BRITISH ZOOPHYTES. 19 



considered to belong to this sub-kingdom from tbe 

 fact of their possessing a distinct alimentary system, 

 and more especially from the facts that their tentacles 

 perform the functions of gills and are respiratory 

 organs, and that they possess a nervous ganglion. The 

 PoLYZOA may be defined* as "Acephalous Mollusca, 

 with free oral tentacles serving as gills, and having the 

 mouth and anus near together, separated or not by the 

 tentacles. They are enclosed in a cell, and usually form 

 colonies by continuous gemmation." Some naturalists 

 assign the Polyzoa to a division of the Mollusca 

 designated the Molluscoida, which includes the Poly- 

 zoa, TuNiCATA or sea-squirts, and Bkachiopoda. 



The Polyzoa are divided into two sub-classes ac- 

 cording to the shape of the lophophore, the name given 

 to the stage which supports the tentacles. The first 

 sub-class is the Holobeanchia, in which the lophophore 

 is either circular or horseshoe-shaped, and the wreath 

 of tentacles is unbroken. In the second sub-class, or 

 Pterobranchia, the lophophore is continued on each 

 side into a process upon which alone the tentacles are 

 borne, which therefore are discontinuous. 



The Holobranchia contain two groups, namely, the 

 EcTOPROCTA, in which the anal orifice is outside the 

 lophophore, and the Entoprocta, in which both the 

 anal and oral orifices are within the lophophore. The' 

 Ectoprocta contain two orders, the Phylactol^mata 

 and the Gymnol^mata. The former of these orders 

 includes most of the fresh-water Polyzoa, and embraces 

 those forms in which the lophophore is bilateral, and 

 the mouth is furnished with an epistome or valve-like 

 organ arching over it. 



* Hincks, British Marine Polyzoa, cxxxv. 



c 2 



