THE ORGANIZATION OF TRILOBITES. 



CHAPTER I. 



ON THE VISIBLE STRUCTURE OF THE BODY OF THE TRILOBITES. 



SECTION I. 



The body of all Trilobites consists of three distinct divisions, which have received the 

 denominations of cajmt, thorax, and abdomen. They may be recognized as Articulata by this 

 characteristic alone. The first two divisions include many associated parts, constituting 

 the cephalothorax ; but these remain separated in the Trilobites, and this circumstance not 

 only greatly facilitates the special examination of their body, but also affords convincing 

 information respecting their afiinity to existing species. Postponing the investigation of 

 their affinity to the next chapter, we shall now consider the remains of the Trilobites, as 

 they are presented for our examination. 



SECTION II. 



The remains of the Trilobites are limited to the shell and its impressions, and no softer 

 part of their body has, or indeed could be preserved. Hence it appears to me certain that 

 all those parts which possess the hardness of the shell, or at least were clothed by any 

 substance as hard, must exist in the impressions of the Trilobites ; and that, on the other 

 hand, those parts which probably existed, but which are wanting in these impressions, did 

 not possess the firmness of the shell, and are absent on that account. If, therefore, as is the 

 case, we no longer perceive the entire abdominal sui'face of the Trilobite body with all its 

 attached organs, we must infer that they had a much softer membraneous covering and 

 consistency, but we can by no means infer that those parts did not exist. This view of the 

 subject is rendered more probable, when we observe the same quality of the abdominal 

 surface and its organs in still existing organic bodies which are similar to tlie Trilobites ; 

 indeed, a more particular comparison of the existing Trilobite remains with the shells of 

 such living animals raises our assumption to a positive certainty, since we also recognize the 

 greatest similarity in the latter. An accurate knowledge of the shell of the Trilobites is, 

 therefore, the first and most important requirement for the observer. 



