Some species are provided with avicularia and vibracula, some with avicularia alone and a few with vibracula alone.
It is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum, and an avicularium like the head of a bird; yet they are almost certainly homologous and have been developed from the same common source, namely a zooid with its cell.
Hence, we can understand how it is that these organs graduate in some cases, as I am informed by Mr.
Busk, into each other.