4 JOINTED ANIMATS. 
two extremes we meet with two pairs in the Millipedes, three in the Insects, and 
four in the Centipedes. 
The appendicular nature of the jaws, then, is the most distinctive feature of 
the animals now under discussion. But if two members of the Arthropoda, say for 
instance a lobster and a centipede, be compared together, they will be found to 
possess many other structural characters in common. Thus the body is bilaterally 
symmetrical, that is to say, if it be cut exactly in half lengthwise, the right and 
left portions will be precisely alike. It is, moreover, divided into a series of 
segments, placed one behind the other in a long series; each segment bearing a pair 
of limbs, which in the centipede are all alike, but in the lobster vary considerably 
in size and structure in different regions of the body. In both types, moreover, 
some of the segments at the front end of the body are modified by fusion, and in 
other ways, to form a head, which is furnished with eyes, and bears, in addition to 
the jaws, appendages that have been transformed into long, many-jointed feelers, 
called antennge. In the lobster, however, there are two pairs of these organs, while 
in the centipede there is but one. 
These external resemblances are correlated with others connected with the 
internal anatomy. The alimentary canal, for instance, traverses the body from 
end to end; and the nerve-chord lying beneath it consists of two adjacent strands 
united together in the separate segments, the points of union being marked by 
swellings called ganglia, from which nerve-threads radiate to the neighbouring 
parts. Above the alimentary canal comes the heart, and this organ, although 
superficially very different in the two types, is yet constructed upon the same 
general plan. In the centipede it is long, tubular, and composed of many distinct 
segmentally-arranged chambers, and furnished with arteries for the distribution of 
blood to the tissues, and with slits or ostia by which the fluid again makes its 
way back to that organ. In the lobster, on the contrary, the heart is short, thick, 
and consists of a single chamber, but is nevertheless provided with the arteries 
and slits as in the case of the centipede. 
The dissection of these two creatures would, however, reveal one fundamental 
difference between them. In the centipede it would be noticed that the body is sup¬ 
plied internally with a rich system of branching tubes which open on the exterior 
by means of apertures placed in the sides of the segments. These tubes are known 
as trachece, and their apertures as stigmata. They, or similar structures, are found 
in nearly all Arthropods that live upon the land and breathe the oxygen in the 
air. They are, in fact, the breathing organs, and analogous to the lungs. The 
lobster has no such system of tubes; for living in the water, and breathing 
the oxygen dissolved therein, this crustacean has need of a different type of 
respiratory 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 characters that have been here briefly alluded to in the description of the 
