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 Peripdtus, 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 Arachnida 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 will be s ifficient to quote two. According 
