HYDROZOA. 191 



c. Thecaphora. — Ex. Sertularia, Campanularia. 



d. Trachymedusce. — Ex. Trachynema. 

 Sub-class 2. Siphonophora (Oceanic Hydrozoa). 



a. Calycophoridce. — Ex. Diphyes. 



b. Physophoridce. — Ex. Physalia. 

 Sub-class 3. Lucernarida (Sea-blubbers). 



a. Calycozoa. — Ex. Lucernaria. 



b. Acraspeda (Discophord). — Ex. Aurelia, Rhizostoma. 

 Sub-class 4. Graptolitoidea (Graptolites). 



Sub-class 5. HYDROCORALLIN/E (Hydrocorallines). 



a. Milleporidce. — Ex. Millepora. 



b. Stylasteridce. — Ex. Stylaster. 



Sub-class 6. Stromatoporoidea (Stromatoporoids). 



Class II.— Actinozoa. 



Order 1. Zoantharia. — Ex. Sea-Anemones (Actim'dce) Corals 



{Madreporaria). 

 Order 2. Alcyonaria. — Ex. Sea-pens (Pemiatididce), Red Coral 



{Cor allium), Organ-pipe Coral {Tubipora). 

 Order 3. Ctenophora. — Ex. Venus's Girdle {Cesium). 



As regards their general distributio?i in time, the earliest known 

 remains of Coelenterates appear in the Upper Cambrian rocks, where 

 the sub-kingdom is represented by ancient types of the Hydrozoa. 



Fig- 75- — Diagrammatic vertical section of a Sea-anemone, a, Mouth ; s, Gullet ; b, Body- 

 cavity ; cc, Convoluted cords (" craspeda ") forming the free edges of the mesentery (w); 1 1, Ten- 

 tacles ; o, Reproductive organ contained within the mesentery. The ectoderm \e) is indicated 

 by the broad external line, the endoderm (c') by the thin line, and the space between that and 

 the ectoderm represents the mesoderm. 



In the Ordovician rocks we find the two great classes of the Hy- 

 drozoa and the Actinozoa thoroughly differentiated, and existing 

 under many and varied types, a fact which would lead us to as- 

 sign a very high antiquity to the early progenitors of this series 



