In Yoldia and Nucula proxima the test consists of five rows of flattened cells, the three median rows bearing circlets of long cilia.
In Yoldia and Nucula proxima the ova are set free in the water and the test-larvae are free-swimming, but in Nucula delphinodonta the female forms a thin-walled egg-case of mucus attached to the posterior end of the shell and in communication with the pallial chamber; in this case the eggs develop and the test-larva is enclosed.
Animal of a male Nucula proxima, Say, as seen when horizon of an inter-filamentar junction, in the other (lower in the figure) at a point where they are free.
In Nucula delphinodonta the test is uniformly covered with short cilia, and there is no flagellum.
In the genus Nucula (fig.
No ciliated An extraordinary modification of the veliger occurs in the development of Nucula and Yoldia and probably other members of the same families.