267 
added throe European species as infesting submerged timber in Victoria.* But as 
an Australian ship-worm, twice as large and with a different shell, is identified with 
a foreign species, these records may require confirmation. 
The Australian species are among the largest in the world, for they reach a 
length of almost 6 feet, and a diameter of more than an inch.f A fossil ship-worm 
from the Cretaceous beds of Queensland is described by Mr. 11. Etheridge as 
reaching a breadth of an inch and a half. J 
A rudimentary pair of valves at that end of the worm which is sunk deepest 
into the wood, have long ceased to fulfil their original purpose of protecting the 
soft body. Assuming a quite different function, they have become clamps to grip 
the wall of the burrow, and thus afford a fulcrum to the foot when scraping at the 
mine face. 
Whereas the crustacean pests already mentioned, actually eat the wood for 
food, the ship-worms feed by suction on silt or animalculfc floating in the water, 
and bore into the wood as they might into some inorganic substance for shelter only. 
Sometimes, indeed, the burrow is sealed at the end and lined for its length with a 
shell Avail, thus excluding the animal from the wood. 
The process of excavation has been a subject of much debate : the following 
conflicting explanations have been suggested : (1) That the wood is destroyed by 
some chemical agent secreted by the worm; but no such acid has been observed, and 
the clean even cut is not like the rough surface half-consumed by a chemical 
mordant. (2) That the hole was rasped out by the blade or the back of the shell; 
but the delicate surface of the shell shows under the microscope no sign of wear. 
(3) That the fleshy foot of the ship- worm, shaped like a pestle and roughened with 
lime spicules, has been the instrument of perforation. This latter appears to be the 
correct explanation. The drill is equally efficient against the hardest or the softest 
wood. A specimen of the boring is shown in Fig. 4. 
In some species the eggs arc extruded into the water, and there fertilised, 
but in other ovo-viviparous species the larvrc are hatched in the gills of the parent. 
These eggs are extremely small, having a diameter of not more than one five- 
hundredth of an inch. So prolific are they, that Professor Sigerfoos estimated that a 
large ship-worm might produce 100,000,000 eggs. 
Having hatched, the larvae exist for probably a few weeks as a free-swimming 
organism, they then settle down on some wooden structure, moult their larval 
features, and commence to burrow. A month later the animal has grown from 
microscopic dimensions to a length of a few inches, and probably attains its full size 
within a year. 
White the Crustacea work at or near the surface of the sea, the ship-worms 
may descend to any depth. One was found boring a deep sea cable, and another 
was dredged by the Challenger Expedition from a depth of 1,400 fathoms. 
* G.itliffand Gabriel Proc. Roy. Soe. Viet, xxviii, 19!.'>, p. 171. 
t Hedley Proc. Austr. As?oc. Adv. St-i., 1901. p. 231. 
t Etheridge Hoc. Austr. Alus. iv. 10 il, pp. 201-204,'pU 34 35. 
SijjTfoos Bull. U.S. Bur. Fish, >:xvii, 1<;07, p. 190. 
