386 CHARACTERS OF INVERTEBRATE ANIMALS 



an insect, into well-defined head, thorax, and abdomen, for the 

 parts commonly considered as equivalent to the first two of these 

 are fused together into one mass known as the cephalo-thorax, 

 which consists of at least six segments, for it bears six pairs of 

 appendages. The remainder of the body is made up of twelve 

 rings or segments, the last five forming a narrow tail which is 

 carried bent up over the body, and bears at its tip a pear-shaped 

 spine on the sharp end of which open two poison-glands. Here 

 we have an example of a true "sting", like that of a bee or 

 wasp, as contrasted with the biting arrangements found in 

 serpents, bugs, and gnats. 



The appendages of a Scorpion differ strikingly from those of 

 an insect. Instead of antennae, three pairs of jaws, and three 

 pairs of walking-legs, we find two pairs of grasping organs and 

 four pairs of walking - legs, while feelers or antennae are not 

 present as such. The first pair of these appendages are short, 

 strong, forwardly- directed nippers (chelicerae), and they are 

 followed by a very large second pair (pedipalps), which end in 

 strong pincers much like those of a lobster and used for seizing 

 prey. The bases of these appendages are adapted for biting. 

 The possession of eight walking- legs is as characteristic of an 

 Arachnid as that of six is of an insect, and this affords the 

 simplest means of distinguishing the members of the two classes. 



The two segments which immediately succeed the cephalo- 

 thorax also bear structures which are interpreted as appendages, 

 the first being fused into a small plate (operculum) notched 

 behind, while the others are comb-shaped organs (pectines) which 

 probably have a tactile function. 



An ordinary insect breathes by means of air-tubes which 

 open to the exterior by a series of pores on each side of the 

 body, but the respiratory organs of a Scorpion consist of four 

 pairs of "lung -books" which open by a corresponding number 

 of oblique slits placed on the under side of the segments following 

 the one upon which the comb-shaped organs are borne. Each 

 of these breathing organs consists of a cavity into which a large 

 number of thin plates project, these being packed together in 

 a way which suggests the leaves of a book, hence the name 

 lung-" book". An insect, again, typically possesses a pair of 

 large compound eyes, and simple eyes may be present in addition. 

 Here the eyes are all simple and are arranged on the upper side 



