GENERAL CHARACTERISTICS. 5 



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 specialised members towards 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. Never- 

 theless, underlying all the modifications of structure — however extensive these may 

 be — there is a common plan of organisation 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 

 locomotion ; 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 masticating 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 oesophagus with the brain, which is lodged in the head. The heart, lying 

 above the alimentary 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— (1) Insects (Insecta, or Hexapoda) ; (2) Centi- 

 pedes (Chilopoda) ; (3) Millipedes (Diplopoda) ; (4) Spiders, Scorpions, Ticks, etc. 

 (Arachnida); (5) King-crabs (Gigantostraca) ; (6) Crustaceans (Crustacea): (7) Pro- 

 totracheata (Peripatus). 



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

 centipedes, and millipedes, for example, may be placed together as Tracheata, 

 characterised by the possession of tracheae and of a single pair of antennae. 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 limbs, and in having no antenna', strikingly resemble the 

 Arachnida. Peripatus is very peculiar, but shows signs of a distant relationship 

 with the centipedes, although in many anatomical features it is not very far 

 removed from the worms. 



Distinctive The term insect, although originally and, according to the mean- 



Characteristics ing of the word, correctly employed in a wide sens,, to embrace 

 ofinsects. a]1 those anmia l s i n which the body is externally divided into a 



