NATURAL HISTORY OF THE SHIP-WORMS. 197 
their development. As will be described later, individuals become sexually mature 
in a month after they have attached, and those which attach in August must bear ripe 
sexual products latef in the season, so that the breeding period would seem to extend 
throughout the warmer months. 
ATTACHMENT OF THE LARVA. 
During its free mode of life, the ship-worm larva has gradually developed into 
the larva typical of marine lamellibranchs. There is a bivalve shell into which the 
whole creature can be withdrawn for protection; a large swimming organ, the velum, 
by means of which the larva swims freely in the water; a long, powerful foot, by 
means of which it crawls actively over surfaces; and the internal organization 
peculiar to advanced lamellibranch larve. At the end of this larval development, 
in fact, the ship-worm larva is a typical small bivalve, except that it possesses the 
swimming organ. 
Throughout the summer (or at least from May till the middle of August) at 
Beaufort, if one examines fairly clean, unprotected wooden structures submerged 
in the water, very small bivalves will be found crawling actively over the surfaces. 
These are very minute and are easily recognized as ship-worm larve that have 
just settled upon the wood. The larva moves rapidly in search of a favorable 
place for attachment, and this is usually in some minute depression or crevice 
in the wood, though it may become attached to perfectly smooth surfaces. It 
seems to possess no organ of special sense for the purpose, and yet it is able to deter- 
mine what places are favorable for its future life and to avoid those which are not. 
Once it has chosen a point for attachment it throws out a single long byssus thread, 
thus securing itself to the surface of the wood, and very soon loses its velum, so 
that it can no longer lead a free-swimming life. Once attached, the larva begins 
to clear away a place for its burrow by scraping away the surface of the wood with 
the ventral edges of its shell valves. Such small particles of wood and other sub- 
stances as are thus collected are cemented together over the larva so as to form a 
sort of conical covering for protection. This formed, the further transformation 
of the larva into the small ship-worm begins and progresses rapidly. The foot 
becomes a pestle-shaped organ which assists the shell in burrowing. The shell 
valves lose their power of opening at the ventral side and, by the development. of 
knobs on the ventral and dorsal portions of both valves, are able to swing upon 
each other at right angles to the former direction. Meanwhile, because of the 
rapid growth of the valves on their ventral edges, the shell gapes at both anterior 
and posterior ends, for the protrusion of the foot in front and the siphons (and 
later the body) behind; and on the external surface of the valves at the anterior 
edges has been formed the first row of the small teeth which at this and later stages 
are the mechanical agents by which the animal bores into the wood. This trans- 
formation has taken place within two days from the time the larva has settled, and 
afterwards the animal rapidly becomes an elongate ship-worm, enlarging its burrow 
in the wood as it increases in size. 
