LIFE IN PRIMARY PERIOD 221 



body of the primitive Annelid. Articulate Brachiopods 

 attach themselves permanently to the rocks by the extremity 

 of this peduncle, and indeed a very large number of both non- 

 articulate and articulate species glue themselves to the rocks 

 by means of one of their valves. The Inarticulate 

 Brachiopods have a complete digestive tube open at both ends. 

 In the Rhynchonellidae, of which there are living specimens 

 as of Lingulidae, the body is apparently composed of two 

 body-segments, and ends in a ccecum. Among other 

 articulated Brachiopods, the digestive apparatus is reduced 

 to a sac which is prolonged as a thin filament directed toward 

 the hinge. According to the theory of cephalization, these 

 arrangements would clearly indicate an organism undergoing 

 reduction. 



It may be asked how creatures so simply organized, and 

 destined for a life of immobility, were able to multiply as they 

 did during the Primary Period, and become so abundant and 

 varied that they furnish geologists with precise stratigraphic 

 evidence. The non-articulated group is dominant in the 

 Cambrian, and is there associated with the Strophomenidae and 

 the Pentameridae, but after the Silurian numerous families, 

 including the Rhynchonellidae, are added to the former group, 

 and the latter undoubtedly also existed in the Cambrian. 

 Among the new forms we must cite Productus, with a much 

 rounded lower valve, which during the Carboniferous Period 

 attained one decimetre in diameter. The families, genera and 

 species, continue to multiply during the Devonian Period, and 

 it is then that the Terebratulidae, which still inhabit our present 

 waters, make their appearance. The Carboniferous, in turn, 

 is even richer, and it is only during the course of the Secondary 

 Period that the decline begins and continues to become more 

 pronounced down to the present day, when the Brachiopods 

 play an insignificant part. They could not have prospered 

 so well during the Primary Period unless the plankton, their 

 only possible source of food, had then offered abundant supplies 

 for which there was little competition. The Primary, there- 

 fore, must have been the epoch of microscopic floating Algae, 

 Protozoa, and minute embryos. 



This abundance of the plankton in the seas was equally 

 necessary for the evolution of the Crinoids, all of which are 

 fixed Echinoderms, living entirely on the small particles directed 



