HYDROZOA. GYMNOBLASTEA 53 



Well-known living forms are Tubularia, Bougainvillea, 

 and Hydr actinia. The last has been found fossil in 

 Eocene and later deposits ; it forms a crust over the 

 shells of gasteropods, especially those tenanted by Hermit- 

 crabs. The hard part of this crust is chitinous, or rarely 

 calcareous, and consists of laminae separated by irregular 

 or cubical spaces and crossed by vertical pillars ; on the 

 surface are projecting spines. The soft parts form a 

 layer over this skeleton, and consist of a sheet of ecto- 

 derm on the surface, and another sheet next the skeleton ; 

 between these are branching and anastomosing coenosarcal 

 tubes. The skeleton is secreted by the lower ectoderm. 

 From the ccenosarc arise the polyps, which are placed on 

 long vertical stalks and are of four kinds, (1) gastrozooids — 

 the ordinary nutritive individuals, (2) blastostyles, which 

 are individuals specially modified for bearing medusae, 

 (3) dactylozooids — individuals modified for catching prey 

 and having short knob-like tentacles crowded with nemato- 

 cysts, (4) tentacular polyps, which are very slender, with- 

 out a mouth, and occur near the edge of the colony. 



Parkeria, which is found in the Cambridge Greensand, 

 probably belongs to this Order. A few other forms have 

 been described from the Alpine Trias and the Jurassic of 

 southern Europe. 



A. 



ORDER II. CALYPTOBLASTEA 



This Order is distinguished by the presence of hydro- 

 thecae and gonangia (gonothecae). (Fig. 12, 7, 10.) 



The arrangement of the polyps and hydrothecse on the 

 hydrocaulus varies considerably in different genera. Some- 

 times they are placed on stalks as in Obelia (fig. 12) and 

 Campanularia\ in many others they are sessile. They 



