ARACHNIDA. 537 



aorta, from each chamber of the heart, and from the posterior aorta in 

 Scorpionidae : lateral vessels also exist in some Araneidae (Epeira). In 

 Limuhts 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 para- 

 sitic 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 lirnbs 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 (Mygale) or the second pair lead into a system of tracheal 

 tubes. Same 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. Limulus 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 ; 



