276 THE APODID^: PART n 



although apparently larger in proportion to the size 

 of the body. The rostrum is, however, clearly a new 

 structure. How can we explain its origin, at the 

 anterior edge of an Apus-like head ? 



According to our theory the anterior antennae once 

 pointed backwards, as do those of Apus. In 

 Ceratiocaris, however, we find them almost at the 

 anterior end of the head. It is not difficult to show 

 that this migration would almost necessitate the 

 formation of a rostrum. 



One variation on the primitive Apus type would 

 certainly be a species using their antennae forwards as 

 organs of sense. . Just as, in Apus, the eyes travelled 

 forwards, so, in process of time, the antennse might 

 tend to move forwards, but, by way of protection for 

 these, at first, delicate organs, we may suppose them 

 to have moved forwards in slight grooves on each 

 side of the median line. As they moved forwards 

 they may have become more and more developed, 

 not only as sensory organs, but as appendages, until 

 they projected freely from the front (as typical 

 Crustacean antennae). The rostrum is the remains 

 of the middle wall between the two grooves. It is 

 clear that such grooves could not exist on the under 

 surface of the head of an Apus without forming 

 a primitive rostrum. According to this view, the 

 rostrum was originally a necessary accompaniment 

 of the migration of the antennae from the sides of 

 the labrum to the front of the head. The articulation 

 of this rostrum was a secondary acquirement not in 

 itself difficult to imagine. 



