128 



THE ORIGIN AND EVOLUTION OF LIFE 



Cambrian strata members of this order differentiated into at 



least three widely distinct famiUes. 



The worms, including swimming and burrowing annulates, 



are represented in the Bur- 

 gess fauna by a very large 

 number of specimens, com- 

 prising nineteen species, dis- 

 tributed through eleven 

 genera and six families. 

 Most of these are of the 

 order Polycha?ta, as, for ex- 

 ample, Worthenella cambria, 

 in which the head is armed 

 with tentacles, while the 

 segmented body and the 

 continuous series of bilobed 

 parapodia are very clear. 

 When compared with such 

 typical living polychaetes as 

 Nereis virens and Arabella 

 op alma (Fig. 25), we have 

 clear proof of the modern 

 relationships of these mid- 

 Cambrian species, as well as 

 of Cambrian sea-shore and 

 tidal conditions closely 

 similar to those of the pres- 

 ent time. A specialization 

 toward the spiny or scaly 

 annulates at this period is 



emphasized in such forms as Canadia spinosa (Fig. 25), a slowly 



moving form which shows a development of lateral cha^tae and 



Fig. 25. Worms (Annulata) of the IMiddle 

 Cambrian and Recent Seashores. 



Canadia spinosa, a mid- Cambrian form (after 

 Walcott) with overlappinj^ groups of scale- 

 like dorsal spines, resembling those of the liv- 

 ing AphroditidcE, such as Polyno'e sqiiamata. 



Worthenella cambria, a worm of mid-Cambrian 

 times (after Walcott) , compared with Nereis 

 virens and Arabella opalina, recent marine 

 worms. 



