POLYZOA AND TUNIOATA. 
233 
slender joint, an organ which has been aptly compared 
to the naked head of a vulture. This organ has a beak 
with two mandibles, tho lower of which alone is moveable, 
opening and shutting like that of a bird, but with far 
greater width of gape, as the lower mandible can bo opened 
till it extends behind in the same line with the upper. 
Each edge of the mandible (in the first-named species) is 
furnished with five strongly projecting teeth; the lower 
has a single tooth at its point, which fits into the notch 
between the terminal pair of the upper. The whole back 
of the head is transversely wrinkled. 
The movements of this strange appendago are in keep- 
ing with its curious structure. The whole head ordinarily 
sways to and fro upon the slender joint of the poll at inter- 
vals of a few seconds ; but besides this motion, which is 
even, though rather quick, tho lower mandible which com- 
monly gapes to its utmost extent, now and then, at irregu- 
lar intervals, closes with a strong sudden snap, much like 
the snapping of a turtle’s jaws, and presently again opens 
and leisurely resumes its former expansion. We may dis- 
tinctly see the muscles which move the lower mandible ; 
they occupy the position of the palate and extend back 
to the inner surface of the skull, if wo may borrow such 
terms from the organ which this organ mimics, it is very 
interesting to witness these singular motions, and it is 
scarcely possible to observe them without believing that 
the animal exercises an active volition in performing them. 
But the observation of these “ bird’s heads ” suggests 
curious questions. Do they form a part of the polype s 
organisation 1 Why, then, are they found attached to 
some cells of a specimen while others are destitute of 
