IT is the enormous over-development of the siphons that has enabled the shipworm to pursue its destructive activities. The shipworm bores into the timbers of piers and breakwaters and, before the days of copper-sheathing, was a source of considerable trouble to shipping. It consists of quite a small body proper, enclosed in the usual bivalve shell, from which extend backwards a pair of long siphons. In some species the siphons are up to two or three feet long. The manner in which the boring of timbers is carried out has not yet been fully elucidated so that it is only possible to say what the shipworm does and not how it does it. The tunnels are driven into the wood with the grain and a piece of timber well-infested is excavated with longitudinal tunnels, many of them separated from each other by the thinnest partitions of wood imaginable, so that what appears to be a mighty baulk of timber is really a fragile mass, ready to fall to pieces at the buffeting of the waves.
A close relative of the shipworm, the piddock, bores into limestone rocks, and evidence of its work may be seen in the limestone pebbles, sometimes picked up on the shore, which are riddled with holes into which the finger can be inserted with ease. The piddock, however, has not undergone the change seen in the shipworm. It is a normal bivalve with the front ends of the shell ornamented with rows of rasp-like teeth, to assist in the boring, and a pair of siphons only slightly elongated. Some piddocks bore into driftwood, and samples of their handiwork may, again, be picked up on the shore.