PHYLUM ARTHROPODA 



623 



vitr 



in a median vagina, which opens on the exterior by a median genital 

 aperture at the base of the abdomen. One, two, or three receptacula 

 seminis are present, and either open into the vagina or independ- 

 ently on the surface. In the male there are two elongated tubular 

 tcstes with two narrow, and often greatly coiled, efferent ducts, which 

 unite in a short median vas deferens, the aperture of which is on the 

 base of the abdomen between the stigmata of the first pair. The 

 pedipalpi of the male (Fig. 507) are modified to act as intro- 

 mittent organs ; the terminal segment is swollen, and contains a 

 twisted tube (sph.) into which the sperms from the reproductive 

 aperture are received in order to be transferred in the act of 

 copulation to the reproductive aperture of the female. The eggs 

 of spiders are laid in nests or cocoons, and are usually guarded 

 by the mother, some- 

 times carried about 

 by her. 



In their mode of 

 life the Arachnida 

 present almost as 

 great a diversity as 

 the Insecta. Some 

 Acarida are parasites 

 throughout life. Most 

 of the other groups 

 of Arachnida are pre- 

 daceous preying for 

 the most part on In- 

 sects or other Arach- 

 nids. To capture the 

 Insects which consti- 

 tute their food the 

 majority of Spiders 

 construct a web 

 formed of the threads secreted by the arachnidium. The 

 primary function of the threads formed from the secretion of 

 the spinning organ is to constitute the material for the manu- 

 facture of a cocoon for enclosing the eggs, and in some Arachnids 

 this is the sole purpose to which they are devoted. In others 

 there is added a nest for the protection of the eggs and of the 

 parent itself; this in many cases becomes a permanent lurking 

 place which the Spider inhabits at all seasons, and from which it 

 darts out to capture its prey ; in the Trap-door Spider, the nest has 

 a closely fitting hinged lid. In very many Spiders the secretion 

 is used mainly to form the web by means of which, the prey is 

 snared, with the addition frequently of a nest in which the Spider 

 lies in wait. A subsidiary function of the threads is to aid in 

 locomotion, the Spider being enabled by means of them to let 



FIG. 519. Section of the central eye of Euscorpius. 

 Letters as in preceding figure, pigm. cells containing 

 pigment ; vitr. vitreous body (a specialised part of the 

 ectoderm). (After Lankester and Bourne.) 



