458 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Genus ASPIDISCA Ehr. 1830. 
(Perty ’52; C1.& Lacli. ’58; Stein ’59; Quennerstedt ’65, ’67, ’69; Mereschowsky '79; Kent ’81; Biitschli ’88.) 
Small, colorless, and rigid forms, with nearly circular to oval contour. The left side is usually 
straight, or at least but slightly convex. The right side is much more convex, and the right margin is 
considerably thickened. The ventral side is flat, the dorsal surface convex, with from one to several 
longitudinal ridges which run more or less parallel with the right edge. The peristome is limited to 
the left edge, where it forms a small depression which may or may not reach the anterior border, but 
which in no case runs around the anterior margin. The left peristome margin in some cases grows 
over the peristome depression toward the right, thus making a sort of cover for the peristome. In the 
posterior region is a deep depression, from which 5 to 12 cirri take their origin. Seven or 8 cirri 
are placed in the anterior half of the ventral surface and are arranged more or less in rows. The 
anus is on the right side in the region of the anal cirri (Stein). The contractile vacuole is generally 
on the right side and similarly located. The macronucleus is a horseshoe-shaped body. Movement 
rapid, somewhat in circles, and 
rather uniform. 
Fresh and salt water. 
Aspidisca hexeris Quenner- 
stedt ’67. Fig. 56. 
The carapace is elliptical, 
about 1 1 times as long as broad, 
roundedatthe extremities. The 
left border of the carapace bears 
a spur-like projection. The ven- 
tral 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 com- 
pressed they quickly become 
fibrillated, and then look like ex- 
tremely old and worn brushes. 
These cirri are placed in depres- 
sions 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 pro- 
gressively 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//; diameter 48//. 
This form was incorrectly mentioned as Mesodinium sp. hy 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 1 central one. Tire 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. liexeris, 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 
