THE DEVELOPMENT OF ELECTRA CRUSTULENTA (PALLAS) 263
Figure 8. C. barroisi, settled larva.
Scale equals 100μ.
In life the apical organ is distinct and does not appear to be
ciliated. Preserved larvae, even when previously narcotised, have
a very similar appearance to that figured by Lohmann (1911, fig.
8) in which the apical organ is not apparent. The pallial lobes
are abundantly ciliated and a strong current was seen. The
edges of the valves have rounded protuberances which are associ-
ated in position with the inhalant and exhalant apertures (fig. 2).
In active swimming the apical organ was directed forward and
the valves were opened wide. Although the gut could not be
clearly seen, frequent contractions were observed during which
the valves were swiftly closed and forward movement momen-
tarily ceased. The larvae also frequently rested on the glass of
the container and rotated with apical organ at the centre. This
may have been a preliminary to settling (Atkins, 1955: 442), but
none was actually observed to take place after this behaviour.
Larvae were placed in an aerated tank of water from the
Creek and kept at a temperature of 11° C. with natural lighting.
A feeding culture of Phaedodactylon was added at the rate of 20
drops every 24 hours. Perspex frames each holding six 3" x 11/2"
glass slides in a horizontal position above one another at intervals
of 1" were immersed in the tank. In both 1959 and 1960 the
frames were transported soon after settlement to the British
Museum (Natural History) and kept at an average temperature
of 15° C. In 1960 aerating apparatus was used.
Larvae had settled on the slides within 24 hours of collection
and after 4 days single ancestrulae, recognizable as E. crustulenta
by the presence of the characteristic calcareous opercula, were