12 Segments of Abdomen 



"arciili genitales " (PI. I., tigs. 1 I and 1 m, m), are therefore seen very distinctly in almost 

 the whole of their extent. They surround the orifice laterally, in front, as well as above. 

 With regard to the morphological value of these "arculi genitales" we refer to the later 

 portion of this chapter, which is devoted to morphological questions and in which we propose 

 to discuss the relations of the abdominal segments in the whole order. 



We would mention here only, that it can .scarcely be denied that the genital orifice 

 belongs to that portion of the "first" sternite which is situated behind it. 



The preceding description of abdomen applies to the adult animals. We would therefore 

 add, that although we have had opportunities of examining earlier stages of but a few species 

 (Stylocellus sulcatiis n. sp., Purcellia illustrans n. gen., n. sp., and Pettalus brevicauda Pock.), 

 these have in several respects afforded interesting particulars. One of these is that in young 

 specimens a larger or smaller number of the abdominal segments are free, with regard to 

 which we refer to the special (Latin) description of these three species. Another interesting 

 point is the appearance of the dorsal surface in young animals. The dorsal surface of 

 Stylocellus sulcatiis shows a longitudinal groove, in which a narrow strip of soft skin is found 

 in the young on the first seven tergites, so that these plates here really are divided, whilst 

 the eighth and ninth are entire. In a young Purcellia illustrans the first five tergites 

 are divided longitudinally by a narrow strip of soft skin, the succeeding ones being entire; 

 on a somewhat older specimen the first tergite is also undivided. In the young of Pettalus 

 brevicauda figured on PI. III., fig. 3 ff, the si.x first tergites are divided, the first less dis- 

 tinctly than the others. 



How very much the .statements of even recent authors vary concerning the number of 

 abdominal segments in Phalangium, the genus amongst Opiliones which is most numer- 

 ously represented in Europe, will appear from the following data. In 1843 Tulk (p. 163) 

 reckoned ten tergites, counting as he did, the last tergal plate of cephalothorax as belonging 

 to abdomen, and interpreting operculum anale as an independent segment ; at the same 

 time, however, he only counted five sternites, which number apparently is the right one in 

 this and allied genera. In 1872 Balbiani (PI. I., fig. 6), on the contrary, counted only 

 seven segments (tergites) on an almost mature embryo, with three thoracic ones, because 

 he reckoned the first tergite of abdomen as belonging to cephalothorax (pp. 9 — 10), and 

 quite overlooked operculum anale. In 1887 Weissenborn, simply following Tulk, stated the 

 segments to be ten, whilst Bernard (p. 361) in 1894 — 1897 counted only six segments — a 

 result which seems to us unintelligible, as he does not explain how he arrived at that number. 

 The numbers which we have cited were, however, based merely on one-sided investigations 

 of the genus Phalangium, which must be .said to be the one in which it is more difficult 

 than in any other to ascertain the number, at any rate of the sternites. In this respect 

 those authors who attempted to solve the problem through a comparative study of a larger 

 number of representatives of the order, were more favourably situated. 



When Sorensen in 1873 (o) attempted a morphological interpretation of the structure 

 •of Opiliones, he came to the conclusion that abdomen in Palpatores and Laniatores consisted of 

 eight segments, which result was essentially based on the number of tergites. He did not 

 ■count operculum anale as a tergite because he missed the true view of the morphological 



