with Observations on the General Arrangement of the Articulata. 291 



Comparative Anatomy of the Head. 



We will now compare the anatomy of the head in the different families of 

 Chilopoda, in demonstration of the views I have advanced, respecting the 

 number and order of the segments which enter into its composition in all the 

 higher Articulata. 



First, then, of the Geophilidce. The four subsegments observed in the em- 

 bryo of Geophilus, at the bursting of the shell, and then in the act of uniting 

 to form the future cephalic segment, are faintly indicated in the perfect animal 

 (fig. 15.). They are numbered on the accompanying drawings, in their proper 

 order, from one to four. The first or antennal subsegment has almost dis- 

 appeared in this genus, and its boundary is indicated only by a very faint 

 transverse line; but its appendages, the antennae, are developed to a greater 

 extent than those of any other segment of the body, and their divisions are 

 multiplied in the exact ratio of the number of parts which usually enter into 

 the composition of the appendages of other segments in Geophilus. This 

 number is exactly doubled in the antennae, so that the formative principle 

 developed in the first segment of the head is expended on the appendages, 

 which become hypertrophied ; while the segment from which they originate, is, 

 in a corresponding ratio, arrested in its development. This change commences 

 at an early period of the embryo, since at the bursting of the shell, when the 

 appendages of all the other segments exist but as very minute tubercles, the 

 antennae are of great size, and are equal in length to the whole of the four 

 cephalic segments. 



The second subsegment does not give origin to moveable appendages, but is 

 the seat of the important function, — vision. This segment is always more 

 developed in Geophilus than the first, and is almost entirely occupied by the 

 great centre of the animal functions and instincts, — the brain. 



The third subsegment is developed to a greater extent than the second, and 

 gives origin to appendages which are the first moveable parts of the organs of 

 nutrition, — the internal maxillce. 



The fourth subsegment is enlarged to a much greater extent than either of 

 the others, and is equal in length to the whole of the three anterior segments. 

 This greater size appears to have some reference to the development and 



VOL. XIX. 2 Q 



