108 COELENTERATA—CNIDARIA CLASS II 
smaller tubes. Both series of corallites are provided with numerous horizontal 
tabulae ; and, as in Heliopora, the autopores are produced by the coalescence of 
anumber of adjoining coenenchymal tubes. In the Heliolitidae, however, 
twelve well-developed septa are uniformly present. The walls of the corallites 
are composed of homogeneous, compact, calcareous matter, and exhibit the same 

Heliolites porosa, Goldfuss. Devonian; Hifel. 4, Corallum, natural size. 
A, C D, Portion of outer surface 
enlarged. C, Longitudinal section, enlarged. 
histological structure as the Favositidae and Chaetetidae. Owing to these 
anomalies the systematic position of the Heliolitidae remains uncertain. 
Heliolites, Dana (Fig. 192). Corallum massive, nodular, or ramose. Auto- 
pores with twelve more or less strongly developed pseudosepta, though 
occasionally represented by rows of spinules, and frequently with central 
columella. Siphonopores without septa, and multiplying by fission or inter- 
mural gemmation. Abundant from Ordovician to Devonian. 
Plasmopora, EK. and H. Like Heliolites, but having walls of the siphonopores 
incomplete, and tabulae of contiguous tubes fused together so as to form a 
vesicular tissue. Ordovician to Devonian. 
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 
locomotive functions. Of great interest is the prevailing alternation of genera- 
tions, in which process fixed polyp stocks give rise to a generation of free- 
swimming Medusae, the eggs of which develop in turn into polyps. 
The Hydrozow are all aquatic, and with few exceptions are inhabitants of 
the sea. They are commonly divided into the two following sub-classes :— 
Hydromedusae and Acalephae. 
' Huxley, T. H., The Oceanic Hydrozoa. London, 1859.—<Agassiz, A., North American Aca- 
lephae (Ill. Cat. Museum Comp. Zool. Cambridge, II.), 1865.—Hincks, 7., Natural History of the 
British Hydroid Zoophytes, London, 1868.—Claus, C., Untersuchungen iiber die Organisation, ete., 
der Medusen. Leipzic, 1883. Cf. also references on pp. 109, 114, and 121. 
