Ix ANATOMY 257 
is usual in Arthropods, is largely lacunar, but 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 aquatic Limulus 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 shpped before the mouth, 7.e. those of the 
eyes and of the chelicerae. 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 
