GENERA OF FRESH-WATER POLYZOA 505 



other. This greater variation of species of the dominant genus is 

 in complete accordance with the general law enunciated by Darwin 

 that " wide-ranging, much diffused, and common species vary most." 



While the ordinary forms of Plumatella consist of branching 

 colonies, which are either completely adherent to their substratum, 

 or grow in a more or less erect manner, another habit which is 

 assumed by this genus is so different from the first that it has 

 been considered to mark a distinct genus, Alcyonella. The Alcyo- 

 nelloid form (Fig. 246, A) consists of closely packed tubes which 

 stand more or less at right angles to their substratum, which they 

 may cover with a dense mass an inch thick, and with a superficial 

 area of several square inches. But in spite of this difference, it is 

 possible that A. fungosa is only a variety of an ordinary Pluma- 

 tella form. Whether this is so or not, a typical Plumatella may 

 in places take on an Alcyonelloid habit ; and parts of an Alcyo- 

 nella may become so lax in growth as to resemble a Plumatella. 



The British genera of fresh -water Polyzoa may. be distin- 

 guished from one another by means of the following table : 



Zooecia perfectly distinct from one another. Lophophore circular. 



Statoblasts absent . . . . .2 



Colony formed of branching tubes composed of confluent zooecia 3 

 Colony gelatinous, not obviously formed of branching tubes. 



Lophophore horse-shoe shaped . . . .4 



(Colony consisting of a stolon from which new zooecia originate. These 

 may give rise to new stolons, or directly to new zooecia Victorella 

 Branches composed entirely of club-shaped zooecia, each of which may 

 give off two zooecia near its upper end Paludicella (Fig. 250) 



' Tubes hyaline or opaque, usually containing numerous oval stato- 

 blasts (Fig. 251, B), most of which have a ring of air-cells. 

 Lophophore horse-shoe shaped. 



(a) Tubes divergent . . Plumatella (Fig. 246, B; 



(6) Tubes parallel with one another Alcyonella form of 



Plumatella (Fig. 246, A) 

 Tubes cylindrical, usually dark brown. Statoblasts (Fig. 251, A) few, 

 without air-cells. Lophophore circular . . Fredericella 



Colony hyaline, usually divided into three or four short lobes. 

 Ectocyst thick. Statoblasts (Fig. 251, C) pointed at each end, with 

 a broad ring of air-cells . . . Lophopus (Fig. 248) 



Colony slug-shaped, crawling on a flattened sole. Ectocyst rudi- 

 mentary. Statoblasts (Fig. 251, D) circular, with marginal hooks. 



Cristatella (Fig. 247) 



Colonies consisting of small rosettes, many of which are attached to 



a thick basal layer of hyaline ectocyst. Statoblasts circular, with 



marginal hooks. (Not recorded as British) . Pectinatella 



