HYDROIDA. 195 



can be protruded from the terminal opening or " aperture " of the 

 hydrotheca (fig. 76 b, Ji). In some cases, not only are the nutritive 

 polypites protected within " hydrothecae," but the generative buds or 

 gonophores are also provided with a horny or chitinous covering, 

 constituting what are known as "gonangia." Lastly, in the group of 

 the Hydrocorallines, and in a few other cases, the colony has the 

 power of secreting carbonate of lime, and thus of giving rise to a 

 calcareous skeleton, which in many cases is superficially similar to 

 the " corallum " of certain of the Actinozoa. 



With regard to the distribution of the Hydrozoa in space, the great 

 majority of the organisms included under this head are marine, but 

 in a few cases {e.g., Hydra) the organism is found in fresh water. 

 As to the general distribution of the class in time, the Fresh-water 

 Polypes (Hydrida), the Oceanic Hydrozoa (Calycophoridce and Phy- 

 sqphoridce), and the Calycozoa {Lucernaria, &c.) have left no traces 

 of their past existence, as might have been anticipated from their 

 want of hard parts. The Trachymedusce and Acraspeda (Jelly- 

 fishes and Sea-blubbers) are equally destitute of hard structures, and 

 their absence from the palaeontological record might have been 

 confidently predicted. Under favourable circumstances, however, 

 these soft-bodied organisms are capable of leaving impressions in 

 soft mud or sand, by which their past existence may be determined ; 

 and such impressions have been recognised in deposits even as 

 ancient as the Cambrian. The great group of the Graptolitoidea 

 is not only entirely extinct, but is wholly restricted to the older 

 Palaeozoic rocks. The equally great group of the Stromatoporoidea 

 is also extinct, and is principally confined to the earlier Palaeozoic 

 deposits, though fossils which are probably referable here are also 

 found in the Mesozoic rocks. The Corynida and Thecaphora, both 

 largely represented in recent seas, have left but few and imperfectly 

 connected traces of their existence in past time ; and some of the 

 forms usually referred to these orders, and especially the more 

 ancient ones, are of more or less dubious affinities. The recent genus 

 Hydractinia, among the Corynida, occurs, however, in rocks as old 

 as the Cretaceous. Lastly, the existing group of the Hydrocorallines 

 is represented by fossil forms, which begin as early as the Trias. 



In the following more detailed summary of the main divisions of 

 the Hydrozoa and of their principal fossil forms, those groups which, 

 as above mentioned, are not known to be represented in past time 

 are left out of account. 



Sub-class Hydroida. 



The sub-class of the " Hydroid Zoophytes" comprises a large 

 number of Hydrozoa in which the organism is attached, or is cap- 



