198 BULLETIN OF THE 
which were identified as belonging to the genus Lwmbriconereis, were col- 
lected at low tide from this locality. They were found attached to the surface 
of the mud in the form of gelatinous clusters glued together in spherical masses 
of slime, which, when the flats are covered with water, wave to and fro with 
passing currents. In size and general external appearance, they closely 
resemble similar clusters of mollusk eggs found in the same place. The worm 
embryos, however, have a more greenish color than that of the mollusk, and 
can after a little practice be distinguished by the unaided eye. 
The segmentation of the Arenicola egg, which is identical with that of Lwm- 
briconereis, has been described and figured by several naturalists, so that the 
changes which take place in the egg up to the formation of the planula are well 
known. The series of larval forms which are described and figured in this paper 
opens with one where a segmentation of the ovum is completed, and extends to 
a larva in which certain generic structures of Lwmbriconereis are well marked.* 
In Fig. 1 we have represented the egg at that period in its development 
when the smaller spheres, “ micromeres,” have partially grown about the larger 
*macromeres,” two of which seen in profile appear at one pole. When the 
pole at which the macromeres lie is seen from above, so that they occupy the 
centre of the circle of vision, four macromeres surrounded by the encroaching 
micromeres can be easily seen. When viewed in profile, as in the figures 
given, only two of the larger spheres appear. In an egg a little older, the form- 
ing micromeres encroach still more on the pole at which the macromeres 
approach the surface of the ovum, until ultimately the latter are wholly sur- 
rounded by the smaller cells. 
The youngest of the planula series has a spherical and slightly ovate form, 
bearing at one pole a clear projection easily distinguishable from the remaining 
parts of the embryo. This projection may be called a cephalic prominence. 
The whole interior of the larva is occupied by large nucleated cells, which are 
easily seen through a transparent outer layer. On the pole opposite that capped 
by the transparent cephalic projection which has been mentioned, there has 
been differentiated from the outer surface a thin layer which marks the begin- 
ning of the body of the worm. The cap-shaped cephalic prominence at the 
upper pole is the first appearance of ahead. The whobe external surface which 
lies between these prominences of the larval body is a broad ciliated zone, 
which occupies the greater part of the external surface of the worm. 
The first important additions to a simple larva girt by a broad band of cilia, 
which we have just described, is the formation, at either pole, in the clear 
spaces which we have mentioned, of small prominent pigment spots. Five of 
these are formed at the cephalic, and four at the caudal pole. Those which 
appear in the cephalic prominence are three in number, placed one medially, 
with one on each side near the pole, and one on each side near the equatorial 
* These larve resemble closely the young Arenicola marina (piscatorum) of Max 
Schultze (Abhand. d. Natur. Gesell., 1856), or those of A. cristata, Stimp., described 
by Dr. E. B. Wilson, op. cit. The larve doubtfully referred to Lumbriconereis by 
Claparéde and Metschnikoff (op. cit.) are generically different from mine. 
