ARANEIDA. 



813 



joints and end in two, or in three claws ; the metatarsus or joint 

 before the tarsus of the 4th leg bears in some forms a brush- 

 like series of hairs, the 

 calamistrum (Fig. 532) , 

 which are correlated with 

 the presence of an acces- 

 sory spinning organ, the 

 cribellum (Fig. 535). 



Most spiders breathe 

 by means of two lung- 

 books, which open by 

 stigmata situated one on 

 each side of the genital 

 pore on the anterior ven- 

 tral surface of the ab- 

 domen (Fig. 526). Each 

 stigma leads into a space 

 which is largely filled by 

 a series of leaf-like plates 

 resembling the leaves of 

 a book. Within the sub- 

 stance of these plates the 



FlQ. 532. a Leg of the fourth pair of Amavrobi-us 

 ferox ; Ca calamistrum ; b end of foot of Philaeus 

 chrysops with two claws and pencil consisting of 

 spatulate hairs (S) ; c end of foot of Epeira 

 diademata ; K web-claws : Tk ambulatory claw ; 

 Gb toothed bristles (accessory claws) (after O. 

 Hermann). 



blood circulates whilst the 

 air plays between neigh- 

 bouring plates, and thus the 

 blood is aerated. If, as in 

 the Theraphosae, there be 

 four lung-sacs the second 

 pair lie close behind the 

 first. From the develop- 

 mental history of the scor- 

 pion there is little doubt 

 that these lung-sacs repre- 

 sent highly modified appen- 

 dages, perhaps something 

 like the gill-bearing appen- 

 dages of Limulus which 

 have sunk into the body (p. 801). When only one pair of 

 lung-books is present, there is generally a trachea! system 

 consisting of two or four main tracheae which open by two 



Fio. 533. Pedipalp of Tegenaria guyonii, the 

 large house-spider. 1 Coxa ; 2 gnathobase, 

 the so-called "maxilla"; 3 trochanter ; 4 

 femur ; 5 patella ; 6 tibia ; 7 tarsus ; 8 

 palpal organ. After Warburton. 



