164 STUDIES IN EVOLUTION 



of the trilobites were manifestly closer to the Entomostraca 

 and Malacostraca than to other arthropods, and therefore com- 

 parisons were drawn with these sub-classes of the Crustacea. 



In the following remarks only the main points of difference 

 between the views held by Professor Kingsley and myself are 

 dwelt upon : — 



If the trilobites are true crustaceans, as conceded, it is then 

 fair to expect a more or less close agreement between the lar- 

 val forms of both. In my paper on " The Larval Stages of 

 Trilobites" {American (reo^o^is^, September, 1895) I endeav- 

 ored to show this close agreement, and concluded that the 

 protaspis stage of trilobites could be homologized with the 

 nauplius larva of higher Crustacea. Professor Kingsley notes 

 the following differences : (1) The differentiated median and 

 pleural regions ; (2) the segmented cephalic region; (3) the 

 absence of a median eye : and (4) paired eyes. 



As to the first, I do not think the differentiation is much 

 greater than in the nauplii of Apus^ Ci/dops, Lucifer, and 

 others in which there are side regions. The pleural regions 

 cannot be considered as highly specialized characters, since 

 they are common to many groups, and each segment is con- 

 sidered as primarily consisting of tergum, pleura, and sternum. 



(2 ) The segmentation of the protaspis is very feeble in the 

 earliest stages, and is evidently emphasized from the fact that 

 the fossils are viewed as opaque objects and exhibit strongly 

 any inequalities of surface features, while living nauplii are 

 studied as translucent objects. Furthermore, any such dif- 

 ference cannot be real, since the nauplius shows its true seg- 

 mented nature in its paired appendages. 



(3) The apparent absence of a mecUan eye in the trilobite 

 protaspis could be taken as of some value were it not that 

 the fossils are not more than one millimetre in length, and 

 even under the most favorable conditions could hardly be 

 expected to show such small features as ocelli. Moreover, 

 the median eye may have been marginal or ventral, and there- 

 fore would not be seen in the fossil, which preserves only the 

 dorsal crust. 



