ANNULOSA ! MYBIAPODA. 207 



another, and each bears a single pair of limbs. In some cases, 

 however, each segment appears to be provided with two pairs 

 of appendages. This is really due to the coalescence of the 

 somites in pairs, each apparent segment being in reality com- 

 posed of two amalgamated somites. This is shown, not only 

 by the bigeminal limbs, bnt also by the arrangement of the 

 stigmata, which in the normal forms occur on every alternate 

 ring only, whereas in these aberrant forms they are found 

 upon every ring. 



The head always bears a pair of jointed antenna?, resembling 

 those of many Insects, and behind the antenna there is gene- 

 rally a variable number of simple eyes. 



The young, in some cases, on escaping from the egg, possess 

 nearly all the characters of the parents, except that the number 

 of somites, and consequently of limbs, is always less, and 

 increases at every change of skin ('moult' or 'ecdysis'). 

 In other cases, there is a species of metamorphosis, the embryo 

 being at first either devoid of locomotive appendages, or pos- 

 sessed of no more than three pairs of legs, thus resembling 

 the true hexapod Insects. In these cases the number of legs 

 proper to the adult is not obtained until after several moults, 

 the entire process being stated to occupy in some species as 

 much as two years, before maturity is reached. 



The Myriapoda are divided into two orders, viz. the Chilo- 

 poda and the Gliilognatha. 



ORDER I. CHILOPODA. This order comprises the well-known 

 Centipedes and their allies, and is characterised by the number 

 of legs being rarely indefinitely great (usually from 15 to 20 

 pairs), by the composition of the antennas out of not less than 

 14 joints (14 to 40 or more), and by the structure of the 

 masticating organs. These consist of a pair of mandibles with 

 small palpi, a labium, and two pairs of * maxillipedes,' or foot- 

 jaws, of which the second is hooked, and is perforated for the 

 discharge of a poisonous fluid. There is not more than one 

 pair of legs to each somite, and the last two limbs are often 

 directed backwards in the axis of the body, so as to form a 

 kind of tail. 



Scolopendra (fig. 60), Litliobius, and Geopliilus are common 

 European genera of this order. The ordinary Centipedes of 

 this country are perfectly harmless, but those of tropical 

 regions sometimes attain a length of a foot, or more, and these 

 are capable of inflicting very severe, and even dangerous, 

 bites. 



ORDER II. CHILOGNATEA. This order comprises the Milli- 

 pedes (lulidce) and the Gaily worms (Polydesmus). The order 

 is characterised by the great number of legs, each segment 



