236 Mr. C. P. Sigerfoos on the Pholadide. 
rapidly to great size, and is very destructive. The sexes are 
separate and of about equal proportions. I found specimens 
sexually mature during the spring and until the middle of 
July. The breeding-season probably lasts all summer. ‘The 
egos are cast into the water, and, if the sexual products are 
mature, they are extruded through the siphons when the 
animals are taken from their tubes. The eggs are of about 
the same size as those of Pholas, but more opaque and of 
somewhat slower cleavage. The segmentation of the egg and 
the derivation of the germ-layers are step by step asin Pholas, 
so that to describe them would be but a repetition of every 
detail—planes of cleavage, establishment of bilateral symmetry, 
gastrulation and division of the entoblast-cells, double origin 
of the mesoblast, ciliation and rotation of the embryo, per- 
sistence of the crescent-shaped blastopore as the mouth. 
The accompanying figure represents a longitudinal section 
of an embryo of T. norvegica with eight entoblast-cells. The 
apical end is to the right. The posterior mesoblast, pm, lies 
in the cleavage cavity, as does also the smaller anterior meso- 
blast, am, upon the entoblast-cells. The four entoblast-cells 
shown are of subequal size. The shell-gland is just beginning 
to form. 
Teredo (Xylotrya) jimbriata. 
This species is equally as abundant at Beaufort as the 
preceding, but of smaller size. The sexes are separate and 
of about equal proportions. ‘The eggs develop in the water, 
but are not extruded as are those of 7. norvegica. They are 
easily fertilized artificially and develop rapidly. The embryos 
are hardy in aquaria. Except for the greater opacity of the 
egg, I think it impossible to distinguish the segmenting ege 
and embryo from those of Pholas. All of the features sum- 
marized tor Pholas are true for this species. 
Teredo navalis. 
As is well known, the eggs and embryos of this species are 
retained in the gills till the larva have reached a somewhat 
