234 Mr. C. P. Sigerfoos on the Pholadide. 
other two species have been little studied. They are very 
abundant at Beaufort, and in the warm waters of our southern 
coast grow with great rapidity, so that they are extremely 
destructive to all kinds of woodwork. 7. norvegica may, 
under favourable conditions, attain a length of 4 feet in two 
years in hard piles. Its burrow is almost or quite an inch in 
diameter, so that a few of them may soon ruin a heavy pile. 
The present note is published through the courtesy of the 
U.S. Fish Commission. A fuller account of the natural 
history of the ship-worms will be published by the Com- 
mission. 
Pholas. 
Pholas is found in the stiff mud between tide-marks. It 
spawns the latter part of April and during May, so that the 
breeding-season is limited to a period of a few weeks. The 
sexes are separate and found in about equal proportions. 
The eggs are rather transparent, and hence favourable for 
study among marine Lamellibranchs, though their small size 
and the uniformity in the size of the micromeres make them 
unfavourable objects for studies of cell-lineage. Artificial 
fertilization is easy and the embryos develop with great 
uniformity. Development is very rapid, and on warm days 
the embryos may be free-swimming within three hours of the 
time the eggs are fertilized. 
The first two planes of cleavage are meridional, giving 
rise to the four-cell stage which has so often been described 
and figured for various Lamellibranch eggs. The next plane 
of cleavage is equatorial, giving rise to an eight-cell stage, 
consisting of a large macromere and seven micromeres of 
almost, if not quite, equal size. A sixteen-cell stage and a 
seventeen-cell stage are formed just as Lillie has recently 
described for Unio (Journ. Morph., Jan. 1895). In fact, his 
description for Unio of the formation of the blastomeres to 
the seventeen-cell stage applies strictly also for Pholas, except 
in the size of the micromeres. 
After the seventeen-cell stage I could not follow cleavage 
with certainty as to the lineage of the cells. ‘The axes of the 
embryo are approximately indicated by the position of the 
polar bodies, which persist till after the embryo has become 
free-swimming. ‘The posterior mesoderm arises as a single 
cell, as usually described, and soon afterwards the macromere 
divides into right and left entoblasts. Soon afterwards the 
mesoblast also divides into right and left halves, and bilateral 
symmetry is established. 
The two entoblasts soon divide into anterior and posterior 
