18 On the Morphology of the Arachnida. 



EXPLANATION OF THE PLATES. 

 Plate I, 



Fig. 1. Diagram to show fundamental plan of Arachnid structure. 

 I.-VI., ceplialotlioracic appendages ; c, carapace ; m, median, 

 and /, lateral eyes ; 1-12, abdominal somites ; t, telson ; .r, anus ; 

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



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

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

 four pulmonary sacs which correspond to y-^ of fig. 1. (N.B. 

 The coxae of the posterior two cephalothoracic appendages have 

 been omitted so as to show the genital operculum and the 

 pectines. 



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

 striction between cephalothorax and abdomen ; 1-4, anterior 

 four tergites 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. Schizonofus. Lettering as in fig. 3. c', supernumerary cephalo- 

 thoracic tergite. 



Fig. 5. Diagi-am of Phrynus to show the absence of the telson and the 

 reduction in size of the posterior abdominal tergites and 

 sternites. 



Fig. 6. Diagram of the anterior five abdominal somites of Thelyphonus or 

 Phrynus, to illusti'ate 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 ; op, sternum of the 

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

 sacs and of the genital orifice ; ^j^, anterior, ]}-, posterior pul- 

 monary sacs. 



Plate II. 



Fig. 7. Liphistius desulfor, to show the persistence of most of the abdo- 

 minal tergites and of the two anterior 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 jilate of the anus. The appendages marked 

 e, {■ in fig. 1 persist as the spinning-mammillae, m^, 7if. 



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

 appearance of the external segmentation, the persistence of parts 

 of the sterna as respiratory opercida (s\ s-), and the migration of 

 the mammillae to the posterior end of the abdomen. 



Fig, 9. Diagram of a Dipueumonous Spider (Dysdera), to show the 

 replacement of the posterior pulmonary sacs by a tracheal 

 tube (s^). 



Fig. 10. Diagram of another Dipueumonous Spider (Fpeira), to show the 

 migration of the tracheal tubes to the hinder end of the 

 abdomen. 



Fig, 11. Diagram of one of the Pseudoscorpiones (Garypus), to show the 

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

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

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



Fig, 12. Stylocellus Javanus, one of the Opiliones, showing the disappear- 

 ance of the anterior abdominal somites (after Thorell ; slightly 

 modified). 3-12, the tergites ; 4-11, sternites ; s, stigma on the 

 4tli sternite. 



