118 



COELENTERATA— HYDROZOA 



PHYLUM II 



also in Lias and Upper Jurassic. C. radians Fisch., is an important rock- 

 builder in the Russian Lower Car- 

 boniferous, especially near Moscow. 



Dahlia E. and H. Silurian. Tetra- 

 dium Dana. Ordovician and early Sil- 

 urian. Pseudochaetetes Haug. Upper 

 Jurassic; Europe. P. polijjyor us (Qnenst.). 



Fu:. 184ti. 



Chaetetes septosus Fleni. Lower Car- 

 boniferous ; England. A, Transverse 

 section parallel to upper surface. B, 

 Vertical section both enlarged ; ji, Pro- 

 jecting spines representing uncompleted 

 fission (after Nicliolson). 



Fig. 1846. 



Chaetetes radians Fischer. Lower Carbon- 

 iferous ; Moscow, Russia. Portion of longi- 

 tudinally fractured corallum, natural size. 



Geological Range of the Tabulata. 



With but few exceptions the Tabulata are restricted to Paleozoic for- 

 mations, and from the Ordovician to the Carboniferous systems inclusive 

 occur in considerable profusion, being associated with the Tetracoralla and 

 certain Hydrozoa (Stromatoporoids) in the building of large coral reefs. Of 

 the various families constituting this group, the systematic position of which is 

 uncertain, the Halysitidae are limited to the Ordovician and Silurian, and the 

 remainder, including the genus Chaetetes Fischer, are represented continuously 

 from the Ordovician to the Carboniferous. In the Middle Cambrian shales 

 of British Columbia, Walcott has recently discovered a remarkably well- 

 preserved actinian, named Mackenzia, which appears to belong to the family 

 Edwardsiidae, and to be closely related to the genus Edwardsia. 



Class 2. HYDROZOA Huxley. Hydroids and Medusae.^ 



Sessile or free-swimming polyps or polyp stocks, without oesophageal tube, and with 

 simple gastrovascular cavity not divided into radial pouches. 



The Hydrozoans are organisms which rarely secrete hard parts, and hence 

 are ill-adapted for preservation in the fossil state. The ramifying polyp stocks 

 are usually inferior in size to those of the Anthozoa, and possess always a 

 simpler structure ; dimorphism or polymorphism is, however, exhibited by the 

 different individuals, some of which perform solely vegetative, and others only 

 reproductive or protective functions. Of great interest is the prevailing alterna- 

 tion of generations, in which process fixed polyp stocks give rise to a generation 

 of free-swimming Medusae, the eggs of which develop in turn into polyps. 



' Huxley, T. If., Tlic Oceanic Hydrozoa. London, l^^^. — Agassiz, A., North American Aca- 

 lephae. 111. Cat. Miiscuni Conip. Zool. Cambridge ii., 1865. — Hincks, T., Natural Historj' of the 

 British Hydroid Zoophytes. London, 1868. — Clans, C.,, Uutcrsuchungen iiber die Organisation, etc., 

 der Medusen. Leijizic, 1883. 



