18 On the Morphology of the Arachnida. 



EXPLANATION OF THE PLATES. 

 Plate L 



Fiy. 1. Diarrram to show fundamental plan of Arachnid strncturo. 

 L-VI., cephalothoracic appendages; c, carapace; m, median, 

 and /, lateral eyes ; 1-1 -, abdominal somites ; t, telson ; x, anus ; 

 a-(, abdominal appendages ; g, generative aperture. 



Fig. 2. Semi-diagrammatic figure of a Scoi-pion. Lettering as in fig. 1. 

 a, genital operculum ; 8, pectines ; 3'-6', sterna bearing the 

 four pulmonary sacs which con-espond to y-( of fig. 1. (N.B. 

 The coxa; of the posterior two cephalothoracic appendages have 

 been omitted so as to show the genital operculum and the 

 pectines. 



Fuj. 3. Fig. of a Thelyphonns. Lettering as above, w, waist or con- 

 striction between cephalothorax and abdomen ; 1-4, anterit)r 

 four tergltes of abdomen, represented on the ventral side by 

 three sternites, l'-2', 3', 4' ; 10-12, posterior three abdominal 

 somites forming the movable stalk for the antenniform telson. 



Fig. 4. Schizoiiotus. Lettering as in fig. 3. c', supernumerary cephalo- 

 thoracic tergite. 



Fig. o. Diagram of Phrynns to show the absence of the telson and the 

 reduction iu size of the posterior abdominfil tergites and 

 sternites. 



Fig. G. Diagram of the anterior five abdominal somites of Thelyphonus or 

 Phrgnus, to illustrate the hypothesis that explains why the 

 pulmonary sacs of the 3rd and 4th somites open behind the 1st 

 and 2nd sterna. 1-5, abdominal somites ; oj), stenium of the 

 1st (P 2nd) forming the operculum of the anterior pulmonary 

 sacs and of the genital orifice ; ;^\ anterior, p-, posterior pul- 

 monary sacs. 



Plate II. 



Fig. 7. Lijjhistiits desiiltor, to show the persistence of most of the abdo- 

 minal tergites and of the two auicrior sternites. The 1st ter- 

 gite forms the dorsal plate of the abdominal pedicle, and the 

 12th the dorsal plate of the anus. Similarly the 12th sternite 

 persists as the ventral plate of the anus. The appendages marked 

 €, ( in fig. 1 persist as the spinning-mammilla;, wi^, m^. 



Fig. 8. Diagram of one of the Mygalomorphous Araneaj to show the dis- 

 appearance of the external segmentation, the persistence of parts 

 of the sterna as respiratoiy opercula (s^, s'-), and the migration of 

 the mammillfe to tlie posterior end of the abdomen. 



Fig. 9. Diagram of a Dipneumonous Spider {Dgsdcra}, to show the 

 replacement of the posterior pulmonary sacs by a tracheal 

 tube (.«-). 



Fig. 10. Diagram of another Dipneumonous Spider (Epeira), to show the 

 migration of the tracheal tubes to the hinder end of the 

 abdomen. 



Fig. 11. Diagram of one of the Pseudoscorpiones {fiarypu,<i), to show the 

 terga and sterna of the abdomen, also the presence of the stig- 

 mata in tlie 3rd and 4th somites and tlie single generative ster- 

 nite (g) corresponding to the 1st and 2nd tergites. 



Fig, 12. SfglocoUus Jfivamis, one of tlie Opiliones, showing the disappear- 

 ance of the anterior abdominal somites (after Thorell; slightly 

 modified). 3-12, the tergites ; 4-11, steniites ; .<!, stigma on the 

 4th sternite. 



