THE LIGAMENT OF MYA ARENARIA (MYOIDEA) REVISITED

The ligament of the bivalve Mya arenaria was previously described as b eing composed of two layers: a fibrous and a lamellar one, the fibrous layer being more extensive. Other features described, such as the thr ee-dimensional extent of the layers and their mode of attachment to th e chondrophores, have been found to be inaccurate to a certain extent. The ligament is therefore redescribed here. It is conical in appearan ce and attached to the left chondrophore only at its margin, its inter ior being occupied by an epithelium, which is an extension of the mant le isthmus. The lamellar layer is a pyramidal body which merges into a nd constitutes the posterior extension of the fibrous layer. The whole structure is internal and may rotate around the dorsoventral axis, th us being suited to allow rocking of the valves. This kind of movement is used when Mya contracts the anterior adductor muscle to eject water from the mantle cavity during burrowing. In this respect the ligament is unique among those known in bivalves.