LARVAL STAGES OF TRILOBITES 198 



may be described as such most necessary organs. The third 

 pair perhaps belongs to this category, because as mouth parts, 

 generally provided with masticatory processes, they serve not 

 only with the others for locomotion, but also for conducting 

 food to the oral aperture." 



Another point in favor of the original pentamerous compo- 

 sition of the cephalic portion of the nauplius or protonauplius 

 is the dorsal shield which is present in many forms, and is 

 considered (vide Bernard n ) as a dorsal fold of the fifth seg- 

 ment. So that, in reviewing the nauplius structures, we find 

 here and there evidences of the entire series of head segments. 



Now, since the protaspis fulfils the requirements by hav- 

 ing five well-developed cephalic segments, and is besides the 

 oldest crustacean larva known, it is believed that, in so far, 

 at least, it represents the primitive ancestral larval form for 

 the class. 



The nauplius, therefore, is to be considered as a derived 

 larva modified by adaptation. 



Other variations in the characters of the nauplius occur, 

 but as they have clearly originated (a) from the parasitic 

 habits of the adult, (6) from embryonic conditions, or (c) 

 from earlier inheritance, they need not enter into considera- 

 tion here. Such, for example, are (a) the absence of an 

 intestine in Sacculina, (5) the absence of the median eye in 

 Daphnia and Moina^ and (c) the bivalve shell in Cypris. 

 The larval stages of other, and especially later and higher 

 groups of arthropods, offer more considerable differences and 

 need not enter into this discussion, which is aimed chiefly to 

 establish the genetic relationship between the protaspis of 

 trilobites and the nauplius of recent Crustacea. 



Summary. 



Barrande first demonstrated the metamorphoses of trilo- 

 bites in 1849, and recognized four orders of development, 

 which are now shown to be stages of growth of a single 

 larval form. 



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