The supra-oesophageal nervous centre supplies the eyes and integument: it is connected at the sides and behind the oesophagus to a single nerve mass in Linguatulina, Acarina, Araneidae, Pedipalpi, and Solifugae, from which pass off nerves to the limbs, etc. In Araneidae the posterior nerve to the abdomen has often a ganglionic swelling at the junction of that region to the cephalothorax. In Scorpionidae there is a ventral cord with seven ganglia, the first of which corresponds to the fifth abdominal somite, the preceding parts, including the chelicerae, being supplied from the sub-oesophageal nerve-mass. In Limidus there is an oval oesophageal collar with 3-8 transverse commissures, which supplies the appendages of the cephalothorax, the chilaria, and the genital operculum, and an abdominal cord which gives off five groups of nerves to the branchial feet, the last pair supplying the metasoma and post-anal spine as well. A stomatogastric system exists in Araneidae, Scorpionidae, and Phalangida. Eyes are absent in Linguatulina and some Acarina. They are present in other Arachnida, and vary in number from two (Acarina, Tardigrada) to twelve. They are often grouped in a characteristic manner. In Spiders they are arranged in 2-3 rows across the cephalothorax dorsally and anteriorly.
The Scorpionidae and Xiphosura have a pair of central dorsal eyes, and one (Xiphosura) or more pairs placed near the lateral margin of the cephalothorax. The lateral eyes in Scorpionidae and all the eyes of Limuhis are monostichous; the central eyes of the former group and other Arachnids, so far as known, diplostichous. With the exception of the lateral eyes of Limuhis; which are polymeniscous, or more probably composed of a number of distinct ocelli, Arachnidan eyes are always monomeniscous. Vitrellae are absent. The eyes of Limulus and Scorpions are retinulate. Each retinula in the central eyes of the latter consists of five cells surrounding a five-fluted rhabdome. The grouping of the cells is not so distinct in the lateral eyes. In Limulus the central eyes have groups of five retinular cells, but the grouping is sometimes quite irregular; the lateral eyes have retinulae with ten cells, each retinula corresponding to a single lens (supra). Curious differences of structure have been observed in certain Spiders between the anterior and posterior eyes of one and the same set1. In Araneidae auditory hairs occur on the palpi, tibiae and tarsal joints, and a sensory organ, probably olfactory or gustatory, on the anterior surface of the basal joint ( = maxilla) of the palpi or second pair of limbs.
1 In a few Spiders a chitinoid plate, the cribellum, lies in front of the spinnerets. It is perforated by the ducts of innumerable glands. Its presence is correlated with that of the calamistrum, a single or double row of long wavy hairs on the dorsal aspect of the penultimate tarsal joint of the fourth pair of ambulatory limbs. One of the calamistra is rapidly vibrated over the cribellum, and draws out the secretion from the glands in the form of threads, used to strengthen the web, to assist in forming the cocoon for the eggs, and sometimes perhaps in making a domicile. See Bertkau, A. N. 48, I, 1882.
The digestive tract takes a straight course. The stomodaeum is muscular in Linguatulina. The part that traverses the oesophageal nerve collar is generally contracted, the anterior oral end as well as the posterior end being frequently dilated, and in the Araneidae (and others) the latter forms the 'sucking stomach.' Salivary glands, paired or unpaired, are generally present. They are absent in Limulus. The mesenteron is of considerable length. Its thoracic portion in Araneidae is furnished with five pairs of caeca which enter the bases of the limbs. It has lateral tubular outgrowths or glands, sometimes forked or ramified, and then forming a continuous (?)mass in Scorpionidae and Limulus. The numbers of these glands vary. The proctodaeum varies in length and size. Its rectal end is dilated in Araneidae, and with the exception of Linguatulina and Limulus its anterior end is provided with a pair of Malpighian or renal tubes, which are often branched. The anus is ventral at the base of the telson in forms which possess that structure, in others it is terminal and more or less ventral according to the shape of the tergal region of the abdomen.
The food of Arachnids is liquid, either the juices of plants (some Acarina) or of animals; but Limulus feeds upon soft worms, etc, which it sucks into its wide pharynx and there crushes.
1 The differences in question are e. g. in Epeira that the visual cells or retinophorae of the anterior eyes are very numerous and slender, and their retinidia terminal; of the posterior eyes relatively few, stout, and their retinidia distinctly axial, whilst the retinophoral nuclei are consequently situated to the outer side of the retinidia, as in the polymeniscous eyes of Insecta and Crustacea. There can be little doubt that the retinidia of the posterior eyes are compound, and probably double as in Mollusca. In Lycosa and Salticus the ommateum of the posterior eyes is feebly concave, and does not extend laterally as far as do the vitreous cells. The outer ends of the retinophorae in Lycosa are swollen, and contain the nuclei; whilst in Salticus they are lengthened out and bent horizontally, so that the nuclei form a circumferential layer. Graber states that the vitreous cells = corneal hypodermis, are present in the lateral eyes of Scorpions. Patten regards the eyes of Limulus and Scorpions as of a peculiar type altogether, and as divergent in the same direction, Limulus more than Scorpions. They start with the fusion of a number of ocelli, which is not the case with the compound eyes of other Arthropods. It may be noted that the basement membrane of the hypodermis extends across the eye beneath the corneal hypodermis (= vitreous cells) in the eyes of Spiders, the central eyes of Scorpions, and of Limulus. In the last-named there are also a number of intrusive connective tissue elements.