The Spider. 25 



many points show. It has no " antenniB." In most species eight 

 eyes exist, and even when there are but two, these are never placed 

 literally upon the head. It possesses eight legs, Avhile true insects 

 never have more than six. There is an entire absence of wings. 

 The head is united with the thorax, so that the body seems to be 

 formed of two principal divisions. 



II. Nervous System of Insect compared to the Spider. — But a 

 greater distinction lies in the " nervous system." In insects the 

 organ of sensation, which corresponds to the brain in man and 

 animals, consists of a series of knotted nerves, connected by a 

 double nervous cord, which commences in the head, and passes 

 backwards through the body. These knots are termed "ganglia," 

 and are themselves composed of two halves, from which nerves 

 branch off to all parts of the body. Now, whereas the insect has 

 from six to ten ganglia in various parts of the body, the spider has 

 but " two," which being more concentrated than those of the insect 

 are, consequently, of a higher order. 



III. Circulation in Spiders and Insects compared. — Again, as 

 regards the circulation, the mechanism in the spider is far superior 

 to that of the insect. The latter have a simple dorsal vessel to 

 propel the nutritive fluid through the body ; spiders have a heart 

 furnished with veins and arteries, which gives the blood a perfect 

 circulation. 



iV". Respiration Compared. — The respiration is performed by 

 a different kind of apparatus from that of the insect. In the latter 

 the "spiracles" (which are small canals opening from a number of 

 apertures along the body) lead into other canals which conduct the 

 air to the large " tracheae," which run along the sides of the body. 

 These " tracheae^' are the two main canals, and from them smaller 

 ones branch off, and subdivide, so as to pierce the whole body. 



V. Respiration in the Spider. — In the spider each spiracle 

 leads to a little chamber, of which the lining membrane is arranged 

 in a number of folds, like the leaves of a book, so that a large 

 surface is exposed to the air in a small space, and the air which is 

 admitted by the spiracles instead of being carried over the body by 

 canals, like the arrangement in insects, proceeds directly to this 

 chamber. These points of difference show that the spider and 

 insect are not to be reckoned in the same class. 



VI. Eggs of the Spider. — The arachnida do not undergo 

 " metamorphosis." The female when she lays her eggs constructs 

 cocoons for them, and the young come forth from the eggs perfect, 

 with the exception of the two forelegs, which are not developed 

 until a few days after birth. Looking at these eggs, and comparing 

 them collectively with the size of the female spider, it appears 

 almost incomprehensible how they can be contained in so small 



