328 



ARACHNOIDEA AND PALMOSTRACA. 



A three-chambered heart, containing colourless blood, 

 lies within a pericardium near the dorsal surface of the 

 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 Mygale 

 (Fig. 141). 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 sur- 

 face 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 

 posteriorly a little in front of 

 the anus. They are movable 

 organs, perforated by numer- 

 ous (often many hundred) fine 

 tubes or "spinning spools." 

 The tubes are connected 

 with numerous compressible 

 glands secreting liquid silk. 

 There are various kinds of 

 *-'3^VZ?^!7t&& glands, and both the amount 

 chamber ; x., posterior wall ; s t., and the nature of the secre- 



stigma; ch., one of the interlamellar .- * ,, . , 



chambers. tion are under the spinner s 



control. The spinnerets arise 

 from modifications of abdominal appendages, and the glands 

 are ectodermic invaginations. 



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 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 



