HYDROIDA. 197 



usually, the colony develops hard structures ("polypary" or 

 " ccenosteum "), which may have the form of an external chitinous 

 layer, or, in some instances, is composed of carbonate of lime. Re- 

 production takes place by fixed generative buds, or by the develop- 

 ment of free reproductive zooids ("gonophores "). 



Of the orders of the Hydroid Zoophytes, the only three which 

 have fossil representatives are the Corynida, the Thecafihora, and 

 the Trachymed?isce. 



Order i. Corynida. — The " Tubularian Zoophytes " or Corynida 

 are mostly composite, forming more or less plant-like, or encrusting 

 colonies, fixed to foreign objects proximally, and usually furnished 

 with a skeleton or " polypary." The skeleton is mostly in the form 

 of a chitinous investment, which encloses the ccenosarc (fig. 78, a), 

 but which is not developed into definite cups or " hydrothecae " for 

 the reception of the individual polypites. In some cases, however, 

 the skeleton consists of zooidal tubes, in which the polypites were 

 lodged, united by a general clathrate or tubulated ccenosarcal skele- 

 ton, as in the genus Hydractinia and its fossil allies. In the living 

 genus Bi?neria, also, the polypary forms an investment for the poly- 

 pites, and is even prolonged upwards as far as the bases of the 

 tentacles fig. 78, b). Though the skeleton of the Corynida is usu- 

 ally chitinous in composition, it is occasionally calcareous (as in 

 some species of Hydractinia and in the genus Parkeria). 



There are many recent types of the Corynida, such as the com- 

 mon Pipe-corallines (Tubularia), which, from the nature of their 

 skeleton, might quite well have been preserved in the fossil condi- 

 tion. The only recent genus, however, of which any fossil repre- 

 sentatives are known is Hydractinia, the colonies of which are well 

 known as forming crust-like investments upon shells. The species 

 of Hydractinia form chitinous — or rarely calcareous — crusts upon 

 the exterior of the shells of Gastropods (fig. 79, a), the shell thus 

 affected being often ultimately more or less largely dissolved away, 

 and replaced by the substance of the parasite. The skeleton of 

 Hydractinia, when fully grown, consists of numerous close-set ver- 

 tical columns ("radial pillars," fig. 79, d p), which are united at 

 irregular intervals by horizontal "laminae," the number of these, 

 and the consequent thickness of the crust, varying in different indi- 

 viduals or in different species. The successive " laminae " are separ- 

 ated by narrow intervals or " interlaminar spaces" (fig. 79, /), which 

 are broken into irregular chambers by the ascending "radial pillars." 

 The " laminse " themselves are not continuous membranes, but are 

 formed by the anastomosis of numerous horizontal connecting-pro- 

 cesses or "arms" (fig. 79, e c), which spring from the radial pillars 

 at tolerably regular intervals. The " laminae " are thus more or less 

 cribriform, the apertures in their substance serving as tubes for the 



