CHARACTERS OF ARACHNIDANS. 61 



rally globular, soft, and attached to the thorax by a sort of 

 peduncle, never affords origin to legs. 



4. The skin never possesses the hardness remarked in that of 

 crusta'ceans ; generally it is rather coria'ceous than horny ; some- 

 times it has considerable consistence, and, in all cases, it forms a 

 kind of external skeleton, to which the muscles designed to pro- 

 duce motion are attached. 



5. Most arach'nidans are terrestrial animals, and accordingly, 

 their legs are formed for walking or leaping. These organs are 

 often very long, and are ordinarily terminated by two hooks. 

 Of the senses of hearing and smell in these animals very little is 

 known ; on the upper and anterior part of the body, which repre- 

 sents the head, we find in almost all a certain number, commonly 

 eight, shining points, which are the eyes. They are called simple 

 eyes, to distinguish them from the compound or net-like eyes of 

 insects ; each one consists of a little, transparent cornea, which 

 is convex and without any trace of division ; beneath it we find 

 a small vitreous body, a layer of colouring matter, and the ter- 

 mination of the optic nerve. 



6. The nervous system of arach'nidans (Jig> 56) is composed, 

 1st, of a pair of ganglia situated in the head in front of the 

 oesophagus ; 2d, two nervous cords which pass from this species 

 of brain into the thorax, forming a collar around the oesophagus ; 

 3d, a nervous mass situated in the thorax, beneath the digestive 

 tube, composed of a certain number of ganglia which are com- 

 monly agglomerated; 4th, of one or more abdominal ganglia; 

 and 5th, of nerves which pass from these different ganglia to all 

 parts of the body. 



7. Most arach'nidans are carnivorous. Some have their mouth 

 armed with cutting or sharp jaws, and feed on insects which they 

 seize alive ; some fix themselves on other animals and live by 

 sucking their blood ; these parasites have a mouth formed like a 

 sucker. We distinguish in the apparatus of manducation of the 

 first: 1st, a pair of mandibles, which are generally armed with 

 a 'movable claw; 2d, two jaws bearing articulated palpi; 3d, a 

 small lip without palpi. The digestive canal extends to the 

 extremity of the abdomen ; close to the mouth we find salivary 

 organs which open into the first joint of the mandibles, and 

 appear to secrete a venomous liquid. And biliary tubes, which 

 form a substitute for a liver, are attached to the digestive tube 

 further back. 



4. What is the character of the skin of arach'nidans ? 



5. What is the character of the eyes of arach'nidans ? 



6. How is the nervous system constituted .' 



7. What is the character of the mouth in arach'nidans ? 



6 



