; Kent '81; Gruber '84; Entz
'84; Maupas '83. Buetschli '88.)

Small, colorless (except for ingested food) animals with
characteristic springing movements. The form is usually constant, but
in some cases may be plastic like _Astasia_; it is usually globular
or conical, the posterior end being more or less pointed, the
anterior end broadest. The latter is surrounded by a complete circle
of the adoral zone, the oral end of which passes into a peristomial
depression which extends deep into the middle of the body. The mouth,
with a very small oesophagus, lies at the bottom of the inturned
peristome. The region surrounded by the adoral zone is frequently
drawn out into an anterior process, occasionally bearing a pigment
mass. The ventral surface in some cases bears cilia, which may be
distributed or restricted to a row of large cilia. Trichocysts are
usually present and may be widely spread, limited to the posterior
region, or arranged in a girdle about the middle. The contractile
vacuole is simple, and posterior in position. The macronucleus
is spherical and usually central in position. Movement is rapid
swimming, combined with resting and floating periods, the latter
usually terminated by a sudden leap.

Fresh and salt water; more common in the latter.

Strombidium caudatum Fromentel '74. Fig. 46, a, b, c.

Fromentel described a fresh-water form of this genus with a caudal
appendage. The body is pyriform, broadly truncate on the anterior
end, in the middle of which rises a papilliform process (Schnabel).
On this process is a heap of pigment granules, which, however, are
not constant. A ring of long cirri surround the anterior end and pass
into the peristome, and from the left edge of this line of cirri a
large adoral zone continues down to the mouth. The peristome is
elongate and sac-form, and the mouth lies at the posterior extremity.
With the exception of a caudal filament there are no other motile
organs; this is about half as long as the body, structureless,
hyaline, and sharply pointed. It splits up into a bundle of fine
fibers upon treatment with caustic potash (c). The cirri emerge from
minute hollows in the edge of the anterior border. The cortical
plasm contains peculiar rod-like bodies, which look more like lines
or markings than like rods or trichocysts. The nucleus is large,
spherical, and placed in the center of the body. The contractile
vacuole is posterior.

Length without appendage is about 35 mu;