64 ZOOLOGY 



the mesentery. There are no special sense organs. Nemato- 



eysts occur, and are more complicated than those usually 



found in the Hydrozoa. 



The most fundamental distinction between the Actiuozoa 

 and the Hydrozoa lies in the presence of the mesenteries and 

 the consequent division of the coelenteron into a central 

 chamber and several radiating lateral chambers, which all open 

 below iato the central chamber. In the Hexactinia these 

 chambers are put into direct communication with each other 

 by one or two pores, which pierce the mesenteries near their 

 upper end. The existence of these mesenteries is to some extent 

 foreshadowed by the taeniolae of the Scyphomedusae. In two 

 forms recently described by Danielssen, Fenja and Aeytr, the 

 oesophagus is continued to the aboral disk, where it opens to 

 the exterior by an anus ; thus a body cavity is shut off from 

 the digestive cavity. In Aegir, however, these two cavities 

 communicate by a series of pores which lead from the inter- 

 mesenteric chambers into the alimentary canal near the anus. 

 In Fenja each intermesenteric chamber opens to the exterior 

 by a pore close to the anus. Thus a ring of genital pores js 

 formed. In both genera the various intermesenteric cavities 

 communicate with one another by pores at the oral end of the 

 mesentery. 



The Actinozoa have an apparent radial symmetry which 

 does not hold true for all parts ; but a genuine bilateral 

 symmetry exists. 



The Actinozoa are sub-divided into two orders : (i.) the 

 Hexactinia (Zoanthaeia) and (ii.) the Octactinia (Alcy- 

 onaeia). 



Order 1. Hexactinia. 



Charactbkistics. — Tentacles sirnple ; they and the mesenteries 



are very generally some multiple of six. Single or colonial ; 



when colonial a continuous organic or calcareous skeleton is 



usually deposited hy the ectoderm. 



Actinia mesembryanthemum is a beautiful red sea-anemone 

 common in rock pools round our coasts. When in the 

 expanded condition it may be seen that the animal is in the 

 form of a short cylinder, with one end firmly attached to a stone 



