SPIDERS. 



333 



abdomen. It gives off an anterior and a posterior aorta 

 and lateral vessels ; and the circulation corresponds in 

 general to that of the scorpion. 



In a few forms, united as Tetrapneumones, respiration is 

 effected by four " lung books " ; the large bird-catching 

 Mygale is an example. In the vast majority (Dipneumones) 

 there are two " lung books," and tubular tracheae in addition. 

 The stigmata of the lung books lie on the anterior ventral 

 surface of the abdomen ; the tracheae open posteriorly near 

 the spinnerets, or just behind the openings of the lung 

 books, or at both places. 



The spinnerets (4-6) lie posteriorly, a little in front of the 

 anus. They are movable organs, perforated by numerous 



(often many hundred) 

 fine tubes which pro- 

 ject as " spinning 

 spools." The tubes are 

 connected with numer- 

 ous compressible glands 

 secreting liquid silk. 

 There are various kinds 

 of glands, both the 

 amount and nature of 

 the silk secretion be- 

 ing under the spinner's 

 control. 



The males are usually 

 smaller and often more 

 brightly coloured than 

 their mates. From the paired testes in the anterior part 

 of the abdomen, two vasa deferentia pass to a common 

 aperture beside the openings of the lung books. From 

 the paired ovary two oviducts likewise arise and open 

 into a uterus, whose external aperture is surrounded in 

 the mature female by a complex genital armature or epi- 

 gynium. Here also in most females are the openings of 

 two receptacula seminis, in which the sperms received from 

 a male are stored, and from which they pass by a pair of 

 internal ducts to the oviducts, there to fertilise the ova. 

 Allusion has already been made to the fact that the sperms 

 of the male, after emission, may be stored up in the last 



FIG. 106. Section of Lung book. 

 (After MACLEOD.) 



d, Dorsal ; z/, ventral ; /, lamellae ; /, pos- 

 terior ; a, anterior ; d.c, dorsal chamber ; JT, 

 posterior wall ; st, stigma ; ch, one of the 

 interlamellar chambers. 



