52 INTRODUCTION. 



resemblance to mites, approximate to the arachnides undergo 

 it; some of them however do not suffer this change in all its 

 extent, and it is then called imperfect. The organs of motion 

 are the muscles and the skin, hardened by a horny substance 

 contained within its thickness; there are six articulated feet, 

 and generally four wings, some have only two, while a very 

 small number are apterous. Their motions are greatly varied, 

 and consist in the walk, running, jumping and flight. The or- 

 gans of the sensations are compound eyes, and in many simple 

 ones, generally three in number; antennae and palpi. They enjoy 

 both hearing and smell, but their organs are unknown. The 

 disposition of the nervous system is indicated in 28, and ter- 

 minates anteriorly by a little enlargement or brain, situated 

 on the oesophagus, and is distributed to the eyes and other 

 parts of the head. 



43. The arachnides or octopoda, whose head, deprived 

 of antennae, confounds itself with the thorax, have eight feet 

 and no wings. The alimentary canal begins in the one by a 

 mouth with two lateral mandibles, in the other by a mouth 

 fitted for suction or by a proboscis. The greater number have 

 palpi, are subject to moults or a changing of skin, and not to a 

 metamorphosis. The sexes are separated, generation is ovi- 

 parous, and they have generally visible eyes, which vary in 

 regard to number and situation. 



They present two degrees of organization; the first or sim- 

 plest is that of tracheal arteries, where there are no organs of 

 circulation more apparent than in insects; the organs of respi- 

 ration are distinct branching tracheas. The most complex is 

 that of the pulmonary or branchial arteries, (spiders, tarantula 

 and scorpions.) They have a simple muscular heart, dorsal, 

 elongated, cylindrical, branchial or pulmonary, whence are de- 

 rived the vessels for the respiratory organs, which are pulmo- 

 nary sacs, and thence distributed to the whole body. There 

 is also a liver composed of lobules or grains, collected in clus- 

 ters. The sexual organs are double in each sex. Some of them 

 copulate repeatedly and live several years; the scorpions are 

 ovoviviparous. 



44. The myriapoda or centipedes form a little group of 

 animals, intermediate to the Crustacea which they resemble by 



