THE VIBEACULUM AND ITS OFFICE. 



one avicularium is to be found on a single cell, though many cells do not possess these 

 strange appendages. 



By close examination, it will be seen that the avicularium can be roughly distinguished 

 into three portions ; namely, a base by means of which it is attached to the cell, a rather large 

 head, and a movable spine like the lower mandible of a bird's beak. In those examples 

 where the avicularium is seated directly upon the cell, the only movement is that of the 

 lower mandible, which opens and shuts with a continual motion, as if it were a veritable 

 head of a hungry bird snapping at its food. In those cases, however, where the base is 

 lengthened into a neck, the entire head is endowed with motion, nodding up and down in 

 the most lively manner, very like those wooden birds sold in the toyshops, whose head and 

 tail are alternately raised and depressed by means of strings and a weight. But, whether 

 the head moves, or is still, the jaws continually open and shut, and will often inclose 

 between their parts any small worm that may happen to come across their path, and have 

 even been known to seize each other in their grasp. 



In Plate II. of the polyzoa, fig. N, may be seen an instance where the beak has seized 

 a victim. When the mandibles have thus closed upon any object, they retain their grasp 

 with astonishing tenacity, and when, as sometimes happens, two avicularia have seized the 

 same worm, the unfortunate victim is rendered entirely helpless by the grasp of its foes. 



The purpose of these objects seems to be rather dubious, but two conjectures have 

 been offered, which at all events are worthy of notice. 



According to the opinions of some observers, the avicularia answer the purpose 

 of police, and force intruders to leave the spot where their presence might do harm to the 

 creature on which they are placed. This duty seems, however, to be performed by the 

 vibracula, and we must search for another theory for the true object of the avicularia. 

 Mr. Gosse has put forward a conjecture which is not only highly ingenious, but bears 

 with it the elements of probability. 



" More than one observer," he remarks, " has noticed the seizure of small roving 

 animals by these pincer-like beaks, and hence the conclusion is pretty general, that they 

 are in some way connected with the procuring of food. But it seems to have been 

 forgotten, not only that these organs have no power of passing the prey thus seized to the 

 mouth, but also that this latter is situated at the bottom of a funnel of ciliated tentacles, 

 and is calculated to receive only such minute prey as is drawn within the ciliary vortex. 

 I have ventured to suggest a new explanation. 



The seizure of a passing animal, and the holding it in a tenacious grasp until it dies, 

 may be a means of attracting the proper prey to the vicinity of the mouth. The presence 

 of decomposing animal substance in water invariably attracts crowds of infusory animal- 

 cules, which then breed with amazing rapidity, so as to form a cloud of living atoms 

 around the decaying body, quite invisible in the aggregate to the unassisted eye ; and 

 these remain in the vicinity, playing round and round until the organic matter is quite 

 consumed. Now, a tiny annelid or other animal caught by the bird's head of a polyzoon 

 and tightly held, would presently die ; and though in its own substance it would not 

 yield any nutriment to the capturer, yet by becoming the centre of a crowd of busy 

 infusoria, multitudes of which would constantly be drawn into the tentaculean vortex 

 and swallowed, it would be ancillary to its support, and the organ in question would thus 

 play no unimportant part in the economy of the animal." 



In the engraving a solitary avicularium may be seen perched upon the cell, having its 

 beak widely expanded. Avicularia of various shapes and sizes may be seen upon the 

 sides of several other magnified specimens on this and succeeding plates. 



We now proceed to the vibraculum, one of which organs is seen at fig. K, attached 

 to the left-hand cell, and shown as it appears when bent downwards upon its base. 



The vibraculum is hollow, the interior being filled, during the life of the animal, by 

 a fibrous contractile substance, which enables the organ to perform its curious movements. 

 These movements are very irregular as regards time, but very regular in their directions, 

 each vibraculum sweeping slowly over the whole surface within its reach, first moving in 

 one direction and then in the other, and it is sufficiently notable that these movements 

 will continue for several days after the death of the polype to which it is attached. 



