Morphology of the Arachnida. 5 



Thelyphonidffi and wider. The telson has disappeared. 

 The shortness of the abdomen is brouglit about by the reduc- 

 tion in size of the three posterior somites, thetergaand sterna 

 being represented by very short, closely approximated, trans- 

 verse sclerites. Moreover the anterior two tergites are also 

 much smaller than in Thelyphonus. The increase in the 

 width of the ccphalothorax is accompanied by a recession of 

 the coxse, especially of the fourth and fifth appendages, from 

 the middle line, leaving a membranous space between the 

 anterior and posterior sternal pieces ; this membrane is 

 chitinized from distinct centres. Furthermore, the coxas of 

 the second pair of appendages are freely movable, and not 

 fused, as in Thelyphonus. 



The Aranese or Spiders agree with the Phrynidse in having 

 a distinct constriction between the ccphalothorax and abdomen, 

 in having the coxse arranged radially round the sternum, those 

 of the second pair being freely movable as maxillae ; in having 

 two-jointed mandibles and normally eight (median and lateral) 

 eyes. Moreover in lower forms there are two pairs of lung- 

 sacs, as in Phrynus. Again, although a marked difference 

 between the two groups is the absence of segmentation in the 

 adult Spiders, yet it is interesting to note that during their 

 embryonic condition the ventral surface of the abdomen seems 

 at one time or another to be divisible into eleven sternal areas, 

 as in the adult Phrynus. The first of these is apodous and 

 represents probably, I think, the genital sternite ; the second, 

 third, fourth, and fifth bear a pair of appendages each, while 

 tlie last six are without appendages, as in the Scorpion. The 

 anterior two pairs of appendages disappear in connexion with 

 the formation of the breathing- organs ; the fate of the poste- 

 rior two, however, is quite unlike anything met with else- 

 where in the Arachnida, for they take on the form of dwarfed 

 limbs and constitute the spinning-mammilla3, which, with 

 their appropriate glands, are the most characteristic features 

 of this group. Their presence constitutes the greatest struc- 

 tural break between the Spiders and Pedipalpi. In the 

 higher forms of Spiders the posterior lung-sacs are replaced 

 by tracheal tubes. In the lowest, i. e. Liphistius, the upper 

 surface of the abdomen is furnished with a series of nine 

 tergites, the posterior of which are very much reduced in size, 

 as in Phrynus^ and the spinning-appendages retain their 

 primitive position close behind the lung-sacs. 



Another order of the Arachnida, namely the Pseudo- 

 scorpiones, also seems to me to be tolerably nearly related to 

 the Pedipalpi. It is not unusual to associate this group with 

 the Scorpions, as Mr. Cambridge has done in his article in 



