2966 THE JOINTED ANIMALS 



name is consequently often superseded by the later but more appropriate term 

 Gnathopoda, meaning foot jawed, which refers to a characteristic that is perfectly 

 distinctive of all the species included under the heading. This is the transforma- 

 tion into jaws, or gnathites, as they are sometimes called, of one or more pairs of 

 the appendages that lie at the sides of the mouth, or just behind it. The number 

 of pairs involved in the formation of jaws varies from one to six, the smallest 

 being found in Peripatus, and the largest in crabs and their allies, while between 

 these 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 characteristics in common. Thus the body is bilater- 

 ally 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 consider- 

 ably in size and structure in different regions of the body. In both types, more- 

 over, 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 feel- 

 ers, called antennae. 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 

 in the separate segments, the points of union being marked by swellings called 

 ganglia, from which nerve threads radiate to the neighboring 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 segmeutally-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 

 supplied internally with a rich system of branching tubes which open on the exte- 

 rior by means of apertures placed in the sides of the segments. These tubes are 

 known as trachea, 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 



