234 LIFE, IN ITS INTERMEDIATE FORMS. 
them ? Why do some specimens possess them and not 
others? Why are some species of a genus furnished with 
them while some are destitute of any such appendages, 
though essentially the same in every other respect ? No 
light is thrown on these questions by anatomical exami- 
nation. It would appear that the animal within is quite 
independent, organically, of the “ bird’s head,” for, as Dr 
Reid has remarked, and as wo have personally observed in 
several species, tho “ bird’s head ” continues to move for 
a considerable time after tho death and decomposition of 
the polype. It has boon suggested that the use of the 
organ is to grasp and kill passing animalcules, which then 
may be drawn into the cell by means of the ciliary cur- 
rents of the tentacles ; and this appears not improbable, 
and receives confirmation from the toothed structure of 
the beak. 
The organ in question, with its singular function, may 
minister to the support of the common life in another way 
less direct but not less effective. The seizure of a passing 
animal, and tho holding of it in the tenacious grasp until 
it dies, may be a means of attracting the proper prey 
to the vicinity of the mouth. The presence of decom- 
posing animal substance in water invariably attracts crowds 
of infusory animalcules, which then breed with amazing 
rapidity, so as to form a cloud of living atoms around 
the decaying body, quite visible in the aggregate to the 
unassisted eye, and these remain in the vicinity, playing 
round and round until the organic matter is quite con- 
sumed. Now, a tiny Annelid or other animal, caught by 
the bird’s head of a Polyzoan and tightly held, would 
presently die ; and though in its own substance it might 
