OYSTER-DRILLS IN THE ESSEX ESTUARIES. 305
THE EGG-CAPSULES OF OYSTER-DRILLS.
The oyster-drills mentioned above all lay their eggs in egg-
capsules of the same type, but differing in general shape and
thickness (9). These egg-capsules are yellowish, tubular, vase-
shaped structures about 1-5th inch high and consist of a tough but
pliable semi-transparent horny substance. They are closed after
receiving a number of eggs, but open some weeks later when the
Fig. 2. (reproduced by courtesy of the Editor of "Nature.").
Egg-Capsule of the Three Oyster-Drills now Found in the
Blackwater Estuary, Essex.
1. Urosalpinx. 2. Ocinebra. 3. Purpura (in side view).
1a. do. 2a: do. 3a. do.
(in transverse section, to show relative thicknesses.)
eggs have developed into young individuals. Fig. 2 from a
drawing by Dr. C. Amirthalingam (9), shows the variation in shape
and relative thickness of the capsules in the three species. The
capsule of Urosalpinx is seen to be flattened oval in section, that
of Ocinebra triangular, while that of Purpura is sub-circular, and