436 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Movement consists in rapid swimming, with rotation on its axis, or in creeping by means of its 
anterior cirri, or in sudden jumping, by which it apparently clears a distance of 20 times its diameter 
in one bound. Mouth parts may also be used for attachment to foreign bodies. The moving periods 
alternate with quiescent periods, during which the organisms with their outstretched and radiating 
cirri resemble the lieliozoon Aclinophrys. 
Mesodinium cinctum, n. sp. Fig. 31. 
Body spherical to pyriform, constricted near the middle, the constriction dividing the body into 
dissimilar parts. The anterior part is broadly pyriform, somewhat plastic and hyaline, with an oral 
extremity which is sometimes hollow, sometimes evaginated and convex. Upon this flexible anterior 
part there are four short but distensible tentacles. The posterior part is granular and usually filled 
with food particles; it is well rounded and holds the nucleus and contractile vacuole. The entire 
body is surrounded by a fine cuticle. The nucleus is elongate and extends through the greater part of 
the posterior half. The contractile vacuole lies on one side, near the girdle. The mqutli is on the 
anterior pole in the tentacle region. The motile organs are cirri and cilia, all inserted in the constriction. 
There are two sets of cirri and one of cilia; the latter stand out radially from the girdle and are usually 
in motion. The cirri of one set, the anterior, extend forward about twice the length of the anterior 
half; those of the posterior set closely engirdle the lower half, reaching not quite to the posterior 
extremity. These are somewhat hyaline and are closely approximated, giving the impression of a 
tight-fitting crenulate casing about the lower half. The cirri are sharply pointed, much broader at 
Fig. 31. — Side and top views of Mesodinium cinctum. 
the base, and the two sets are so placed that, looked at from above, they have the appearance of a 
twisted cord. (Fig. 31 b.) Movement erratic; sometimes the animal swims steadily forward with 
mouth in front; again it shoots across the field of the microscope, either backward or forward or 
sideways, through the action of its powerful cirri. It is often quiet, usually mouth downward, and is 
held in place by adhesion of the tentacles. In this position it looks strikingly like a heliozoon. 
Length 35// ; greatest width 30/o Uot uncommon. 
The chief features by which this species is distinguished from the frequently described M. pulex 
of Europe are the number of anterior cirri and the ring of true cilia in place of the central girdle of 
cirri. The European form is described with four anterior bristles; the present form has from 28 to 32. 
The radial cilia differ decidedly from the more powerful cirri and they are not in one plane, so that 
counting is difficult; they are not closely set. The presence of tentacles makes these forms of especial 
theoretical interest, especially in the light of the origin of Sudor ia. 
Genus TIARINA R S Bergh ’79. 
(Claparede & Lachmann ’58.) 
Body subcylindrical, pointed posteriorly, two and one-half times as long as broad; encased in 
covering composed of separate pieces arranged in five girdles. The pieces bear processes which rest 
against neighboring pieces of the girdle. Mouth large, anus terminal near contractile vacuole. The 
macronucleus is simple and round. Salt water. 
