POLYZOA AND TUNICATA. 



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, the 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 be 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 appendage 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, the 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 we 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 



