LARVAL STAGES OF TRILOBITES 187 



older stages and with the nauplius and metanauplius stages 

 of Apus. 



It cannot be doubted that the protaspis had five pairs of 

 limbs on the head portion and one or more on the pygidium, 

 and although these are the main points necessary to prove the 

 argument in the next section, on the nauplius, yet it seems 

 perfectly warrantable and better for graphic purposes to attach 

 the required number of elementary limbs to the ventral side 

 of the protaspis, as represented in Plate V, figure 1. 



There are other organs and structural details occurring in 

 the nauplius and in adult trilobites, which deserve recogni- 

 tion in a restoration of the protaspis stage. First among these 

 is the labrum, or upper lip. Nowhere is this plate so well 

 developed and so striking a ventral feature as among the tri- 

 lobites. There can be no hesitation, therefore, in accepting 

 this as characteristic of the protaspis. 



The trilobites and most recent crustaceans have a metas- 

 toma, or lower lip. This is already developed in the nauplius 

 stage of some Crustacea, as EupJiausia and Peneus, and prob- 

 ably represents an early larval character. It usually appears 

 as a median plate divided into two small plates, or lappets, on 

 each side of the median line, posterior to the mouth, and is 

 thus represented in the restored protaspis. As it occurs on a 

 segment bearing also a pair of legs and has no separate neu- 

 romere, it cannot well be considered as representing a somite. 



An anal opening is found in most nauplii, especially in 

 those of the non-parasitic Crustacea, and in those in which 

 this stage is normal and free-swimming. The protaspis, as 

 representing a free-swimming larval stage of trilobites, there- 

 fore probably possessed an anal opening. 



The only character represented in the restoration which is 

 accepted purely from analogy is the median unpaired eye. 

 This organ is almost universally present in the nauplius, and 

 is regarded as a very primitive character wherever found. 



The next and last structures to be noticed are the free- 

 cheeks and the beginnings of the paired eyes, as shown in 

 Plate V, figure 1 (#, oo). Their existence has already been 



