STRUCTURE OF FLUSTRA HISPIDA. 423 



occasionally encircling the stem. These patches are 

 now and then in such profusion, that the leaf is weighed 

 down by the accumulation of its parasitic occupants. 



On plunging a small branch of the Fucus so iii- 

 crusted in a jar of recent sea-water, and allowing it 

 to remain for a little while at rest, a thin pale blue 

 cloud will be speedily seen to hover over its dark 

 irregular surface, which, on the slightest shock, is 

 instantaneously dispelled, leaving the brownish fleshy 

 substance distinct as before. This experiment may 

 be frequently repeated. The semblance of a cloud is, 

 however, soon perceived, by the aid of a microscope, 

 to be caused by the simultaneous protrusion of a mul- 

 titude of polyps emerging from the cells of the brown 

 polypary, while the play of their numerous pale ten- 

 tacula in motion over the darker ground produces a 

 misty shade. 



The minute and active polyps which thus crowd 

 the surface of this parasitical Bryozoon are, in fact, so 

 timid, that they sink into concealment on the slightest 

 apprehension of danger, and it is only when they are 

 relieved from their alarm that they venture again to 

 display their tentacula (PL VII. fig. 8). 



Viewed by a lens, the surface of the polypary, from 

 which these little beings issue forth, is seen to be 

 covered with numerous projections, indicating the 

 position of so many cells, each of which has its orifice 

 beset by fine spines; but it is difficult to see these 

 distinctly, except in particular positions of the object. 



The various phases attending the evolution of the 

 contracted Polyzoa are easily witnessed. The rough, 

 dark surface of the Flustra remains undisturbed until 



