64 
POPULAR SCIENCE REVIEW. 
and their action is confined to an incessant exercise of the 
lower jaw ; but in some cases they are mounted on a kind of 
joint, on which they swing backwards and forwards in a half- 
rhythmical way, opening .and closing their mouths with pro- 
digious vigour. 
I am not aware that any recent observations have been made 
tending to throw light on the relation which these curious 
organs bear to the economy of the Polyzoan. The capture by 
the avicularia of small worms, which are much given to wrig- 
gling in and out amongst the branches of the colony, has often 
been observed ; and I have been surprised to find an unhappy 
captive of this kind still in durance and still living after the 
lapse of two or three days. The birds’-heads may also be seen 
frequently with a quantity of dirt between their beaks, as if their 
office too might belong to the scavenging department. I have 
referred to them, however, for the purpose of noticing the 
familiar fact, that they continue their movements after the 
disappearance of the polypides, and may often be seen in full 
play v T hen there is no sign of life in the rest of the colony. 
The explanation is that the common nervous system, on which 
they are dependent, no doubt retains its energy for some time 
after the death of the polypides ; and, further, that in a large 
number of such cases, though the cells are tenantless for the time, 
the life of the colony is not really impaired, but a company of 
the “ Groddkapseln ” is on the point of supplying it with a 
new population. 
