POLYZOA 185 
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 Aleyonidiwm, 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 Gymnolaemata 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 mouth is circular, and with no 
apparatus for closing it. Many of them are found 
fossilised, probably because their calcareous skeleton ws 
easily preserved. Crisia, Hornera, etc. 
(ii.) The Crenostomata.—The zooecia are soft, and their 
apertures guarded by a folded frill.  Bowerbankia, 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 
