630 ARACHNIDA. 



THIRD CLASS. ARACHNIDA, (Gr. ^&%v^, arachne, a spider.) 



These animals, including Spiders, Mites, and Scorpions, are 

 separated from Insects on account of their external form, struc- 

 ture and habits. They differ from Insects in having no antennse, 

 in the eyes, which are in most species eight, and even when two 

 in number, are never placed on the side of the head; in the legs, 

 which are usually eight, though in some species six, and in others 

 ten in number ; in the breathing apparatus, consisting of radiated 

 wind-pipes, communicating with a sort of gills inclosed in pouches 

 in the lower part of the abdomen. 



The skin of the Arachnida is in general rather leathery and 

 horny ; like the bones of the larger animals, giving support to 

 the soft parts, and attachment to the muscles, the legs being ex- 

 ternally united to a common breast plate, from which they radiate. 



The greater portion of these animals are carnivorous, and 

 furnished with organs adapted to their predatory life. 



Nerve-knots, or ganglia, make up the nervous system of the 

 Arachnida. These are uniform in their composition, and more 

 concentrated than in the Insects. 



The organ of hearing in these animals is not known ; though 

 it is certain that they hear. The eyes of Spiders and Scorpi- 

 ons, externally formed in exactly the same manner, " are smooth, 

 glittering, and without divisions ; and are as much dispersed as 

 those that are disposed at random over the body. The Wolf- 

 Spider, which catches its prey by leaping on it, has its eyes 

 placed in the same manner." 



Male spiders are uniformly much smaller than the females, 

 being often not one-fourth as large. 



The female spider lays nearly one thousand eggs in a season. 

 These are soft and compressible, before they are laid, lying in 

 the ovarium, or egg-bag, within the spider's body, squeezed to- 

 gether in a flat manner, but when laid, assuming a round form. 

 The eggs are excluded unlike those of birds, from a cavity just 

 behind the breast. Here there is a hook-like organ which the 

 spider can move in such a manner as to direct each egg to the 

 exact spot in the nest cup where it would have it placed. The 

 sense of touch in this organ must be very acute,- as by touch 

 alone it can be guided, the eyes being so situated in the upper part 

 of the head, that they cannot be brought within sight of the nest. 



Latreille arranges the Arachnida into two orders: 



I. Pulmonaria, (Lat. pulmo, a lung,) which have pulmonary 

 sacs or air-pipes for respiration, similar to those of Insects, and 

 from six to twelve eyelets. These include the COMMON SPIDERS, 



