G@COLOGY OF CIRRATULUS TENTACULATUS. 63 
the worm be withdrawn from the mud on the floor of the pool and left on 
the surface, it will immediately coil up and commence to burrow again in 
the manner already noticed. Owing to the downward and forward move- 
ment the gill filaments will tend to stream backwards, and, as the worm 
progresses, being extremely elastic, their distal portions will remain at 
the surface, the filaments stretching till the animal has found its proper 
level. The portions of the filaments remaining in contact with the water 
will thus be available for aeration of the contained blood. 
The question now arises as to how the animal would react should the 
filaments become buried beneath several inches of mud, as must often 
happen. 
!n order to answer this question a specimen of Cirratulus was placed at 
the bottom of a glass vessel 5 inches high by 24 inches in diameter, covered 
with mud to the height of two inches, and the remainder of the vessel 
filled with water. Four hours later a number of filaments were projecting 
at the surface. The following day, when about 20-30 filaments were pro- 
jecting, another 24 inches of mud were added. Two hours later one fila- 
ment had been protruded. This time, by a lucky chance, the anterior end 
of the worm was in contact with the glass close to the top of the first layer 
of mud, and its behaviour could thus be observed. The body of the worm 
itself was practically stationary, some of the filaments of the anterior 
fascicles were yellow and motionless, but numbers which were gorged with 
blood showed remarkable activity, and were gradually yet speedily 
forcing their way upward through sand and mud to the surface, exactly 
as if they were so many individual worms. The extensility and muscular 
activity of the filaments is therefore enormous, the length of some of them 
from their junction with the body wall to the tips exceeding three inches. 
The whole forms a remarkable adaptation to an underground habitat. 
The majority of species of Polycheetes inhabiting the same environment 
are either of small size and able to respire through the body wall generally, 
or they are obliged to mount to the surface to avoid asphyxiation. Cirra- 
tulus is able to live permanently surrounded by the sandy mud where it 
finds its food supply (reference to which will be made later), and by re- 
maining constantly underground is well protected from enemies. The 
large numbers in which Cirratulus occurs is sufficient proof of its success. 
We have seen that when the body of the worm is undergoing pressure 
the filaments are stimulated to great activity by the pressure of blood in 
their vessels, and on seeking an explanation of this phenomenon, we can- 
not help being struck by the fact that in the Cirratulide, the heart-body, 
the function of which has aroused so much curiosity, reaches its greatest 
development. 
The structure and function of the heart-body, in this and in other 
