246 MANUAL OF ZOOLOGY. 



however, each segment appears to be provided with two pairs 

 of appendages (fig. 82). This is really due to the coalescence- 

 of the somites in pairs, each apparent segment being in reality 

 composed of two amalgarnated somites. This is shown, not 

 only by the bigeminal limbs, but also by the arrangement of 

 the stigmata, which in the normal forms occur on every alter- 

 nate ring only, whereas in these aberrant forms they are found 

 upon every ring. 



The head always bears a pair of jointed antennae, resembling 

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

 rally a variable number of simple sessile eyes. In one species 

 (Scutigerd) compound facetted eyes are present ; and in Pauro- 

 piis the antennae are bifid, and carry many-jointed appendages, 

 thus differing wholly from the antenns of Insects, and pre- 

 senting a decided approximation to the Crustacea. 



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 in- 

 creases at every change of skin (" moult " or " ecdysis "). In 

 most 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. It is believed, however, that the legs of 

 these hexapod larvee do not correspond homologically with the 

 three pairs of legs proper to adult 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 three orders — viz., the Chi- 

 lopoda, the Chilognatha, and the 'Pauropoda. 



Order I. Chilopoda. — This order comprises the well- 

 known carnivorous Centipedes and their allies, and is charac- 

 terised by the number of legs being rarely indefinitely great 

 (usually from 15 to 20 pairs), by the composition of the an- 

 tennae 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. The body in all the Chilopoda is 

 flattened, and the generative organs open at the posterior end 

 of the body. 



Scolopendra (fig. 81), Lithobius, and Geophilus are common 



