4 ANIMAL PARASITES. 



are extremely numerous. The hinder three pairs are usually 

 similar ; the anterior is rather a foot-jaw. The last tarsal joint 

 bears one or two claws, or a pedunculate adhesive lobe fin the mites), 

 or internally a sort of comb or a series of bristles (weaving spiders). 



3. The oral organs vary greatly. The principal weapons are 

 the antenneal jaws, which are of the form of a knife, dagger, or 

 scissors, or furnished with a thick process with a sharp claw. 

 Behind these are the scissor-like or many-jointed maxilla, with 

 very slightly developed palpi at the base, and, beside these, soft, 

 puffed, sucking lips. In the mites, these oral organs stand upon 

 an elongated proboscis with a thick base. 



4. The nervous system in the majority is much fused together. 

 The mites have only a ventral ganglion with a simple cesophageal 

 band, and without cephalic ganglia; the spiders have a cephalic 

 ganglion, an enormous thoracic ganglion, and a ventral ganglion, 

 which is usually small, but rarely deficient ; the scorpions have a 

 ventral chain of ganglia. 



5. The organs of the senses usually consist only of simple 

 eyes, to the number of 2 5, seated laterally or in groups upon 

 the cephalothorax or upon the back. The optic nerve is dilated 

 into the form of a beaker, and surrounded by dark pigment 

 membrane; the vitreous body is globular, and the cornea roundish. 

 Some of them are blind. The true spiders appear to possess the 

 senses of hearing and smelling. 



6. The intestine is divided into a thin, horny, subsequently 

 muscular, oesophagus, and a straight intestine, opening behind, 

 without a stomach (scorpions and Crustacea) ; or the same parts 

 occur with a stomach and all sorts of csecal appendages, which often 

 distribute themselves through the whole body, even into the palpi 

 and claws (the other Arachnida) ; salivary glands, a liver, in the 

 form of a granular coat of the intestine, or large lobate masses, 

 and urinary organs, as thin branched tubes, are present. 



7. The organs of respiration are wanting in many of the lower 

 forms ; in the higher ones they are delicate branched air-tubes, 

 with stigmata arranged in pairs, or flat air-sacs, which receive 

 the air through an opening in the belly, and in their interior 

 contain a number of plates, like the leaves of a book (that is to 

 say, a series of compressed tracheal stems). 



8. The circulatory organs present a tubular, many-chambered 

 heart, from which arteries are given off; they only occur where 

 the respiratory organs are developed. 



