HYDROZOA. 



153 



No. 486. Lithostrotion Canadense, Casieluau. 



This is one of the most 

 widelj' distributed and 

 characteristic fossils of 

 the Sub-Carboniferous, 

 marking characteristic- 

 ally the horizon of the 

 St. Louis Group, and 

 ranging from Northern 

 Iowa to Alabama. 



Henr}^ Co. , Iowa. 



No. 487. Streptelasma corniculum, Hall. 



This genus is related to Zaphrentis, and should be regarded, says Hall, as a 

 sub-genus. Cincinnati Group, Waynesville, Ohio. 



No. 488. Zaphrentis corniculum, E. and H. 

 Corniferous, Falls of the Ohio. 



CLASS HYDROZOA. 



Tlie inenibers of this class are various and complicated, but 

 they are briefly characterized as having the digestive cavity and 

 the body or somatic cavity identical, and the reproductive 

 bodies external. In its simplest form the body is a sac, attached 

 at its aboral end, composed of three cell-layers and provided 

 with hollow tentacles. A nervous system is rarely present as 

 a ring about the center of the disk. Nettling-cells (cnidse) are 

 characteristic. 



The class includes the Hydroidea, as the fresh-water Hydra^ 

 and its marine allies, the hydroid zoophytes ; the Discophora, 

 which include the Sea-Jellies ; and the Siphonophora, which are 

 swimming colonies of zooids of various functions, as the Portu- 

 gese Man-of-war (Physalia). The GraptoHtes are regarded as a 

 sub-class. 



Most members of the class multiply by continuous gemmation; 

 and some secrete a polypidom which may resemble true coral. 

 Many Sea-Jellies are only the free reproductive zooids (gonophore) 

 of hydroid forms ; and these furnish the best example of alter- 

 nation of generation with gemmation. 



The class is represented in the lowest fossiliferous rocks. 



