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The foot first appears when bivalve larvae are about 200 μm in length, and becomes functional in crawling and attachment at ~260 μm shell length. This is the pediveliger stage of development, which immediately precedes settlement and metamorphosis (see Figure 5.10). The ciliated foot is proportionately very large and sock shaped, and is made up of layers of circular and longitudinal muscles surrounding a capacious haemolymph space. A byssal duct opens at the ‘heel’ of the foot, and a byssal pedal groove extends forward along the ‘sole’ from this opening. The groove is embedded in secretory tissue, which produces the different byssal thread components (see later). While swimming, the foot is fully extended, and periodically the velum (larval swimming organ) is withdrawn and the larva sinks to the bottom and begins to crawl. If the substrate is unsuitable (i.e. does not stimulate the secretion of byssus), the foot is withdrawn and the larva once again swims off (Lutz & Kennish 1992). This cycle can be repeated many times over a period of a few days. In Mytilus, when a suitable substrate is found, the larva continues to crawl for some time, gradually ceases movement, protrudes the foot and quickly secretes a single byssal thread. In the newly attached mussel larva, this thread can be repeatedly broken and reformed before final settlement takes place. As the mussel grows in length, more and more attachment threads are secreted; this is not surprising, as larger individuals are subject to greater mechanical stress than smaller ones. To resist dislodgement, mussels cluster their threads in the direction of applied forces (e.g. facing ebb and flow of tide). The adhesive in mussel larvae differs from that of adults, resembling the mucous secreted by other benthic marine species at the larval stage (Petrone et al. 2008). The green crenella, Musculus discors, is unusual in that the byssus threads that are used to fix it to the substrate are woven into a nest or cage surrounding the shell, similar to a ball of twine. Eggs in mucous strings are retained within this nest, which may incorporate a variety of macroalgae (Merrill & Turner 1963).