446 THE VIBRACULUM AND ITS OFFICE. 



in others. Only 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 move- 

 ment 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, how- 

 ever, 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 v ave ventured to suggest a new explanation. 



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

 may be a means '" ? the proper prey to the vicinity of the mouth. The pres- 



ence of decompf ' substance in water invariably attracts crowds of infusory 



animalcules, wh . .^eed with amazing rapidity, so as to form a cloud of living 



atoms around tne 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 move- 

 ments. 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. 



