DEFINITION OF THE CLASS ^ 3 



lated limbs. But in all these instances there is a period 

 of life when the creature possesses, though it subsequently 

 loses, the characters which determine its place in classifi- 

 cation. 



Under the Arthropoda are included five classes, two of 

 which are of very prominent importance in the economy 

 of the world. The five classes are the Crustacea, Pycno- 

 gonida, Arachnida, Myriapoda, and Insecta. A sixth class, 

 the Onychophora, is sometimes added for the sake of the 

 peculiar genus Peripcltus, but for the present it may be as 

 well to give this the rank of an order among the myria- 

 pods, a class represented by the familiar but unfavoured 

 centipede. The Arachuida contain spiders, scorpions, 

 mites, as well as some other less commonly known groups. 



The Pycnogonida (or Pantopoda), the sea-spiders, at 

 one time included in the Crustacea and at another time in 

 the Arachnida, have some remarkable peculiarities, inas- 

 much as the ovaries of the female are found as a rule not 

 in the trunk of the body, but in the thighs of the legs, and 

 when the eggs are laid they are usually carried about not 

 by the mother but in packets upon the oviferous feet of 

 the male. 



The Insecta are so strikingly distinguished by the 

 special number of their legs that this class sometimes 

 receives the name Hexapoda, the six-footed animals. 

 Beetles, bees, bugs, flies, fleas, moths, spring-tails, ear- 

 wigs, grasshoppers, and gnats, in countless profusion people 

 the globe, sometimes disputing possession with man him- 

 self or rendering his life a burden, at other times offering 

 him service direct or indirect of no mean value. It is in 

 this class, and in this class only, that the present state of 

 science reckons the number of species not only by scores 

 of thousands but by hundreds of thousands, and even by 

 millions. The class which stands nearest to the Insecta 

 in the multitude of known species is that of the Crus- 

 tacea, but the interval is so vast that, properly speaking, 

 the Insecta are in this respect first with no second. 



Of the numerous definitions which have been given of 

 the Crustacea, it w411 be s ifiicient to quote two. According 



