526 THE ANIMAL KINGDOM. 



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 Acarind) or of animals ; but Limulus feeds upon soft worms, 

 &c., which it sucks into its wide pharynx and there crushes. 



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, &c. An entosternite or chitinoid fibre-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 Arachnida. In Scorpio, Mygale (Ara- 

 neidae\ in Phalangidae, Galeodes (Soliftigae), 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 oeso- 

 phagus and unite into a median artery running backwards dorsally to the 

 ventral nerve-cord. In Limulus the corresponding vessels enshcath 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 Seor- 

 pionidae. Lateral vessels are given off from the base of the anterior 



