LslRVAL STAGES OF TRILOBITES 191 



appendages; body usually unsegmented; anteriorly there is 

 a single median eye, and a large labruni, or upper lip; an 

 alimentary canal bent anteriorly, and ending in an anus near 

 the posterior end of the Ijody; a dorsal shield; the second 

 pair of antennte are innervated from a sub-oesophageal gan- 

 glion. Frontal sense organs and a rudimentary metastoma 

 are sometimes present. The trunk and abdominal regions 

 are not generally differentiated. 



Balfour 1 remarks of the nauplius that: "In most instances 

 it does not exactly conform to the above type, and the diver- 

 gences are more considerable in the Phyllopods than in most 

 other groups." This variation is indeed quite marked among 

 nearly all the groups besides the phyllopods, and furnishes 

 the facts for the conclusion that the hexapodous condition is 

 not primitive. 



On Plate V are represented some of the leading types of 

 nauplius structure, taken chiefly from the excellent compila- 

 tion by Faxon. 20 Bearing in mind the typical and average 

 characters of this larva, some of the variations will be briefly 

 reviewed. 



The nauplius of Apus, represented in Plate V, figure 2, 

 shows the rudiments of five trunk segments, which in a later 

 stage (figure 3) develop phyllopodiform appendages belong- 

 ing to the sixth, seventh, and eighth pairs of lindjs. They 

 are the anterior trunk appendages, and appear at a time when 

 the fourth cephalic pair is a mere rudiment while the fifth 

 is entirely undeveloped. The fourth and fifth pairs of head 

 appendages evidently must have some existence, though 

 undeveloped in the nauplius. The physical conditions of 

 nauplius life probably do not require them, and they there- 

 fore remain for a time quiescent or undeveloped. 



In figures 4, 5, 8, and 6, respectivelj^ of Branchipus, 

 Artemia, Lejjtodora^ and Limnaida^ the first pair of append- 

 ages becomes progressively shortened, until, in the last, 

 they almost disappear. Leptodora (figure 8) and Lcpidurus 

 (figure 7) also have rudimentary trunk segments and append- 

 ages (?/). Figures 9 and 10, of Daphnia and Moina (ivom 



