THE INSECTS 2967 



organ analogous to the gills of fishes. These it possesses in the form of delicate 

 plumes attached to the bases of the walking-legs and the sides of the body just 

 above them; and although concealed from view and protected from injury by a 

 large plate, these gills are yet freely exposed to the water in which the animal 

 spends its existence. Gills resembling those of the lobster in function, and also 

 substantially in structure, are found in almost all Arthropods that live in the sea. 



The characteristics that have been here briefly alluded to in the description of 

 the anatomy of the centipede and lobster will be found to be equally discernible, if 

 other prominent types of Arthropoda be examined. Differences of course will be 

 found to exist; but, on the whole, the plan of structure that has been sketched is 

 true for all the classes. For instance, in all of them, except the Centipedes and 

 Millipedes, there is a tendency in the more specialized members toward an increase 

 in size of the limbs in the front half of the body, accompanied by a corresponding 

 dwindling of those in the hinder part. Thus a crab and a spider walk upon four 

 pairs of legs placed just behind the head, and an insect upon three; and in the case 

 of the insect the legs of the hinder region have entirely disappeared, while the 

 larger number of them have similarly vanished in the spider and the crab. There is 

 also a tendency in the higher members of each class for the ganglia of the nervous 

 chord to lose their segmental arrangement, and to become concentrated together in 

 one large mass, placed near the seat of the greatest muscular activity. Neverthe- 

 less, underlying all the modifications of structure however extensive these may be 

 there is a common plan of organization which may be regarded as typical of the 

 Arthropoda. This may be briefly sketched as follows: The long bilaterally- 

 symmetrical body is divided into a series of approximately similar segments, each 

 bearing a pair of similar and segmented limbs. These limbs are the organs of loco- 

 motion; but some of those at the front end of the body, where comes the mouth and 

 the organs of vision, take on the function of jaws, and are used for seizing and mas- 

 ticating food instead of for progression. The nervous system consists of a double 

 ventral chord with ganglionic enlargements .in each segment, and the first ganglia 

 of this ventral chain are connected by means of a chord on each side of the oesopha- 

 gus with the brain, which is lodged in the head. The heart, lying above the ali- 

 mentary canal which runs from one end of the body to. the other consists of a 

 series of chambers, one for each segment of the body, and is provided with arteries 

 for the distribution of the blood, and with slits or ostia for receiving it back again. 



The Arthropoda are divided into the following classes, the chief characteristics 

 of which are described further on (i) Insects (Insecta, or Hexapoda); (2) Cen- 

 tipedes (Chilopoda); (3) Millipedes (Diplopoda); (4) Spiders, Scorpions, Ticks, 

 etc. (Arachnida); (5) King crabs (Gigantostraca); (6) Crustaceans (Crustacea); 

 (7) Prototracheata (Peripatus). 



It is possible, however, to group these into larger divisions. The insects, cen- 

 tipedes, and millipedes, for example, may be placed together as Tracheata, charac- 

 terized by the possession of tracheae and of a single pair of antennas. The 

 Crustacea stand alone in having two pairs of antennae and in breathing with gills. 

 By means, however, of the extinct class of the Trilobites, they are connected with 

 the king crabs; and these in possessing only six pairs of well-developed anterior 



