22 Eev. T. Hlncks on the Polyzoan Avicularium. 



sionally this structure is varied by the development of a chiti- 

 nous expansion along each side of the spine, by which it is 

 converted into a kind of flapper. 



The avicularium with more or less elongated mandible is 

 a step towards the second of the appendicular organs with which 

 the CheilostomatouS Polyzoa are furnished, the vihraculmn. 

 The latter, in its most highly specialized form, is the termi- 

 nus of one of the branch lines before referred to. There can 

 be no reasonable doubt that it is a derivative from the avicula- 

 rium, and not an independent modification of the oral valve of 

 the zooecium*; for the steps by which the one appendage passes 

 into the other are easily traceable. The observations whicli I 

 am about to record crown the evidence, by exhibiting within 

 the history of a single species the leading stages of the trans- 

 formation. They also illustrate in a very striking way the 

 instability of avicularian structure, and the liability to varia- 

 tion which is one of its chief characteristics. 



The criterion by which it has been proposed to distinguisli 

 the vibraculum from the avicularium (the absence of a beak) 

 is, of course, a purely arbitrary one ; for the mandible takes 

 on the specifically vibracular function before the beak has 

 vanished. In the highest form of vibraculum the beak is 

 retained, but is so modified as to supply a deep terminal notch 

 or cleft, in wdiich the seta is suspended without liindrance to 

 the freedom of its play. In tliis form the function is most 

 specific and is clearly defined ; planted close to the orifice of 

 the cell, its seta sweeps energetically at intervals over the 

 front and dorsal surfaces, and helps to secure freedom of 

 egress and action to the polypide. 



I come now to the observations which it is the chief object 

 of this paper to record. 



Amongst the species in which the avicularium is furnished 

 with an elongated mandible is the ubiquitous 

 Mtcroporella ciUata, Pallas, which has well nigh Fig. 1. 

 accommodated itself to all climes and circum- 

 stances. It exhibits, however, this peculiarity, 

 that the condition is not constant : in some 

 cases the avicularium is of the ordinary type 

 (woodcut, fig. 1) ; in others the mandible is more 

 or less prolonged into a straight and slender 

 spine (woodcut, fig. 2). The prolongation is 

 usually moderate ; and, so far as hitlierto observed, 

 it is an extension merely, without any further 

 modification. 



* Darwin, ' Origin of Species/ 6th edit. p. 194. 



