NATURAL CLASSIFICATION OF THE TRILOBITES 121 



stage of embryonic development before the escape of the 

 young); the mesembryo, or bias tosph ere ; the metembryo, or 

 gastrula; the neoembryo, or planula-like stage; and the 

 typembryo, when the first distinctive features make their 

 appearance. The first embryonic stage recognized in the 

 trilobites can be referred to the phylembryo, as defined by 

 Jackson, 22 when the animal may be clearly referred to its 

 proper class. Since this period is distinctive for each class 

 of animals and usually bears a separate name, it has been 

 termed by the writer 5 the protaspis stage of trilobites. It 

 closely approximates the protonauplius form, or the theoret- 

 ical, primitive, ancestral larval form of the Crustacea. Like 

 the homologous nauplius of modern higher Crustacea, it is 

 the characteristic larval type common to the class. The 

 nauplius is therefore considered as a derived larva modified 

 by adaptation. 



The post-embryonic stages of ontogeny have received the 

 names nepionic, for the infantile or young; neanic, for the 

 immature or adolescent; ephebic^ for the mature or adult; 

 and gerontic, for the senile or old. When especially applied 

 to trilobites, the nepionic stages may include the animal 

 when the cephalon and pygidium are distinct and the thorax 

 incomplete. There would thus be as many nepionic stages 

 as the number of thoracic segments. The neanic stages 

 would be represented by the animal with all parts complete, 

 but with the average growth incomplete. Final progressive 

 growth and development of the individual would fall under 

 the ephebic stage. Lastly, general evidences of senility 

 would be interpreted as belonging to the gerontic stage. 



Application of Principles for Ordinal Divisions. 



In other classes of animals above the lower coelenterates, 

 the ph}'lembryonic stage is the starting-point from which 

 correlations are made, and out of which all the higher groups 

 are developed by a series of changes along certain lines. 

 The protoconch represents this period in the cephalopods 



