494 



POLYZOA 



a zone not more than two feet below the surface. Certain species 

 show a perference for floating leaves, such as those of water-lilies, 

 where they are not liable to be dried up by alterations in the 

 level of the water. Some forms (e.g. Plumatclla, Fig. 246) are, 

 however, able to withstand being dried for some time. Most 

 species prefer shady places, and accordingly settle on the lower 

 sides of leaves and sticks. Others (e.g. Cristatella, Fig. 247) 

 have 110 objection to the direct rays of the sun. Most forms 

 prefer still water, but one or two are found in running water. 



Freclericella is a common constituent of the deep-water fauna 

 of Swiss Lakes (down to over forty fathoms) ; and reaches there 

 a size considerably larger than the 

 shallow-water form of the same species. 

 Paludicella is common at thirteen 

 fathoms. These two genera, with 

 Plumatella, have been found in absolute 

 darkness, under a pressure of 2^5^ 

 atmospheres, in the Hamburg aque- 

 duct. The Polyzoa and other organisms 

 growing in the water-supply of Ham- 

 burg were accused of being concerned 

 in the spreading of cholera, during the 

 recent epidemic, by choking up the 

 water-pipes, and creating obstructions 

 which formed a favourable nidus for 

 the development of cholera-germs. 



The colony may take the form of a 

 series of delicate, branching tubes 



FIG. 246. A, Plumatella (Alcyo- . ' fo 



funyosa Pall., Naples (Plumatella, Fredericella\ of more mas- 



(fresh water) ,, small part , of a sive aggregations of parallel tubes (as 



mass, natural size : B, Pluma- . eo & ^~ 



tdia repens L., R. Yare, on in the Alcyonelloid forms of Pluma- 

 rai e ef f aWatei " my ' natU " tMa \ f gelatinous masses of vary- 



ing size (Lophopv*, Cristatella). 



Cristatella mucedo (Fig. 247) is remarkable for its power of 

 moving from place to place ; it consists of an elongated mass of 

 greenish, gelatinous substance, which, in its fully developed state, 

 may reach a length of eight inches or more, with a transverse 

 diameter of three-eighths of an inch. It has a flattened sole on 

 which it crawls, while the graceful plumes of its numerous 

 polypides protrude as a delicate fringe from its upper side. 



