Lankester and Bourne figure the hypodermic basement membrane as passing beneath the lateral eyes of Limulus and Scorpions. In Spiders a membrane also extends in a similar way beneath the eye, and is stated to be continuous with the hypodermic basement membrane. The rhabdomes in Lankester and Bourne's account of Limulus and Scorpions would correspond on Patten's view to the vitrellae so called of Insecta, etc. See note, p. 452, where Patten's terms are explained.
The coelome is more or less filled by the contained viscera, and in the higher forms by an abundant connective tissue as well as muscles of the appendages, etc. An entosternite or chitinoid fibro-cellular plate much resembling cartilage, lies between the anterior extremity of the digestive tract and the central nervous system. Muscles are attached to it. It has been found in Apus among Crustacea, but its presence and large size is characteristic of Arachnid a. In Scorpio, Mygale (Araneidae), in Phalaitgidae, Galeodes (Solifugae), and Limulus, a paired ductless coxal gland lies in the thoracic region; in the first-named in relation with the coxae of the last pair of limbs, in Mygale and Limulus with the coxae of the last four pairs of limbs. It has been found also in other Spiders; and in Atypus, two apertures, one at the base of the first pair of legs, the second at the base of the third pair, exist in connection with it. So too in Scurria, where a reservoir for the secretion is appended close to the aperture. In the young Limulus there is an aperture on the coxae of the fifth pair of limbs.
The Oribatidae among Acarina have a very similar gland, which perhaps opens externally.
The blood-plasma contains haemocyanin in quantity, and is blue-coloured when oxygenated in Scorpions (Androctonus) and Limulus. It contains amoeboid corpuscles. A heart is absent in Linguatulina and Acarina, except some Gamasidae, in which a single-chambered heart with long aorta and bivalved inlets lies in the posterior part of the abdomen. A three-chambered heart with three pairs of lateral valved ostia is found in Araneidae and Phalangidae. It is eight-chambered in Scorpions and Limulus. It lies within a pericardium or sinus in the two last-named, and in some Araneids at least, into which open venous passages bringing back blood from the respiratory organs. Alary muscles are attached to this pericardium. There is an anterior aorta which supplies the eyes and limbs, and in Scorpions gives off two vessels which surround the oesophagus and unite into a median artery running backwards dorsally to the ventral nerve-cord. In Limulus the corresponding vessels ensheath the oesophageal nerve-collar as well as the nerves which it gives off and the ventral cord.
The heart has a posterior aorta in Araneidae and Scor-pionidae. Lateral vessels are given off from the base of the anterior aorta, from each chamber of the heart, and from the posterior aorta in Scorpionidae: lateral vessels also exist in some Araneidae (Epeira). In Limulus there are four pairs of such vessels, which unite at their external ends into a lateral longitudinal trunk on each side. These two trunks fuse posteriorly, and are continued backwards as a single vessel. The capillary system is well developed in Scorpionidae and Limulus. The venous blood passes to the respiratory organs from a median ventral sinus. Respiration is cutaneous in Linguatulina, some aquatic and parasitic Acarina, and Tardigrada. There is a pair of stigmata leading to a tracheal system of tubes between the third and fourth pair of limbs in most Acarina, between the last pair of limbs in Phalangidae, and behind the fourth pair of limbs in Solifugae. Stigmata in other Arachnida are confined to the ventral aspect of the abdomen. One pair on the third to the sixth somite inclusive in Scorpions lead into long sacs with foliated walls.
The Pedipalpi have similar structures on the third and fourth somites, and among Araneidae there are either two pairs of such sacs (Mj/gale) or the second pair lead into a system of tracheal tubes. Some Araneidae possess in addition a single stigma in front of the spinnerets leading into two branched or four simple tracheal stems. There are two pairs of abdominal stigmata leading to tracheae in the Pseudoscorpionidae, and also in the Solifugae. Limidus is branchiate, and the respiratory organs consist of parallel leaf-like folds borne upon the posterior faces and near the outer edges of the last five pairs of abdominal limbs.
The Tardigrada are hermaphrodite, and possess an azygos ovary and two testes which open into the proctodaeum. The testis may be unpaired or paired, the ovary always unpaired, in Linguatulina, but the vasa deferentia and oviducts are double. The latter open into a long vagina or uterus, in which the ova develope into embryoes; its aperture lies on the ventral aspect in front of the anus. The former are terminated each by a copulatory cirrus which can be protruded from the single anteriorly placed male aperture. The Acarina and Phalangidae have as a rule a ring-like testis and ovary. The fertile portion of the glands may be more or less extended or restricted. The single genital aperture is on the abdomen ventrally and anteriorly, even between the last pair of limbs in some Acarina. The Phalangidae possess a protrusible copulatory organ and ovipositor. In the 'Scorpionidae the two testes consist of two, the single ovary of three, longitudinal tubes, united by cross anastomoses. The ducts open by a single aperture at the base of the operculum on the first somite of the abdomen. The testes of the Araneidae are two in number, tubular, with the termination of the tube sometimes dilated into vesiculae seminales. They open between the stigmata.
The ovaries are usually two also, but they sometimes unite into a ring (Segestria, Oletera): the oviducts fuse into a single vagina. The testis and ovary of Limulus are retiform; the ducts are double, and open separately on the posterior aspect of the operculum or first pair of abdominal appendages. The Scorpion has a copulatory organ at the end of each vas deferens, whilst in Limulus the spermatozoa are shed into the water. The male Spider retains the sperm in a special receptacle developed on the inner aspect of the last joint of each palpus. This structure consists of a chitinoid protecting envelope which often assumes complicated forms, open at the apex and lodging a coiled tube. From this apparatus the sperm is transferred to the female receptacula. Accessory glands, vesiculae seminales, and re-ceptacula seminis are often found except in Limtdus. The receptacula seminis (rarely single) possess in most Araneidae independent openings in front of the female genital aperture.