704 



ARTHROPODA 



PHYLUM VII 



Final pro^^ressive growth and development of the individual would fall under 

 the'epheblc stage. Lastly, general evidences of senility would be interpreted 

 as belonging to the gerontic stage. 



Morphogeny. During the protaspis stage, several moults take place before 



Fig. 1351. 



Proetus jxirviuscuhis Hall. 

 Ordovician. A, Protaspis 

 mnch enlarged. B, Adult 

 slightly enlarged. 



Fio. 1352. 



Acidaspis tuberculata 



Conrad. Devonian. A, 



Protaspis enlarged. B, 



Adult reduced. 



Fig. 1353. 



Dahnanitina socialis (Barr.). 

 Ordovician. A, Protaspis en- 

 larged. B, Adult reduced. 

 (Figs. 1348-1353 after Beecher.) 



the complete Separation of the pygidium or the introduetion of thoracic Seg- 

 ments. These bring about various changes, namely, the stronger annulation 

 of the axis, the appearance of the free cheeks on the dorsal side, and develop- 

 ment of the pygidium by the introduetion of new appendages and segments, 

 as indicated by the additional grooves on the axis and limb. In the earliest, 

 or Cambrian genera, the protaspis stage is by far the simplest expression of 

 this period to be found. In the higher and later genera, the process of 

 acceleration or earlier inheritance has pushed forward certain characters until 

 they appear in the protaspis, thus making it more and more complex. 



Taking the early protaspis stages in Solenopleura, Liostracus or Ptychoparia, 

 it is found that they agree exactly with the foregoing diagnosis in its most 



elementary sense. Since they are the 



characters shared in common by all larvae 



^T?\ I MJ 1!^^ ^^ ^^^^ stage, they are taken as primitive, 



Wjl 'yW ^r ^^^ accorded that value in dealing with 



I *^ adult forms possessing homologous features. 



^ Therefore, any Trilobite with a large elon- 



gate cephalon, eyes rudimentary or absent, 



free cheeks ventral or marginal, and glabella 



long, cylindrical, and with five annulations, 



would naturally be placed near the begin- 



ning of any genetic series, or as belonging 



to a very primitive stock. 



Next must be considered the progressive 

 addition of characters during the geological 

 history of the protaspis, and the ontogeny 

 of the individual during its growth from 

 ., . , condition. It has been shown by Beecher that 



there is an exact correlation to be made between the geological and zoological 



nrofTi'if?.^ important structures not especially noticeable in all stages of the 

 o m anvn l"" ?^' \^''^ ^^^""^ ^^^^^^«^ themselves in the meta- 



Visual areas of the eyes, when such are present, their appearance on the dorsal 



Fio. 1354. 



Sfto hirmta Barrande. Cambrian ; Skrey 

 Bohemia. A, Protaspis. B-F, Nepionic stages 

 of development (after Barrande). 



the larval to its mature 



