704 



ARTHROPODA 



PHYLUM VII 



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. 



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



13ol. 



I'lXiitus parviusculus Hall. 

 Ordovician. A, Protaspis 

 iimch enlarged. B, Adult 

 slightly enlarged. 





'-Y,-.' 



Fin. 1352. 



Acidas2)is tuherculata 

 Conrad. Devonian. A, 

 Protaspis enlarged. B, 

 Adult reduced. 





Fig. 13")3. 



Dahnanitina socialis (Barr.). 

 Ordovician. A, Protaspis en- 

 larged. B, Adult reduced. 

 (Figs. 1348-1353 after Beecher.) 



I 



the complete separation of the pygidium or the introduction 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 introduction 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 Pfychoparia, 

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

 ^ j> c elementary sense. Since they are the 



chai'acters shared in common by all larvae 

 at this stage, they are taken as primitive, 

 and accorded that value in dealing with 

 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 

 Cambrian • skrey addition of characters during the geological 



"iopmlnHSBanfndVr'''""''''''^^^^ ^^"^^^'^ °^ ^^^ protaspis, and the ontogeny 



of the individual during its growth from 

 the larval to its mature condition. It has been shown by Beecher that 

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

 succession of first larval stages and adult forms, and therefore both may be 

 reviewed together. 



The first important structures not especially noticeable in all stages of the 

 protaspis are the free cheeks, which usually manifest themselves in the meta- 

 or para-protaspis stages, though sometimes even later. Since they bear the 

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



Fio. 13.04. 



.Sao hirsuta Barrande. 

 Bohemia, 

 of de 



