POLYZOA 1 8s 



In some of the Gymnolaemata there is a pore by means of 

 which the sea-water can be admitted into the coelom when it is 

 desirable to expand the polypide. This pore may be guarded 

 by a circlet of bristles, which tend to prevent the entrance of 

 grains of sand or other foreign bodies. In certain individuals 

 of some species of Alcyonidium, etc., there is a ciliated canal 

 which leads from the coelom to the exterior ; this is known as 

 the " inter-tentacular organ," it probably serves as an exit for 

 the generative cells, at any rate spermatozoa have been observed 

 to leave the body by this channel. In most species, however, 

 the generative products escape only by the dying down of the 

 polypide. 



The G-ymnolaemata are divided into three classes, according 

 to the character of their zooecia and the nature of their cell 

 mouths when the polypides are retracted. 



(i.) The Cyclostomata. — These have tubular zooecia, always 

 calcareous. The cell raouth is circular, and with no 

 apparatus for closing it. Many of them are found 

 fossilised, probably because their calcareous skeleton is 

 easily preserved. Crisia, Hornera, etc. 



(ii.) The Ctenostomata. — The zooecia are soft, and their 

 apertures guarded by a folded frill. JBowerbanJcia, one 

 of this sub-order, has a muscular gizzard armed with 

 teeth, situated between the oesophagus and the 

 digestive stomach. Paludicella, one of the few 

 freshwater members of the Gymnolaemata, belongs 

 here, and Alcyonidium. There is some reason to 

 suppose that the Phylactolaemata are derived from 

 this group of the Gymnolaemata. 



(iii.) The Cheilostomata. — This is the largest subdivision, 

 and its members are clearly characterised by the posses- 

 sion of a lid or operculum, which closes the mouth of 

 the zooecium when the polypide is retracted. The 

 zooecia are calcareous. 



This subdivision exhibits a considerable degree of poly- 

 morphism. Some of the individuals of the colonies are modified 

 to form structures known as avicularia, resembling in shape a 



