border of the carapace bears a
spur-like projection. The ventral cirri are short and thick, and are
very characteristic of the species. When moving slowly they look much
like nicely-pointed paint brushes, but when the animal is compressed
they quickly become fibrillated, and then look like extremely old and
worn brushes. These cirri are placed in depressions in the ventral
surface and each one appears to come from a specific shoulder. At the
posterior end an oblique hollow bears 6 unequal cirri placed side
by side. The extreme right cirrus is the largest, and they become
progressively smaller to the opposite end. Dorsal to these lies the
contractile vacuole. The peristome is in the posterior half of the
body and an undulating membrane extends from it into the oesophagus.
The dorsal surface is longitudinally striated by 5 or 6 lines, which
are usually curved. The nucleus is horseshoe-shaped and lies in the
posterior half of the body. Length 68 mu; diameter 48 mu.

[Illustration: Fig. 56.--_Aspidisca hexeris_.]

This form was incorrectly mentioned as _Mesodinium_ sp. by Peck '95:

In the figure given by Quennerstedt there are only 7 ventral cirri.
In the Woods Hole form there are 8, 7 of which are anterior, 6 of
them about one central one. The eighth cirrus is by itself, near the
base of the largest posterior cirrus. These cirri, in spite of their
size, are easily overlooked and more easily confused, but by using
methylene blue they can be seen and counted.

Aspidisca polystyla Stein. Fig. 57.

This species is similar to _A. hexeris_, but is smaller, very
transparent, and without the spur-like process on the left edge of
the carapace. The chief difference, however, lies in the number of
anal cirri. These are 10 in number and they are arranged obliquely as
in the preceding species, with the largest one on the right and the
smallest on the left. The ventral cirri are 8 in number, and are
arranged in two rows, one of which, the right, has 4 cirri closely
arranged, the other having 3 cirri close together and one at some
distance, near the largest anal cirrus. The peristome, contractile
vacuole, and nucleus are similar to the preceding. Length 36 mu; width
22 mu.

[Illustration: Fig. 57.--_Aspidisca polystyla_.]

Stein assigns only 7 ventral cirri to this species, but he also
describes 2 very fine bristle like cilia (p. 125) and pictures them
in figs. 18, 19, 20, and 21 of his Taf. III in the same relative