

 tain outgoing as well as ingoing fibres, from the so-called 

 mandibles (m). 



Owing to the suctorial habits of these fierce and 

 predatory creatures, the oesophagus is very narrow ; and 

 as a consequence, the oesophageal cords are very short, so 

 that the brain is unlike the arrangement which obtains 



H 2 



