FLUSTRA. 15 



of the margin, not exceeding an eighth part of its width. They are per- 

 ceptible only when the object is thrown into the shade ; and in certain posi- 

 tions I have thought other two could be indistinctly seen. — Plate IV. 

 figs. 2, 3, 13. 



Ascidian hydra, which are whitish to the eye, issue from the cells, un- 

 folding about 14 or 16 tentacula. Towards one side of the body, the re- 

 turning tube of the excretory canal, which terminates under the tentacula, 

 becomes very conspicuous. No farther peculiarities distinguish them, 

 farther than their complex organization, besides the quick and lively 

 action of all their race. — Plate IV. fig. 1. 



The edges of the stem are somewhat more compact than the middle, 

 but no distinct marginal reinforcement, like the yellow band so strong in 

 the Flustra carbasea, guards it from injury. 



The whole product is of wood-brown colour, darker towards the root, 

 and lighter towards the extremities. It is generally founded on old shells, 

 generating there most luxuriantly ; and it occurs in vast profusion, greater 

 perhaps, than any of our other northern zoophytes. In their meagre voca- 

 bulary of marine nomenclature, the Scotish fishermen denominate it 

 sea-caff. — Whether to be interpreted chaff, or something ivaste and worth- 

 less, I know not. No other name than sea-weeds, flowers, or simply growth, 

 is applied by them to the numerous tribe of zoophytes. 



Ferruginous patches of various intensity overspread the leaves of some 

 of the Flustra foliacea at certain seasons of the year. These are shewn 

 by the microscope to be derived from large spherical corpuscula occupy- 

 ing the cells, as in the Flustra carbasea. Only one belongs to each. — Fig. 3. 



In order to determine the precise nature of such patches, I consign- 

 ed a fine specimen thus profusely stained with some of large extent, to a 

 capacious vessel half full of sea-water, and next day shut it up in the dark. 



Resuming inspection ten days afterwards, the specimen was found as 

 it had been left ; the water limpid, the subject in its place, every thing in 

 statu quo. 



However, I transferred the latter to another vessel, which was com- 

 pletely replenished, and now exposed immediately to the light. 



In three hours, at least ten thousand yellow spherules had here quit- 



