7io 



THE POPULAR SCIENCE MONTHLY. 



Among Arachnida forms of the nervous system exist which agree 

 in many respects with those belonging to members of the class last 

 described these resemblances being in the main associated with cer- 

 tain general similarities of external form or configuration of body. 

 Thus in scorpions the arrangement of the nervous system is not very 

 dissimilar from that belonging to the prawn and its allies, since the 

 thoracic ganglia have coalesced with one another and with the ante- 

 rior abdominal ganglia, so as to form a large stellate nervous 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 maximum of con- 

 centration. In addition to the abdominal and thoracic ganglia having 

 all fused into one another and with the sub-cesophageal ganglion, we 



Fro. 4 Nervous System op a 

 Ckab (Paitnurus). 



F:o. 5. Head and Nervous System of a Spider 

 {MygaU ). 



find the large mass thus composed (Fig. 5, s) brought into extremely 

 close relation with the cerebral ganglia or brain (c). They are con- 

 nected by means of two stout commissures, one on each side of the 

 very narrow oesophagus, whose small size is attributable to the sucto- 

 rial habits of these carnivorous and predatory creatures. The cap- 

 tured fly is not eaten, its juices are sucked by the fierce spider by 

 whom its life has been taken. 



The bilobed brain of the spider receives nerves on each side (o), 

 corresponding in number with the ocelli which the animal may pos- 

 sess. It also receives two large nerves (m) from the so-called mandi- 

 bles, which are organs presumably developed from modified antennas. 

 These large nerves probably contain outgoing as well as ingoing fibres. 



