532 COMPARATIVE ANATOMY CHAP. 



The Pseudoscorpionidce have tubular tracheae with 2 pairs of stigmata, which lie in 

 the 2d and 3d abdominal segments. In Cheiridium there is only one pair of stigmata, 

 which has perhaps arisen by the fusing of the two pairs found in other Pseudo- 

 scorpionidce. 



The ramified tracheae of the Phalangidce are said to open through a single pair of 

 stigmata, lying ventrally at the anterior end of the abdomen, which is closely applied 

 along its whole breadth to the cephalo-thorax. 



Among the Cyphophthalmidce, Cyphophthalmus is said to have only one pair of 

 stigmata on the under side of the first abdominal segment. Gibbocellum (Fig. 372), 

 on the contrary, has 2 pairs of stigmata lying laterally and ventrally in the 2d 

 and 3d abdominal segments. The anterior pair of stigmata leads into richly branched 

 tracheae, whose two principal trunks unite into an unpaired median trunk in the 

 cephalo-thorax. The posterior pair of stigmata lead into tufted tracheae. Each 

 stigma is covered by a plate pierced like a sieve, and each pore in this plate represents 

 the aperture of a tracheal tubule. 



In many Acarina, especially in the parasitic and marine Acarina, tracheae are 

 wanting. When they are present they open out through one pair of stigmata, 

 which are placed very unusually. This pair of stigmata generally lies near the coxal 

 joints of the last pair of extremities, but often much further forward. It occasionally 

 lies on the dorsal side, and sometimes above the base of the chelicerae. This arrange- 

 ment is not at present understood. In certain Acarina short tubes or sacs connected 

 with apertures in the outer chitinous integument have been considered as the rudi- 

 ments of tracheae. 



The Linguatulidce are devoid of tracheae. The Microthelyphonidce also are said 

 to have no special respiratory organs. If this be established, it must not be considered 

 in the Acaridce the original arrangement. 



The Tartaridce are said to possess lateral apertures supposed to be stigmatic in 

 the 2d, 3d, and 4th ventral rings, thus having 6 in all. 



A review of the position of the respiratory organs and their apertures in the various 

 divisions of the Arachnoidea shows us that not only do several abdominal segments 

 possess stigmata, but that these may occur, as is shown by the example of the 

 Solpugidce, in the thorax also. Leaving out of consideration the anterior position 

 of the stigmata in certain Acarina, which are a very one-sidedly developed Arachnoid 

 group, and evidently, excepting the Linguatulidce, the furthest removed from the 

 racial form, we are justified in assuming that the, to us, unknown racial form of the 

 Arachnoidea possessed a larger number of stigmata x and of tracheae connected with 

 them than any Arachnoid form now living. This presupposes that the book-leaf 

 tracheae are modified tubular tracheae. 



VIII. Sexual Organs. 



In all Arachnoidea the sexes are separate. The sexual organs lie 

 in the abdomen. The testes and ovaries are either paired or single. 

 The paired condition must be the more primitive. The ovaries in 

 very many Arachnoidea appear as tubes beset with spherules or sacs, 

 and so have a grape-like appearance. The eggs arise only in the sacs, 

 which may be called egg -follicles, and they thence enter the ovarian 

 tube, which serves only as a duct. 



With very rare exceptions the ducts of the sexual organs are 

 paired. These unite in their terminal portion, and open externally 



1 See footnote on page 516. 



