CCELENTERATA — HYDROZOA TI5 



has resulted in driving off most of the hydrogen, oxygen, and 

 nitrogen, thus leaving as fossil only a film composed mostly of 

 carbon. 



It was very probably similar to Sertularia in the character 

 of its food, its digestive, excretory and nervous systems, and in 

 its assimilation and respiration. 



Its reproduction was likewise both sexual and asexual, and it 

 seems to have lacked the jelly-fish stage. The union of a sper- 

 matozoon and ovum gave rise to the first polyp which secreted 

 the earlier protective cup, called the sicula. From this by bud- 

 ding developed the colony (rhabdosome) . 



In the genus Diplograptus (Ordovician-Silurian) the cups 

 (hydrothecae) are arranged in tw^o rows along the stem (whence 

 the name from Greek diploos, double). In a side view, the vir- 

 gula shows as a raised ridge running through the middle of the 

 colony, while each cup has a thick outer edge owing to the com- 

 pression of its entire width. 



D. foliaceiis is abundant in the Utica (Ordovician) shales of 

 eastern North America and in the Ordovician of Europe. 



1. Sketch a portion of the colony, enlarging it three times. 

 Label cup, virgula, sicula (if present). 



2. In life what part of the animal was probably visible outside 

 the cup when feeding ? 



3. Give in tabular form resemblances to and differences 

 from Sertularia. 



4. What is the chemical composition of the fossil ? What 

 produced this change from its composition when living ? 



General Survey of Order Graptolithida 



Extinct, colonial Hydrozoa, fixed to some object upon the sea- 

 bottom, or floating upon the surface attached to drifting sea- 

 weed or to a pneumatocyst of their own manufacture. They 

 secreted a protecting and supporting skeleton of chitin, with 

 the separate individuals lodged in pits upon, or cups along the 

 margin of, the chitinous stalks. 



