THE CAMBRIAN PERIOD 



499 



Vermes and echinodermata. Sea worms left evidence of their 

 abundance by borings, tracks, etc. (Fig. 375). A few cystoids, the 

 forerunners of the beautiful crinoids (stone lilies) were present, 

 but fossil crinoids have not been found. 



4:^ 



Fig. 375. Cambrian Vermes: borings and trails, a, a natural surface of 

 sandstone seen from above, showing annelid borings, with mounds of 

 sand heaped about their mouths and with trails leading away from some 

 of them; b, a horizontal section, showing perfect borings, as well as 

 abandoned borings whose sides have been forced in during the forma- 

 tion of new borings, giving a cresentic cross-section. Although the 

 animal is not shown, the name Arenicolites woodi Whitfield, is assigned 

 to it. 



Coelenterata. The coelenterates were represented by hydrozoa 

 (graptolites and medusae) and anthozoa. The eccentric freaks of 

 fossilization are nowhere better illustrated than here. Relics of 

 graptolites, among the most delicate of animal forms, and of 

 medusae, among the softest of animals, were preserved, while some 

 much less easily destroyed forms left scant record of themselves. 

 The graptolites, now extinct, were slender, plume-like organisms, 

 consisting of a series of" indurated cells, in which the individual 

 zooids lived, attached to a common slender axis which united the 

 colony. The whole colony appears to have floated free in the sea 

 (Fig. 376, e). The secret of their preservation probably lies in the 

 fact that, being floating forms, they often settled in quiet and rather 

 deep waters off-shore, where fine silts accumulated, and where the 



