HYDROIDA. 



203 



Order 2. Thecaphora. — This order includes the Sea-firs and 

 their allies (Sertularida and Campanularida), in which the organism 

 is fixed, and consists of a more or less plant-like colony (fig. 84, a), 

 composed of numerous polypites united by a common stem or 

 i; ccenosarc." The coenosarc usually consists of a main stem or 

 ' ; ''hydrocaulus,' , with many branches, and it is fixed to foreign bodies 

 by an adherent base or " hydrorhiza." The colony produces a 

 strong corneous or chitinous outer investment (the " periderm " or 

 " polypary "), which not only invests the ccenosarc, but is also pro- 

 longed into cup-like receptacles, which enclose the individual poly- 

 pites, and are known as the " hydrothecae/' The reproductive 

 zooids (" gonophores ") are either set free as Jelly-fishes (as in the 

 Campanularians generally), or are developed within urn-like recep- 



which are of larger size than 



tacles (" gonangia " or " gonothecae "] 



Fig. 85. — Dendrograptus Halliamis. a. Portion of the frond, natural size ; b, Portion of a 

 branch, enlarged ; c, The footstalk and some of the principal branches, natural size. Upper 

 Cambrian (Potsdam Sandstone). (After Hall.) 



the ordinary hydrothecae (fig. 84, b and c). In this latter case, the 

 reproductive zooids are not liberated from the parent colony as 

 independent organisms. In the typical Sertularians, the hydrothecae 

 spring directly from the sides of the coenosarc and are not supported 

 on stems, and they may be biserial (as in Sertularia and Difihasia, 

 fig. 84, a), or they may, as in Plumularia and its allies, be de- 

 veloped on one side only of the divisions of the ccenosarc. On the 

 other hand, in the Campanularians the polypites are stalked and ter- 

 minal, each being placed at the end of a division of the ccenosarc. 

 The existing Thecaphora are not only very abundant and very 



