FOSSIL HYDROZOA. 153 



TABLE OF THE DIVISIONS OF THE CCELENTERATA. 



CLASS A. HYDROZOA. The walls of the digestive sac not separated 

 from those of the general body-cavity, the two coinciding with one 

 another. Reproductive organs in the form of external processes of the 

 body- wall. 



Sub-class I. HYDROIDA (Hydroid Zoophytes). 

 Order 1. Hydrida.Ex. Hydra. 

 Order 2. Corynida. Ex. Tnbularia. 

 Order 3. Thecaphora. Ex. Sertularia, Campanularia. 

 Order 4. Thecomedusce. Ex. Stephanoscyphus. 

 Sub class II. SIPHONOPHORA (Oceanic Hydrozoa). 

 Order 5. Calycophoridce. Ex. Diphyes. 

 Order 6. Physophoridce. Ex. Physalia. 

 Sub-class III. DISCOPHORA (Jelly-Hshes). 



Order 7. Medusidce. Ex. vEgina. 

 Sub-class IV. LUCERNARIDA (Sea-blubbers). 

 Order 8. Lucernariadce.Ex. Lucernaria. 

 Order 9. Pelagidce. Ex. Pelagia. 

 Older 10. Rhizostomidce. Ex. Rhizostoma. 

 Sub-class V. GRAPTOLITID.E (Graptolites). 

 Sub-class VI. HYDROCORALLIN^E. Ex. Millepora, Stylaster. 

 CLASS B. ACTINOZOA. Animal with a differentiated digestive sac 

 opening below into the body-cavity, but separated from it by an inter- 

 vening " perivisceral space," which is divided into compartments by a 

 series of radiating vertical partitions or " mesenteries," to the faces of 

 which the reproductive organs are attached. 



Order 1. Zoantharia. Ex. Sea - anemones, Star - corals, Brain- 

 corals. 



Order 2. Alcyonaria. Ex. Sea-pens, Fan-corals, Sea-shrubs, Red- 

 coral, Heliopora, Heliolites. 

 Order 3. Rugosa. Ex. Cyatl.ophyllum. 

 Order 4. Ctenophora. Ex. Venus's Girdle. 



With the exception of one or two Graptolites (Dendro- 

 f/raptus), which are known in the Upper Cambrian, the 

 Ccelenterata are first well represented in the Lower Silurian ; 

 and when we consider that we find the two great classes 

 thoroughly differentiated, and existing under many and 

 varied types, at this early period, we are forced to conclude 

 that the first appearance of the sub-kingdom must really 

 have been at a much more ancient epoch. The Crelenterates 

 are extremely abundant and important as fossils, some large 

 groups being wholly or largely unrepresented at the present 

 day ; but owing to the fact that other large groups (such as 



