192 



CCELENTERATA. 



of animals. From the beginning of the Ordovician rocks onwards, 

 the Ccelenterates are very abundant and important as fossils, the large 

 groups of the Graptolitoidea and Stromatoporoidea being wholly 

 extinct, and having no close relatives now in existence. Owing 

 to the fact, however, that other large groups (such as the Lucer- 

 narians, the Oceanic Hydrozoa, and the Ctenophora) are almost or 

 altogether without hard parts, and therefore only capable of pre- 

 servation in the fossil condition under very exceptional circum- 

 stances, the geological history of the sub-kingdom is very imperfect. 



Class I. — Hvdrozoa. 



The class of the Hydrozoa comprises those Ccelenterates in tvhich 

 the walls of the body enclose a simple undivided cavity (the " cozlenteric 

 cavity "), which acts both as a body-cavity and a digestive cavity. An 

 oesophageal tube is not developed ; but the upper end of the alimentary 



Fig. 76.— A, Diagram of Hydra, vertically bisected, and greatly magnified ; b, Diagram to 

 show the structure of a compound Hydrozoon (Sertularia). cc, Ectoderm ; en, Endoderm ; m, 

 Mouth, opening into the ccelenteric space or body-cavity ; t, Tentacle ; /, Nutritive polypite ; go, 

 Reproductive bud or "gonophore," consisting of a capsule enclosing a central axis to which re- 

 productive polypites are attached ; cs, Ccenosarc ; rh, Hydrorhiza ; pe, Periderm ; h, Hydrotheca. 



tract may be prolonged into radiating canals united by a peripheral 

 ring. The reproductive organs are external buds, and are often devel- 

 oped in specially modified zooids (fig. 76). 



