ARTICULATA. 119 



which is followed by the abdomen, and this is either distinct or united in a 

 single piece with the former. These animals are not subject to a perfect 

 metamorphosis, but they have, in some cases, a partial one. They have 

 neither wings, antennae, nor upjier lip ; the number of feet is eight, affixed 

 to the sternum or lower side of the ccphalothorax ; the moutli is provided 

 with a pair of mandibles or chellcera which usually project well in front, a 

 pair of jaws or maxillfE supporting palpi, and a lower lip. The cJielicera 

 are considered by Latreille not to be equivalent to the mandibles of the 

 Crustacea and insects, but to correspond to the internal antennas of the 

 former, and modified into predaceous organs ; a view which, if correct, 

 would deny proper mandibles to these animals, although these organs have 

 an important place in the Articulata as a whole. Each of the two maxill?e 

 bears a palpus or articulated organ much like the ordinary feet, but smaller, 

 and like the ordinary feet of the Crustacea. (See j9?. 78, figs. 35-40.) In 

 Scorpio {j)l. ^7^ figs. 51, 52), the palpi are cheliform, and much larger and 

 stronger than the feet, resembling the first pair of true feet in the crab and 

 lobster. There is an affinity between Scorpio and the Xiphosura or genus 

 Limulus^ in the mandibles, which are cheliform, each ending in a pincer. 

 The feet correspond to those of insects, the round basal portion or coxa 

 being joined to the thigh ov femur by means of a short interposed tvoclianter.^ 

 the femur being followed by the tibia., and this by the tarsus., which has 

 three articulations in the more typical forms. 



The eyes are simple and smooth, and their number varies from two to 

 eight. Their number and relative position are much used as generic characters. 



The respiratory oi-gans are of two kinds. The first (which resemble those 

 of the Crustacea) are formed of numerous internal gills or laminte contained 

 in internal pouches, and answering to lungs. The second kind are com- 

 posed of tracheae or air tubes (as in insects), connected with two spiracles; 

 Those with the former are termed pulrnojiary., and the latter tracheary 

 aracJinidans. B(:)th kinds may occur united, and the Podosoraata {Pycno- 

 gonum., &c.), like some of the louver Crustacea, have no means of respira- 

 tion except the skin. 



The abdomen is generally soft and more or less globular in form, and it 

 bears the spiracles, anal and generative apertures (the latter being near the 

 base beneath), and the spinnerets in those forms which spin webs. The 

 skin is generally of a tough leathery texture, and the muscles are attached 

 to its internal surface, thus assimilating it to the external skeleton of the 

 Crustacea and insects. 



Most of the AracTinida feed upon fresh animal food, as insects, which 

 they take alive, either in their nets, or by running or suddenly leaping upon 

 them. Some, as the ticks, are parasitic upon various animals ; whilst others, 

 as some of the mites, feed upon vegetable matter, being found in flour and 

 figs. Some mites infest dried meat and cheese. 



The Araclinida are mostly oviparous ; the young, upon leaving the ^g'g., 

 are active, and resemble the adult, although some have a pair of feet less, 

 which are finally acquired after several moultings, for, like the Crustacea, 

 the members of this class change their integument from time to time. 



323 



