BOOK 7. JOINTED ANIMALS 



CHAPTER I 



THE CRAB AND SCORPION GROUPS 



T 



BY W. F. KIRBY, F.L.S. 



iHIS section of animals is often called a " sub-kingdom," 

 and differs from back-boned animals in having the frame- 

 work of the body outside. That is, instead of a skeleton, 

 Crabs, Spiders, Centipedes, and Insects are protected either by a 

 hard shelly casing, or by a tough skin, to which the muscles are 

 attached ; and this arrangement renders them much stronger and 

 much less susceptible to injury, in proportion to their size, than 

 vertebrate animals. They have cold blood, generally of a white 

 colour ; and their bodies and limbs are usually composed of a 

 considerable number of separate joints. 



The group is a very large one, and it is probable that there 

 are at least 300,000 different 

 kinds of insects already known, 

 while the total number of spe- 

 cies now existing is estimated 

 by different entomologists as 

 from two to ten millions. It 

 is, therefore, no exaggeration 

 to say that every word of our 



Phttt tf E. Cmntld 



BARNACLES 



A species which commonly attaches 



itself to ships' bottoms and 



floating wreckage 



brief account of the Insects 

 represents from ten to twenty 

 known species at least. The 

 other classes of the group are 

 also very numerous. Our ac- 

 count must necessarily be very 

 short; the characters of the 



principal classes of the Jointed Animals are referred to in 



their places. 



CRABS, LOBSTERS, SHRIMPS, WOOD-LICE, BARNACLES, ETC. 



Jointed animals are generally provided with one pair 

 of long jointed organs, called " antennae," a naturalised 

 word derived from the Latin, in which language antenna 

 means a sail-yard. They are often called " feelers," and 

 usually fulfil this function at least; but they are also 

 frequently organs of smell, and sometimes probably of 

 hearing and other senses. One peculiarity of the CRAB 

 and LOBSTER group is that they are generally furnished 

 with two pairs instead of one pair of these organs. 



670 



Phttct, (T. P. Dand,, F.Z.S., Rtgtnt't Part 



A PAIR OF BARNACLES 



The larger size of this photograph exhibits more 

 structural details than the last 



