CHAPTER XVI 



PHYLUM POLYZOA 



THIS group includes a great number of species, the individuals 

 of which are so small as to be barely visible to the naked eye, 

 but they are all colonial in their habits and the colonies usually 

 attain a fair size. These colonies take many shapes, some branching 

 like a tree, others being flattened like a leaf, while others again 

 are discoidal ; often they are encrusting, that is to say they form 

 a layer on some seaweed or rock, for the majority of them are 

 marine. 



If one of these colonies be 

 dried so that the organic mat- 

 ter shrivels up, a hard skele- 

 ton remains, and this is then 

 seen to consist of a number 

 of chambers or "cells," each 

 of which opens to the exterior 

 by an orifice, and, as a rule, 

 communicates with its neigh- 

 bours. The skeleton* may be 

 calcareous, or chitinous, or 

 the colony may be gelatinous 

 in consistency. The dried cell 

 may be open or closed by a 

 lid termed the operculum. 



During life each of these cells lodges part of the body of a person 

 of the colony, the "cell" being indeed the cuticle; part however 

 of the person is not clothed with cuticle and is normally stretched 

 out above the cell the opening in the dried cell is in fact the 

 place where the flexible part of the person begins. At the end of 

 this flexible part is the mouth surrounded by a ring of ciliated 

 tentacles: on one side is the anus. The flexible part is termed 



JFio. 177. Portions of two Polyzoan 

 colonies. Magnified. 



A. Smittia landsborovii. a. Avicularium. 



m. Orifice of cell. o. Ooecium or 

 pouch in which the egg develops. 



B. Tubulipora plumosa. After Hincks. 





