2-4 Rev. T. Ilincks on the Pohjzoan A mcularium. 



forms is suspended. This modification, wliicli is veiy slight 

 in degree, secures to a corresponding extent a freer range of 

 movement. 



In this remarkahle variety, tlicn, the avicuhirium of the 

 normal M. ciliata is replaced bj a well-developed organ with 

 vibracular function, which has made a considerable advance 

 towards the structure of the most specialized vibracula. 

 Placed as they are on the summit of a considerable rising, at 

 a short distance below the orifice, the tall setse command the 

 whole of the oral tract ; and their vigorous sweep must do 

 much to prevent the accumulation of noxious matter within 

 its bounds. 



It should be mentioned that the ordinary forms of the 

 species also occur somewhat abundantly on shells from the 

 Queen Charlotte Islands. Another interesting modification 

 occurs in the same species. On a large colony, obtained by 

 Capt. Cawne Warren, ])robably from the coast of 

 Ceylon or from Bass's Straits, the avicularium 

 is furnished with the spinous prolongation of 

 the mandible, and along each side of the 

 spine a delicate membranous expansion is 

 developed (woodcut, fig. 4) , which completely 

 alters its appearance and fits it for a new func- 

 tion or for the discharge of the old one in a 

 difterent way. The avicularian jaws and the 

 vibracular setaj are replaced in tliis variety by 

 the jlapi)er ; and these varied modifications are 

 embraced within the life-history of a single species. 



We are already acquainted with the changes which occur 

 in the radical fibres of the Polyzoa correspondent with diver- 

 sities of habitat. When the sponge or other soft substance is 

 the site of the colony, they develop a system of hooks, to act 

 as grapnels ; when the smootli, tough frond of the sea- 

 weed, they elaborate adhesive disks for attacliment. It would 

 seem that a like ready adaptability to clianges of circum- 

 stance is also characteristic of the avicularian appendages. 



These observations, besides their morphological interest as 

 throwing a clear light on the genealogy of the vibraculum, 

 bring out very forcibly the instability of avicularian struc- 

 ture, to which I have already referred ; and in the ]>resence 

 of such facts as 1 have now adduced and others like them, 

 I find myself unable to agree with those who assign a liigh 

 value to the appendicular organs for purposes of classifi- 

 cation. 



