672 



ZOOLOGY 



vtir- 



exterior by a median genital aperture at the base of the abdomen. 

 One, two, or three receptacula seminis (rec. sem.) are present, and 

 either open into the vagina or independently on the surface. In 

 the male there are two elongated tubular testes 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. 545) are modified to act as intromittent organs : 

 the terminal segment is swollen, and contains a twisted tube (spA.) 

 into which the sperms from the reproductive aperture are re- 

 ceived 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 pre- 

 sent almost as great 

 a diversity as the In- 

 secta. Some Acarida 

 are parasites through- 

 out life. Most of the 

 other groups of Arach- 

 nida are predaceous 

 — preying for the most 

 part on Insects or 

 other Arachnids. To 

 capture the Insects 

 which constitute 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 manufacture of a cocoon 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, and frequently also 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 itself down safely 

 from considerable heights, and even to float in the air. 



MM ^'■^'" 



Fio. 557. - Section of the central eye of EuscorpiuB. 



Letters as in preceding- figure, pigm. cells containing 

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

 ectoderm). (After Lankester and Bourne.) 



