ANTHOZOA 



243 



directly into the gastro-vascular cavity as it does in the Hydro- 

 zoan polyp. There is a long oesophagus which extends from 

 the edge of the mouth to the centre of the gastric cavit}-. It 

 is in reality a cylindrical continuation of the oral surface and is 

 lined with ectoderm. The gastric cavity is incompletely 

 divided into chambers by folds of the entoderm supported by 

 layers of mesoglea. Some of these mesenterial folds extend 

 from the wall of the column to the oesophagus. These are said 

 to be complete. Others do not reach the oesophagus and are 

 therefore known as incomplete mesenteries. On the free edge 

 of some of the mesenteries there is a thick muscular cord, the 

 mesenterial filament, which is richly supplied with gland and 

 nettling cells. There are no special sense organs and nothing 

 that can be called a central nervous system, though beneath the 

 ectoderm of the oral disc the network of nerve fibres is better 

 developed than elsew^here. A strong circular muscle is usually 

 found just below the edge of the oral disc and in the mesen- 

 teries there are strong longitudinal muscle bands. The gonads 

 lie embedded in the mesoglea of the mesenteries along the free 

 border. The sexes are usually distinct. The larva develops 

 for a time wdthin the body of the parent and escapes as a ciliated 

 planula which becomes fixed and develops directly into the 

 polyp. 



Order 2. — The Octactiniaria are chiefly colonial. The polyp has eight 

 pinnately branched tentacles and eight mesenteries. There is frequently 

 a skeletal structure of horny or calcareous matter. The whip corals, sea 

 fans, organ pipe corals, etc. 



Order 3. — The Ceriantipatharia include the anemone Cerianthus and 

 some colonial forms with a horny skeleton and polyps with si.\ tentacles. 



Order 4. — The Zoanthactiniaria comprise the sea anemones and the 

 madrepore corals. The mesenteries are grouped in pairs. 



531. Class V. Ctenophora. — The Ctenophora are another 

 group of jelly fishes. They are more transparent and watery 

 then the medusae and exceedingly fragile. A common type is 



