150 Ciiuitniali Society of S'atmal History. 



Class. — HVDKOZOA. 



This class comprises those Co-lenterates in which the walls 

 of the bod}' enclose a simple undivided cavity. No oesopha- 

 geal tube is present, but the upper end of the alimentary 

 tract may be prolonged into radiating canals united by a 

 peripheral ring. The reproductive organs are external buds, 

 often developed into specially modified zooids. (Nicholson, 

 Ibid, p. 192.) 



The simplest living type is the fresh-water //vrt'r^r. In this, 

 as in all the other members of the class, the ovum gives rise 

 to a " polypite," often capable of throwing out buds. These 

 generally remain attached to the parent " zooid," and a com- 

 pound organism is produced. Frequently the zooids are 

 differentiated into two sets; the members of one of these 

 supply the food, while the others act as the reproductive 

 bodies of the colony. These last are termed "gonophores," 

 and may remain attached to or become free from the parent. 

 The colonies are either free or attached by a modified end. 

 The ectoderm frequently secrets a chitinous or calcareous 

 outer layer, that may cover only the fleshy stem or " cceno- 

 sarc," or be extended into little cups or " hydrotheccc." When 

 the.se last are present the body of the polypite is contained 

 within the cup, while the tentacles are protruded from the 

 open end. 



But few genera of this class are preserved in a fossil state, 

 though great numbers are now found in the oceans. The 

 bodies are generally soft and illy-adapted for preservation ; 

 and although certain of these soft bodied animals (/. c. Mcdusce 

 or jellyfish) are supposed to have left impressions on the 

 rocks, generally only those secreting a horny, chitinous or 

 calcareous skeleton have been preserved. 



Three sub- classes are represented in the Cincinnati Group 

 in Ohio. These are Hvdroida, Graptolitoidka and 

 Stromatoporoidk.v. The features of these sub-classes are 

 given under their respective heads. 



Sub-class HvnROiD.\. 



A single order, Thkcapiiora, is represented. The general 

 features are : organism attached or at least capable of attach- 

 ment ; branching and plant-like, consisting of numerous 



