214 PAL/EONTOLOGY 



graphy of little-known areas like China and British 

 Columbia, it will be possible to say with greater certainty 

 whether they were inhabited by faunas differing from 

 the strictly contemporaneous faunas of Europe. 



Ontogeny of Trilobites. Owing to the inability of 

 the exo-skeleton to grow with the animal's growth, the 

 adult trilobite, unlike a brachiopod or mollusc, retains 

 no trace of its early form. The only way to trace the 

 development of a trilobite is to find remains of a sufficient 

 number of individuals of all stages of development 

 (whether cases of infant mortality, or merely the skins 

 moulted in healthy development) to be sure they belong 

 to the same species. This has been done in a number 

 of cases, particularly by Barrande in Bohemia and 

 Beecher in North America, and the general course 

 of development has been established. The earliest 

 stage known has been termed the protaspis larva, and 

 is probably the form in which most trilobites were 

 hatched. At this stage, the whole dorsal shield is 

 less than i mm. long, and consists principally of head, 

 the thorax and pygidium being very rudimentary. The 

 trilobation is distinct, and the glabella shows five well- 

 marked somites. The eyes are marginal, probably as an 

 adaptation to the free-swimming life usual in Crustacean 

 larvae. By repeated moults the larva passes through 

 stages in which the post-cephalic part of the body 

 increases in length and in number of somites, gradually 

 attaining the adult state. 



The remaining divisions of the Arthropoda must be 

 dealt with much more briefly. 



