48 



become the two pairs of antennae and the mandibles. The 

 first pair of antennae are comparatively slight, consist of one 



Fig. 17. Phyllopod Nanplius (Apus) when hatched (after Claus). 1, first pair of 

 antennEe ; 2, second pair of antennae ; 3, mandibles ; I, labrum ; o, median eye ; 

 s, segments of the post-cephalic region of the body. 



branch only, and are probably sensory organs. Between 

 their bases is placed a large upper lip or labrum, behind 

 which lies the mouth. The second antennae are very much 

 larger, and are biramous, with a spine projecting from near 

 the base inwards towards the mouth. These appendages 

 are the main organs of locomotion of the Nauplius. The 

 mandibles are rather smaller. They represent the mandi- 

 bular palps of the adult, and are biramous. On the front 

 of the head is placed a median unpaired eye. The post- 

 cephalic region shews traces of five segments and their 

 appendages, and in this respect diff'ers from most Nauplii, 

 which are unsegmented. 



Probably this Phyllopod Nauplius is the nearest form 

 known to the primitive Crustacean, but it must remain 

 doubtful whether or not additional post-cephalic segments 

 with simple biramous appendages were present. From 

 this form the existing group of Phyllopoda was probably 

 evolved very much as we find Apus developing from the 

 Nauplius stage at the present day. The modifications of 

 form and structure take place gradually, without any 

 sudden metamorphosis. This ancestral series of Proto- 

 Phyllopods has formed the main axis of the Crustacean 

 branch from which the various existing groups have been 



