VARIATION IN FORM. 



371 



other trees which dip the extremities of their branches into the water. I have seen the tim- 

 bers of locks quite encrusted with the Alcyonella in many places. 



When carefully removed and placed in fresh Avater, it gradually develops a kind of white 

 downy appearance over its entire surface, which disappears with the rapidity of magic if a 

 hand is moved quickly over the vessel in which it is lying. This downy appearance is caused 

 by the tentacles which protrude themselves in vast numbers, and instantly retract when the 

 creatures are alarmed. 



Even in swallowing its food, the Plumatella displays considerable powers of discrimina- 

 tion, accepting some particles as they pass over the mouth, and rejecting others "as unworthy 

 of reception. Its usual places of abode are under stones, submerged branches, floating leaves, 

 and similar substances. Sometimes it attains a considerable size, spreading over a square foot 

 of surface, and having some branches more than three inches in length. It is in best condition 

 towards the end of summer. 



POLYZOA. 



A. Alcyondla fung&sa. B. Alcyonella fung&sa. x C. Alcyonella fung&sa. (Tentacles of a polype.) D. Ptumatetla repent. 



E. Pttimatdla repfritt. x F. PiumateUa repen*. (Statoblast. x ) Q. Pluinatella repens. (Statoblast, Bide view, x ) 



H. Fredericelta sultana, x I. Fredericetta sultana. (Statoblast. x ) K. Paludicella. L. Paludicella. x 



M. PaludkeUa. (Tentacles, x) N. Paludicella. (Tentacles, x) O. Paludicella. (Tentacle, x x) 



The sign x signifies that the object is magnified. 



A lovely polyzoa is the Fredericella sultana ; the former title being given to it in honor 

 of M. Fr. Cuvier, the celebrated naturalist, and the latter being earned by its graceful and 

 queenly beauty. This is a common species, and is found plentifully in tufts on submerged 

 stones, plants, sticks, and similar objects. It also inhabits rivulets, but seems to prefer toler- 

 ably still waters. As it is tolerably hardy, it is useful to microscopists, who can keep it alive 

 in a common vial of water and place it under the microscope whenever they choose. 



Our last example of the Polyzoa is the Paludicella ehrenbergii. Its peculiar form is not 

 a horseshoe outline, but a funnel-like shape of the marine polyzoa. This arrangement of the 

 tentacles seems to be unique among the fresh-water species ; for, although the tentacles of 

 Fredericella appear at first to assume the circular form, a more careful examination will show 

 that this is not really the case. 



The mechanism by which the floating particles contained in the water are inevitably driven 

 towards the mouth is of a knife-blade shape, on which the cilia is arranged in such a manner 

 that all those of one side point upwards and those of the opposite side downwards. The 

 tentacular plume viewed from the front shows that the arrangement of these organs is really 

 Circular. Only the bases of the tentacles are delineated. 



