LIFE IN PRIMARY PERIOD 211 



of head. These organisms are grouped together under the 

 name Merostomata, signifying that the limbs encircle the mouth. 

 It is worthy of note that in this group, so homogeneous in 

 appearance, Euryptcrns and Pterygotus lasted but a short 

 while, whereas Limitlus has persisted almost unchanged through- 

 out twenty million years. 



Side by side with these organisms lived the Trilobites, in 

 which the first pair of appendages had already been trans- 

 formed into antenme, while the five other pairs preserved 

 the regular structure of the Merostomata, from which there is 

 no reason to separate them. Arising from the abdominal 

 articulations were very small feet surmounted by branchiae, 

 so fragile and delicate that for a long time they remained 

 unobserved. It is puzzling to see how such heavy organisms 

 could make use of them. They lived on the sand and could 

 no doubt descend to great depths, for some species are blind. 

 Others, on the contrary (JEglina), had enormous eyes. Two 

 longitudinal grooves running along the carapace, demarcated 

 a median area, the glabella, which they separated from the 

 genes, or cheeks, on either side of which were placed the eyes, 

 and these grooves extended the whole length of the abdomen, 

 whose terminal articulations were sometimes united in a 

 pygidium appendant to the carapace (Bronteus, Agnostus). The 

 body was thus divided into three longitudinal belts, hence the 

 name of Trilobite. Embryos of Trilobites have been recovered, 

 those of the Cambrian genus Sao in particular, and it has been 

 established that after the carapace and the last segment had 

 been formed, the others were formed one by one in front of 

 the terminal segment, so that the method of segment formation 

 nowadays common to Arthropods, Annelid Worms, and 

 Vertebrates dates back at least twenty million years. That is 

 equivalent to saying that it has never varied, any more than 

 the mechanical conditions which determined it. 



In Primary Times the Trilobites were distributed through- 

 out all the seas. They were especially numerous during the 

 Silurian and Devonian Periods, but in the Carboniferous 

 were represented only by Prcetidse, themselves reduced to 

 two genera, P rectus and Phillipsia, somewhat resembling the 

 Cambrian Paradoxides, the oldest Trilobites known. Trilobites 

 present a large variety of forms. Not only can we distinguish 

 littoral and deep sea species, but also, as one might say, local 



