420 



PHYLUM ARTHROPODA 



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 (Tetrapneumones) respiration is effected 

 by four " lung-books," e.g. in the large bird-catching My gale 

 (Fig. 232). 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 opening of the lung-books, or 

 at both places. 



The spinnerets (4-6) lie just in front of the anus. They 



are movable and perforated 

 by numerous (often many 

 hundred) tubes or " spinning 

 spools," each of which is 

 connected with a compressible 

 gland secreting silk. There 

 are various kinds of glands ; 

 both the amount and the 

 nature of the secretion are 

 under control. The spin- 

 nerets are transformed abdo- 

 minal appendages (a new 



FiCx. 233. 



— Section of lung-book. 

 After Macleod. 



^..Dorsal; t;., ventral ; Z., lameU^ ; p. organ from an old aS is SO 



posterior ; a., anterior ; d.c, dorsal ^ 



chamber ; x., posterior wall ; St., 



the 



stigma ; ch., one of the interlamellar 

 chambers. 



in- 



often the case) ; and 

 glands are ectodermic 

 vaginations. 



Many spiders have in front of the anterior spinnerets a 

 transverse plate or cribrellum perforated by spinning 

 tubes, and from this they comb out a peculiar curled 

 silk with the help of a row of stiff bristles or calamistrum 

 on each posterior leg. 



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 epigynium. 

 Here also in most females are the openings of two recep- 



