This species has a very different aspect at different periods of 
its growth, and it is not without a careful examination, and 
watching the plant as it progresses from its infant state till it 
reaches maturity and gradually passes off into old age and decay, 
that it can be fully understood. Without such examination, I 
should not have ventured to unite the three synonymes here 
brought together, which are regarded by continental authors as 
distinct species; and one of which, C. centralis, is by Kiitzing 
considered so widely separated from the others, as to be placed 
in a different section of the genus. The state called centralis, 
which the lower figure in our plate may be taken to represent, 
does indeed differ widely in aspect from the original arcta, des- 
cribed by Dillwyn, and formerly I considered it sufficiently dis- 
tinct. But amore familiar acquaintance with the plant, aided 
by observations made at different periods of its growth, both 
by Mrs. Griffiths and myself, have fully convinced me that its 
characters entirely depend on age, and that it is merely the fully 
developed state of the species. C. vaucherieformis, on the other 
hand, is the youngest stage of growth. 
Cladophora arcta abounds on all our rocky shores, and ap- 
pears to be equally common throughout the Northern Atlantic. 
Dr. Hooker also collected it in abundance at the Falkland 
Islands, and very probably it is distributed as abundantly in 
all southern latitudes with a similar climate. I believe that it 
always grows upon rocks, within the range of the tide, but nearly — 
at the limit of low water, and in such places it frequently covers 
a considerable surface. When young, its colour is peculiarly 
vivid, and its aspect silky; but as it progresses, the bright colour 
is more and more confined to the top branches, and the lower 
part of the frond becomes coarse and woolly. 
The species most nearly allied to C. arcta, are C. uncialis, and 
C. lanosa, both of which are of much smaller size. 
Fig. 1. CLADOPHORA ARCTA; ina young state. 2. The same, later in the 
season :—both of the natural size. 3. Portion of a filament. 4. Apex. 
5. Joints from the lower part of a filament :—all more or less magnified. 
