190 STUDIES IN EVOLUTION 



and tolerably homonomous segments. The head segment 

 was fused with the 4 subsequent trunk segments to form a 

 cephalic region, and carried a median frontal eye, a pair of 

 simple anterior antennae, a second pair of biramose antennte 

 and 3 pairs of biramose oral limbs, which already served to 

 some extent for taking food. From the posterior cei^halic 

 region proceeded an integumental fold which, as dorsal 

 shield, covered a larger or smaller portion of the trunk. The 

 trunk segments were each provided with one pair of biramose 

 limbs. Besides the median eye there were 2 frontal sensory 

 organs. The nervous system consisted of brain, oesophageal 

 commissures and segmental ventral chord, with a double 

 ganglion for each segment and pair of limbs. The heart was 

 a long contractile dorsal vessel with numerous pairs of ostia 

 segmentally arranged. In the racial form the sexes were 

 separate, the male with a pair of testes, the female with a 

 pair of ovaries, both with paired ducts emerging externally 

 at the bases of a pair of trunk limbs. The excretory func- 

 tion was carried on by at least 2 pairs of glands, the anterior 

 pair (antennal glands) emerging at the base of the second pair 

 of antennae, the posterior (shell glands) at the base of the 

 second pair of maxillte. The mid-gut possibly had segmen- 

 tally arranged diverticula (hepatic invaginations)." 



The characters ascribed to the typical nauplius have been 

 selected mainly on the principle of general average. They 

 do not satisfy the theoretical demands resulting from a com- 

 parative morphological study, nor are they consistent with the 

 accepted requirements of an ancestral type of the Crustacea. 

 Claus ^^ urges that the nauplius is a modified or secondary 

 larval form, and the writer now hopes to further substantiate 

 this view, and partly to reconstruct the nauplius from inter- 

 nal evidence and from its more primitive representative, the 

 protaspis of the trilobites. 



The usual features attributed to the nauplius are: three 

 pairs of appendages, afterwards forming two pairs of antennae 

 and the mandibles ; the first pair is uniramous and sensory in 

 function; the second and third pairs are biramous, swimming 



