ROTATORIA OF THE UNITED STATES. 
279 
STRUCTURE OF THE RATTULID^E. 
The Rattulidce, are Rotatoria, usually of small size, in which the cuticle of the 
body has become stiffened to form a sort of shell, called a lorica. At the anterior 
end is a ciliated area or corona, means of which the animal swims; this may be 
retracted within the lorica. At the posterior end is a small separate joint, known 
as the foot (/, figs. 1, 27, 46, etc.). To the foot are attached one or two bristle-like 
structures, which are called the toes. The internal organs comprise an alimentary 
canal, nervous, muscular, excretory, and reproductive systems, and certain mucus 
glands. In the following account these sets of organs will be taken up in order. 
I. EXTERNAL FEATURES. 
(1) General form. — The more usual form of the body in the Rattulidce, is that 
of a cylinder, or long oval, frequently curved. In some cases the body is much 
elongated, as in Rattulus elongatus Gosse (pi. xii, fig. 102), or Diurella insignis 
Herrick (pi. II, fig. 15); in other cases it is short and plump, as in Diurella porcellus 
Gosse (pi. ii, figs. 19-21). In a few cases (Rattulus lotus Jennings, pi. vn, figs. 65, 66; 
R. nudticrinis Kellicott, pi. VI, figs. 55-57) the body is broad and ovoid in form. 
A striking feature of the animals is their tendency to asymmetry in shape. 
This shows itself in many ways. The body with the toes usually forms a curve, 
concave to the right, convex to the left (figs. 1, 8, 16, 28, 46, 95, 99, 102, etc.). The 
curve is often not simple, but is of such a nature that the body forms a segment 
of a spiral. This is perhaps best seen in fig. 1, of Diurella tigris Muller; it is a 
characteristic which is difficult to represent in a drawing, although often very 
noticeable in the animal itself. As will be seen later, the asymmetry shows itself 
in the form and arrangement of many organs. 
(2) Lorica. — The body is covered with a hardened cuticula, known as the lorica. 
The lorica covers the body completely, being without openings at the sides, but it 
is open anteriorly for the projection of the corona, and posteriorly for the protrusion 
of the foot. The lorica is not so stiff and unyielding in the Rattulidce as in many 
of the Rotifera, usually permitting considerable change of form. Compare, for 
example, the extended form of Rattulus longiseia Sehrank (pi. vm, fig. 67) with the 
contracted form in the same species (pi. vm, fig. 70). In some species the lorica is 
stiffer, not permitting such marked changes in shape. 
Head-sheath. — The anterior part of the lorica is usually set off from the remain- 
der of the body by a slight constriction. This anterior portion, covering the head, 
may be known as the head-sheath (h. s., figs. 1, 3, 8, etc.). It presents a number of 
interesting characteristics, and some that are very important in classification. Only 
in Rattulus latus Jennings is it impossible to distinguish a head-sheath from the 
remainder of the lorica. 
The head-sheath frequently has longitudinal plaits, if they may be so designated, 
which serve for permitting the folding of the head-sheath when the head is retracted 
within the lorica. These are well seen in figs. 3, 4, 58, 59, and 62. These plaits 
seem to be due to alternate longitudinal strips of hard, stiff material, and of soft, 
yielding cnticnla. On the inner surface of the head-sheath are many fine transverse 
muscle fibers (shown especially in figs. 58 and 59, pi. vi). When the head is drawn 
within the lorica, these longitudinal folds are brought together by the yielding of the 
