196 BULLETIN OF THE BUREAU OF FISHERIES. 
BREEDING HABITS. 
As is well known, some species of ship-worms retain their eggs in the gills 
during their embryonic development. This is true of 7. navalis, and even the small 
(an inch or two in length) specimens of this species I have taken have usually carried 
embryos in various stages of development. On the other hand, I have found that 
the eggs of the other two species are laid free into and fertilized in the water. If 
species of 7. dilatata be taken from their tubes, they soon begin to extrude their 
sexual products, if these be mature. The eggs and spermatozoa are extruded from 
the anal or exhalent siphon in a slow, steady stream, which continues as long as 
the reproductive organs contain ripe sexual products. Xylotrya gouldi I have 
observed but rarely extruding its sexual products in this manner, but why there is 
a difference in this habit I have not determined. 
In association with their character of free development in the water, the eggs of 
the ship-worm are very small and very numerous. While they varysomewhat in size, 
they have an average diameter of somewhat less than 3!; mm. (;}5 inch). Very 
large ship-worms may lay great numbers of eggs at one time. In one case I estimated 
the number laid by a large female of 7. dilatata to be one hundred millions. The 
spermatozoa are very minute, and much more numerous than the eggs. The eggs 
of both species that lay their eggs into the water may be fertilized artificially, and 
develop with great uniformity and rapidity in aquaria. The eggs when first laid 
are of irregular shapes, but they soon become spherical and, if fertilized, the polar 
bodies are soon extruded and segmentation begins. Development is very rapid 
and on warm days the embryos become free-swimming within three hours after the 
eggs are laid. Within a day the shell has been formed and the typical lamellibranch 
veliger stage is reached. Beyond this stage I could not rear them in aquaria, though 
they may live for days afterwards. Hatschek has observed (1880) that the vivipa- 
rous larvee of a species studied by him (? 7. navalis) are almost always present in 
only a few stages, and that transitional stages are but seldom found. It seems 
probable that the free-living larvee of Xylotrya and T. dilatata attain one of these 
stages within a short time, and that the unusual conditions in aquaria prevent 
their advance beyond it. 
The mode of life of the larvee and the rate of development beyond the early 
stage attained so rapidly in aquaria have not been determined. What becomes of 
the larve after hatching from the eggs, how and where they live, it is difficult to 
surmise, Though the developed larve are settling on wooden structures constantly, 
I have not taken them and the intermediate stages in the tow-net, and where they 
develop I do not know. The rate of growth of larve of the marine lamellibranchs, 
however, is slow, and I think the larvee of ship-worms when they attach themselves 
must be at least a month old. They may be more, for at this time their development 
is quite advanced and their organization complex. (See the description of the organ- 
ization of the larva, p. 201.) 
The breeding period of X. gouldi and T. dilatata seems to extend throughout 
the warm season. I have found ripe sexual products of both species from early in 
May till the middle of August. At the latter time there seemed no abatement in 
