92 
SUPPLEMENT 
and on this are attached the feet and gills. This distinction 
of large and small wrinkles does not exist in the posterior 
third of the body, where no gill nor foot is discernible. In 
front the feet advance, close up to the head, which itself is 
distinguishable only by the presence of the mouth, but the 
gills do not proceed so far. There are six of these large 
wrinkles in front, which have only feet without gills. There 
are fourteen which support gills, very small at first in front, 
enlarging by degrees as far as the tenth or twelfth, and after- 
wards diminishing. The feet, like those of the other genera 
of this order, are only fasciculi of stiff' and brilliant seta 1 , 
which the arenicola causes to enter, or issue forth at will, by 
means of certain muscles, of which we have already given a 
sufficient notion in our general article. 
The gills arc those parts which most quickly attract the 
eyes of the observer, in the living animal, by the beauty of 
their structure and the changes of colour which are remarked 
in them. These gills are sorts of aigrettes, composed of eight 
or ten principal slips or blades, which proceed from a com- 
mon base, and separate like the radii of a circle, at the same 
time curving gently. Each of these blades or stalks supports 
a dozen of small branches, which are subdivided two or tlnee 
times into smaller branches. All this apparatus cannot well 
be seen but for a very short instant, during which it is ex- 
tended in every direction, and of a fine red colour. In a 
second it sinks in upon itself— the branches are folded, it 
grows pale, and becomes altogether grey. These two states 
thus alternate with each other as long as the animal is in 
good health, and are caused by the blood, which is carried 
into the gills to be respired, that is, to undergo the action of 
the circumambient element, and which afterwards returns 
into the interior of the body. It is thus that respiration takes 
place in all the marine worms with red blood, such as amphi- 
norna, &c. &c. 
