CHAPTER IX. 



OF THE CLASS OF ARACHNIDA. 



738. This class is composed of Articulated animals, which 

 have a great analogy with Insects, 

 and which are equally fitted to live in 

 the air ; but which are distinguished 

 from them at the first glance, by 

 the general form of the body, and by 

 the number of limbs ; and which 

 differ also from those animals, in 

 several important particulars of their 

 internal structure. All the Arach- 

 nida have the head united with the 

 thorax, and are destitute of antennae ; 

 they have four pairs of legs and no 

 wings ; most of them breathe by 

 means of air- sacs, instead of by pro- 

 longed tracheae ; and nearly all of 

 them have a complete circulating 

 apparatus. 



739. The tegumentary skeleton of Arachnida is generally 

 less firm than that of Insects ; and their body is composed of two 

 principal parts, nearly always distinct : one called the cephalo- 

 thorax, because it is formed by the head and the thorax united 

 into a single mass ; the other termed the abdomen, and com- 

 posed sometimes of a series of distinct rings (such as we see in 

 the Scorpions Fig. 447), and sometimes of a soft globular mass, 

 without any evident divisions (as is the case among the ordinary 

 Spiders Fig. 442). 



740. The organs of locomotion are all fixed to the cephalo- 

 thorax, and consist of eight legs, very similar to those of Insects, 

 and nearly always terminated by two hooks. In general theic 



Fro. 435. MYOALK. 



