ANATOMY 



257 



is usual in Arthropods, is largely lacunar, bat in Scorpions and 

 Limulus the arteries form definite channels, and are in fact better 

 developed than in any other Arthropod. 



As a rule the alimentary canal in Arachnids is no longer 

 than the distance between the mouth and the anus ; but in the 

 King-crab, where the mouth is pushed back almost to the centre 

 of the body, there is a flexure in the median vertical plane. 

 Paired glands, usually called the liver, open into the mesenteron ; 

 food passes into the lumen of these glands, and is probably 

 digested there. In many Arachnids these glands extend into 

 the limbs. In those members of the group that have become 

 terrestrial the nitrogenous excreta are separated out by Mal- 

 pighian tubules which open into the proctodaeum ; but coxal 

 glands, homologous with the green gland and shell -glands of 

 Crustacea, may coexist, and in the ac|uatic Livnilus these alone 

 are found. They usually open on the base of one or more pairs 

 of walking legs. 



The endosternite, or internal skeletal plate to which muscles 

 are attached, reaches a higher development in the Arachnida than 

 in the Crustacea. In Scorpions it forms a kind of diaphragm 

 incompletely separating the cavities of the pro- and meso-soma. 



The supra- oesophageal ganglion supplies the two existing 

 segments which have slipped before the mouth, i.e. those of the 

 eyes and of the cheHcerae. The post-oral ganglia in the Acarina, 

 the Pedipalpi, the Solifugae, and the Araneae have fused into a 

 central nerve-mass, from which nerves radiate ; but in Limulus 

 the prosomatic appendages are all supplied from the nerve-ring. 

 The chief sense-organs are eyes of the characteristic Arthropod 

 type, and sensory hairs of a great variety of complexity. 

 Scorpions and Spiders have stridulating organs, and we may 

 assume that they have also some auditory apparatus ; perhaps 

 some of the hairs just mentioned act as hearing organs. 



Arachnids are male and female ; they do not reproduce 

 asexually, and there is no satisfactory proof that they ever repro- 

 duce parthenogenetically. As a rule there is little external 

 difference between the two sexes, except in Spiders, where the 

 male is as a rule smaller than the female, and when adult has 

 the pedipalpi modified for use in depositing the spermatophores. 

 The ovaries and testes are annular, and with their ducts encircle 

 the alimentary canal in Mites and Phalangids ; in Scorpions and 

 VOL. IV s 



