18 The true Jirst abdominal Sternite 



of hard chitine, "arculi genitales" (m in several figures)^ which are always separated from 

 abdomen by a groove-like suture, whilst they are separated from the fourth pair of coxffi by 

 a more or less distinct, often partly quite indistinct, linear impression which is generally deep, 

 but in Ogovia (PI. Ill, iig. 1 d) is so slight that it is not possible to indicate the boundary 

 between the coxae and " arculi genitales " with any certainty. A comparison between the 

 structure in these different groups clearly shows that what appears to be the proximal 

 extremity of the fourth coxa really is the same piece as the "arculi genitales" in Cyphoph- 

 thalmi, and is not a typical part of the coxa at all. In Cyphophthalmi the genital orifice is 

 not closed by operculum genitale and it is therefore easy in their case, without any prepara- 

 tion, to see that the " arculi " turn inwards, on a level with the posterior margin of the 

 third pair of coxae, and meet in the middle line of the body where, as a rule, they are 

 separated from each other by a furrow, in front of the genital orifice (especially plainly seen 

 on PI. III., figs. 2 / and 3 c). In this way the " arculi " enclose this opening on the sides, 

 in front, and at least anteriorly above. The morphological value of these " arculi genitales " is 

 most easily recognized in a not full-grown animal of Cyphophthalmi, for instance Stylocellus 

 sulcatus n. sp. (PL II., fig. 3 h) or Piircellia illustram (PI. III., fig. 4 c). In this stage of 

 development the orifice itself does not exist, but the development of operculum genitale has 

 commenced; in front of which, and partly at either side of it, a flat and rather large area 

 appears, the shape and position of which clearly indicate that it belongs to abdomen. From 

 the rest of the latter it is separated by a strip of thinner skin, which is continued across 

 the body behind the small beginning of operculum genitale. The small lateral protuberances 

 which are seen on this area and have the character of incomplete replicas of those which are 

 found in the adult animal, clearly show that it is the " arculi genitales " that are formed in 

 this area. 



It follows that " arculi genitales " represent the real first sternite. We would point out 

 that this result confirms the correctness as regards Opiliones of our statement on an earlier 

 occasion in treating of Palpigradi (a, p. 232): "As usual in those Arachnida whose anterior 

 segments can be distinguished from each other, the second is the genital one, viz. the one 

 in which the genital organs have their orifice." We were unable at the time to prove the 

 correctness of this view in the case of Opiliones, but we believe that one of us^ has demon- 

 strated it as regards most of the other orders of Arachnida, in opposition to the view held by 

 other authors, that the genital segment is the first. 



As arculi genitales in Cyphophthalmi offer characters by which the sexes can be 

 distinguished, we shall have to return to them again in our chapter on the external sexual 

 characters. In this place we will deal with the arculi of the two other sub-orders. Where 

 operculum genitale is removed in Phalangioida? a transverse piece will be seen (woodcut 

 fig. B), which before was hidden or nearly hidden by operculum genitale, and which anteriorly 

 forms the roof of the genital orifice. The posterior, smaller portion of this piece consists of 

 a firmly chitinized transverse strip which in some cases, for instance in Phalangium, gradually 

 merges into the anterior lobe of the lateral arculi (from wi in woodcut fig. B, a long dotted 



' Being so conspicuous, particularly in the larger species description which is correct in so far that they really are 



of Cyphophthalmi, they have of course not escaped the pregeuital, but otherwise is less fortunate, because lobi 



notice of Thorell (c) when he described the genital orifice, eoxales, properly speaking, is a formation which does not 



but he neither gave them any name, nor proposed any occur in any Arachnidean either on the third or the fourth 



explanation of them (c, pp. 26, 27). Pocock described them pair of legs. 

 ((/, p. 291) as "pregenital coxal lobes of the fourth legs," a - Siirensen, /. 



