NERVOUS SYSTEM GENERATIVE ORGANS 779 



the pro- and meso-soma, occurs again in spiders, and is much 

 more highly developed in the Arachnida than in Apus or in the 

 Decapoda, where only an insignificant plate occurs. Numerous 

 muscles are attached to the endosternite. 



The nervous system consists of a supra-oesophageal ganglion 

 which supplies the eyes and the chelicerae, that is the appen- 

 dages (if the eyes are appendages) of the preoral segments, 

 the two prosthomeres of Lankester's terminology. The nerve 

 ganglia of the postoral segments have in the Acarina, the Pedi- 

 palpi, the Solifugae and the Araneina fused into a central mass, 

 though in the last named group the nerve which runs posteriorly 

 to supply the abdomen bears a ganglion close to the base of 

 the latter. In Limulus the circumoesophageal collar is very 

 large and gives off nerves to the pedipalps and the remaining 

 prosomatic appendages as well as to the genital opercula. The 

 right and left halves of this collar are connected by a ladder-like 

 series of transverse commissures ; behind the collar the ventral 

 cord gives off five nerves to the gill-books and is continued 

 back to supply the metasoma and spine. In scorpions the 

 nervous system is less concentrated and the ventral cord has 

 ;seven discrete ganglia the first of which corresponds with the 

 segment bearing the 3rd lung-book and the last with the 4th 

 metasomatic segment. The nerves to the chelicerae and re- 

 maining appendages, up to and including the 2nd lung-book, 

 arise from the collar or from the sub-oesophageal ganglion. 

 The right and left halves of the ventral cord remain distinct 

 throughout its course. 



The chief sense organs are eyes which vary in number from 

 two, in Acarines and Tardigrades, to twelve. Their grouping 

 is often of systematic importance. Some mites are blind. 

 Auditory organs, which may be presumed to exist in those 

 Arachnids such as the scorpions and spiders which have stridu- 

 lating organs, are apparently represented by sensory hairs usually 

 on the pedipalps or on the walking limbs. 



Arachnids are dioecious, but as a rule there is little external 

 differentiation of sex except amongst the spiders, where the male 

 is often much smaller than the female, and amongst some mites. 

 The Phalangids possess a large extrusible ovipositor and penis 

 and in the spiders the pedipalps are curiously modified and used 

 in depositing the spermatozoa in the female orifice. 



