THE DEVELOPMENT OF ELECTRA CRUSTULENTA (PALLAS) 261
foot of the sea wall, and were exposed at low tide. Borg (1947:
347, text fig. 2) describes a similar mode of growth of colonies
found on the under side and in the interstices between stones in
the Baltic. The estuary at Creeksea is fully tidal and the salinity
may range between 24‰ and 36‰, depending upon the
amount of fresh-water drainage from the land. Generally
the variation is between 31‰ and 35‰ with a 2‰
fluctuation between low and high water. On 17th March 1959 a
five minute tow-netting into a rising tide at mid-stream at the
junction of the Rivers Roach and Crouch did not produce any
Cyphonautes larvae.
The polypides of E. crustulenta specimens from the Creek had
well developed intertentacular organs (fig. 1) in which a ciliary
current was observed. No eggs or sperm were found in the body
cavities of preserved specimens but egg extrusion was observed
twice in live material in 1960. On both occasions the egg was
first seen near the orifice of the intertentacular organ. The Poly-
pide was fully extended but the tentacles were closely apposed
and deflected to a position parallel to the frontal of the zooid.
The intertentacular organ was thus protruded as far as possible
above the surface of the colony. Several small rapid contractions
of the polypide occurred during which the egg was extruded,
becoming constricted as it passed through the intertacular pore.
Immediately after extrusion the tentacular crown expanded and
resumed a normal position. Unfortunately, the liberated eggs
could not be found amongst the colonies being observed and it
was thus impossible to trace their further development.
C. barroisi Lohmann is readily distinguishable from other
species of Cyphonautes by its valves which appear to be covered
in minute sand-grains, and by its small dimensions. The larvae
from the plankton in the Creek have the characteristic "sand
covered" appearance described by Barrois (1877: 235) and
Lohmann (1911: 39), which makes observation of the internal
structures difficult, the gut showing through them only as a darker
brown mass. The grains appear to be attached to the outside of
the valves (figs. 2 and 3) and are irregular in size, shape and
distribution. The smallest measurable particles were 1μ in
diameter, the largest 6-5μ. Although figured here as opaque, the
grains show differing degrees of transparency and vary in colour.
It is very probable that they are indeed minute particles of sand,
incorporated into the surface layer of the valve during growth,
but as very young larvae were not found no observations on their
development could be made.
Nearly all the live Cyphonautes measured were between 160-
200μ broad (the range given by Lohmann); they had probably
reached their maximum size, and had presumably been swimming
and feeding for their full developmental period and were ready to
settle. The largest living larvae observed and measured were
200μ broad and 150μ high (after preservation specimens averaged
170μ broad and 120μ high).