with Observations on the General Arrangement of the Artie ulata. 289 



segment of four parts is more clearly indicated than that of the cephalic. 

 We have already seen that every normal segment of the body is an integral 

 structure ; that it has its separate ganglion of nervous matter ; and that it 

 developes one pair of appendages. When one segment becomes atrophied by 

 the enlargement of another, or disappears by uniting with it, the appendages 

 of the atrophied segment either are not developed at all, or are arrested in 

 their further development. If this arrest takes place in the whole structure, 

 the result is merely a diminished size of the organ; but if it be partial, as 

 regards a portion of the structure, the result invariably is an alteration of form. 

 On the other hand, the appendages of the segment that becomes enlarged are 

 always fully developed, and perhaps also are hypertrophied. Now this is 

 exactly what takes place in the development of the segments that form the 

 basilar portion of the head in Geophilus (fig. 3.) and Scolopendra. At the 

 moment of bursting the shell the whole of the segments that form the basilar 

 region of the head are all equally developed (5, 6, 7, 8), and each one has the 

 rudiments of its appendages, a pair of minute tubercles, at its sides. This also 

 is the case with the other segments of the body, the tubercles of which are 

 afterwards developed into legs. During the very few minutes that elapse while 

 the shell and membranes are being fissured, a change takes place in the basilar 

 segments and tubercles. The first and second segments (5, 6) become less 

 distinct from each other, and, like the cephalic segments, unite; and the 

 tubercles of the second are considerably enlarged. These are the structures 

 which afterwards become the immense forcipated foot-jaws (f, g, h, i), the 

 true mandibles of the perfect animal, and which are the analogues of the 

 strong mandibles of Insects. The tubercles of the first segment (5) also are 

 further developed, and form the elongated palpi. But the third of these seg- 

 ments (7) is more and more encroached on by the second (6), and its tuber- 

 cles, which at first are arrested in their development, gradually disappear. 

 The second segment (6) and its appendages continue to enlarge ; in con- 

 sequence of which the first segment (5) also becomes partially atrophied, and 

 the further development of its appendages is arrested. The third and fourth 

 segments unite, like the first and second ; the third (7) becoming almost 

 entirely obliterated by the encroachment backwards of the second, and its 

 union posteriorly with the fourth. This latter segment is that which remains 



