The cirrus is a bundle of individual cilia protruding from a single cell. This is illustrated when a cross section of the cirrus is viewed using TEM (below). These structures appear to be unique to sphaeriid and corbiculacean bivalves (Way, et al, 1989) and have not been observed in Unionacean bivalves thus far. The physical appearance of these structures also varies among species. For example, the cirri of Polymesoda caroliniana resemble horse tails, giving the appeance that the cilia of these structures are not bound together by any mechanism other than their common cell of origin. However, those of C. fluminea and some sphaeriid (fingernail) clams are much more organized in appearance as though the cilia that compose the currus were bound to one another.
Images of the cirrus and it's cell are below.
Other structures found within the bivalve gill are found here.
Back to image of gill.