BRITISH ZOOPHYTES. 201 



THE POLYZOA. 



Descrijytio'h. 



The Polyzoa are possessed of a much higher degree 

 of organization than the animals ah'eady described. 

 Although to the naked eye many of the Polyzoa 

 resemble some forms of Hydroida in appearance, they 

 are found, wheu examined with the microscope, to be 

 formed on a very different plan. Like the Hydroida 

 each polypide possesses a crown of tentacles, but the 

 functions of these appendages are very different in the 

 two classes. In the Hydroida, as has been pointed 

 out, their functions are tactile ; in the Polyzoa they 

 unite to this function that of respiration, serving the 

 purpose of gills, and on this account, as well as on 

 others, the Polyzoa are dealt with by many naturalists 

 as allied to the Mollusca.* 



As their name implies, the Polyzoa exist in colonies, 

 sometimes connected by a creeping stolon, at others 

 closely packed together, and expanding into a broad 

 leaf-like structure. In some species the cells or zooecia 

 are simple and unadorned, in others they are toothed 

 and girt with spines, and in some cases most richly 

 ornamented. 



The Polyzoa inhabit both fresh and salt water; but 

 the number of fresh-water species is limited, and with 

 the exception of one genus, Paludicella, the fresh- water 



♦ The position of the Polyzoa in the scale of classification is at 

 present very much disputed, and any position to vvhicli the class is 

 assigned can only be considered as tentative. 



