50 



CCELENTERATA. ACTINOZOA. 



and Millepora. They are found only in the Ordovician, 

 Silurian, and Devonian Systems, being most abundant in 

 the last ; frequently they are of considerable importance 

 as rock-builders. Some of the best known genera are 

 Labechia, Stromatopora, Actinostroma, and Clathrodictyon. 



CLASS. ACTINOZOA. 



This includes the corals and sea-anemones. They 

 differ from the Hydrozoa (1) in possessing an alimentary 

 tube, the oesophagus, which is distinct from the coelenteron 

 but at the same time communicates freely with it ; (2) in 

 the ccelenteron being divided up into chambers by vertical 

 radiating partitions known as mesenteries; (3) in the 

 reproductive elements being developed within the polyp 

 and never on a medusa. 



In a typical actinozoan, such as the common sea- 

 anemone Actinaria, the body has a more or less cylindrical 



VLV- 



FIG. 10. Diagram-sections of a sea-anemone. A, vertical. B, horizon- 

 tal, through the upper part of the polyp. C, through the lower 

 part, a, tentacles ; 6, mouth ; c, cesophageal tube ; d, coelenteron ; 



form, and is attached by one end, the other having an 

 opening, the mouth (fig. 10 6), surrounded by tentacles (a). 

 The mouth leads into the oesophageal tube (c), which opens 



