198 BULLETIN OF THE BUREAU OF FISHERIES. 
RATE OF GROWTH WITHIN THE WOOD. 
The ship-worm in-its larval stages develops slowly, but once in the wood it 
grows with remarkable rapidity. During its free life most of its energies are devoted 
to active locomotion and development; after attachment it leads a protected 
sedentary life and its growth is correspondingly rapid. The newly attached larva 
is somewhat less than 0.25 mm. long. In 12 days it has attained a length of about 
3 mm.; 16 days, 6 mm.; 20 days, 11 mm.; 30 days, 63 mm., and 36 days, 100 mm. 
It is thus seen that within two weeks from the time it has settled, the ship-worm has 
increased hundreds of times in volume, and in five weeks thousands of times. Within 
two weeks it has developed its characteristic form. Even in a month specimens 
may contain ripe sexual elements, though normally these seem to be retained till 
larger quantities of spermatozoa and eggs are stored for extrusion at one time. 
I shall describe later what appears to be a change of sex from males to females, the 
male sex being developed in young specimens. I have found males four weeks 
old gorged with ripe spermatozoa, and in every way sexually mature. 
The ages of larger specimens I have been able only to estimate from the time 
the piles and other wooden structures from which they were taken had been in the 
water. In one case I took specimens of Teredo dilatata, 4 feet long and an inch 
in diameter at the anterior end, from piles that had been in the water less than 
two years. This was in July, and in this case it seems the worms had entered the 
wood not earlier than the spring of the preceding year, and hence were little, if 
any, over a year old. 
The rate of growth seems to depend but little, if at all, on the hardness or kind 
of wood. As is well known, ship-worms penetrate all kinds, whether it be soft 
white pine or hard oak. In India there are types that bore into stiff clay. None 
of our species adopt such a habitat, so far as I know, but I have found small, abnor- 
mal specimens of Xylotrya in very rotten wood, and I take it that their abnormal 
character was due to the unusual conditions. In this case they were associated 
with Yylophaga dorsalis and Pholas dactylus. I have observed, however, that in well- 
preserved wood they grow quite as rapidly if it is hard yellow pine as if it is soft 
white pine; so that the rate of growth seems conditioned by food supply and not by 
the ability of the animal, as regards the hardness of the wood, to form its burrow. 
PROTECTIVE ADAPTATIONS. 
The life of the ship-worm in the wood has led to profound changes in the char- 
acter of its external parts and its means of protection. As it enters, the posterior 
part of the body projects more and more beyond the shell, which loses its protective 
character in large measure to take upon itself the purpose of burrowing. In speci- 
mens 2 mm. long the shell is still a quarter of the total length; in specimens 4 feet 
long, the shell is an inch or less in length. With the loss of protection of the soft 
body by the shell other means are acquired. In a general way the wooden wall 
of the burrow offers the protection afforded by the shell in other mollusks. But 
the very delicate tissues of the mantle would be injured by the slightest roughness 
in the surface of the wood. So, as the body elongates, the mantle secretes around 
