618 



ARTHROPODA 



SUB-KINGDOM VII 



head portion predominating ; axis of cranidium with five unuuinl '/,>,, * , <il,l<nninal 

 portion usually less than one-third the length of the shield ; axis with from one to 

 several annulations ; pleural portion smooth or grooved ; eyes, when prwiif, < inferior 

 marginal, or submarginal ; free cheeks, when visible, narrow ami mar;// an/. 



The changes taking place during the growth of an individual are chiefly 

 the following : Elongation of the body through the gradual addition of the 

 free thoracic segments ; translation of the eyes, when present ; modifications 

 in the glabella ; growth of the free cheeks ; and final assumption of the mature 

 specific characters of pygidium arid ornamentation. 



In a classification of the stages of growth, the protaspis has the rank of a 

 phylembryo, and corresponds in value to the protoconch of Cephalopods, the 

 prodissoconch of Pelecypods, and the protegulum of Brachiopods. In its 

 geological history and the metamorphoses it undergoes to produce the perfect 

 Trilobite, accurate information can be gained as to what the primitive char-, 

 acters are, and the relative values of other features acquired during the long 

 existence of the class. 



Of the stages of growth after the protaspis, the nepionic may be considered 

 as including the animal when the cephalon and pygidium are distinct, and the 



FIG. 1272. 



Ptychoparin Kingi, Meek. 

 Cambrian. A, Protaspis en- 

 larged. B, Adult reduced. 



FIG. 1273. 



>'<") // ! r.-iutu, Barr. Cambrian. 

 A, Protaspis enlarged. B, Adult 

 reduced. 



FIG. 1274. 



Triarthrus Becki, Green. 

 Ordovician. A, Protaspis en- 

 larged. B, Adult reduced. 



thorax incomplete. There would thus be as many nepionic stages as there 

 are thoracic segments. The neanic stages would be represented by the 

 animal with all parts complete, but with the average growth incomplete. 



FIG. 1275. 



Proetus parviusculus, Hall. 

 Ordovician. A, Protaspis 

 much enlarged. I',, Adult 

 slightly enlarged. 



FIG. 1276. 



Acidasnis tuberculo.ta, 

 Conrad. Devonian. ^Pro- 

 taspis enlarged. B, Adult 

 reduced. 



FIG. 1277. 



Dalmanites socialis, Barr. 

 Ordovician. A, Protaspis i-n- 

 larged. J>, Adult reduced. 

 (Figs. 1272-77 after Beecher.) 



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 

 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, 



