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 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 reproduce 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 King-crabs they have become retiform. Mites, Scorpions, and Pedipalps are viviparous, their eggs developing in the ovary or in a uterus. Other Arachnids lay eggs, and many Spiders and Pseudoscorpions carry their eggs about with them. As a rule the young is but a miniature of the parent, and the Arachnid, unlike the Crustacean or Insect, undergoes little or no metamorphosis. A certain number of Mites are parasitic in plants and in animals, and a few, together with a few Spiders, have resumed the aquatic life of their remote ancestors. The members of some Orders, such as the Solifugae and Opiliones, are nocturnal, and many are provided with silk-glands and weave webs which reach their highest pitch of perfection amongst the Spiders. At times—especially is this the case with the Mites—enormous numbers of individuals live together, but they never show the least adaptation to communal life, and no individuals are ever specialised to perform certain functions, as is the rule in communities of social Insects. With the two exceptions that we regard the Trilobites as more nearly allied to the Crustacea, and have therefore considered them apart, and have treated the Pycnogonida independently of the Arachnida, we have followed Lankester in his classification, though not always in his nomenclature:— Sub-class 1. Delobranchiata[205] (Merostomata). Order (i.) Xiphosura. Order (ii.) Eurypterida (= Gigantostraca, Extinct).