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BULLETIN OE THE UNITED STATES EISH COMMISSION. 
only trace of it is tlie opening in the loriea, through which it should pass. This 
opening is probably to lie found in all the Rattulidce. 
The position of the dorsal antenna perhaps indicates the position of the dorsal 
median line. This is true at least in other rotifers, and in the Rattulidce the surface 
which bears it is above when the animal is creeping on the bottom. Its place with 
reference to the striated area is therefore of interest. In Rattulus elongatus Gosse, 
R. cylindricus Imliof, and R. bicristatus Gosse, and, indeed, as a rule in the species- 
in which the striated area forms a furrow with its two sides equally developed, the 
dorsal antenna lies in the middle of the furrow. But in Rattulus muco.sus Stokes 
this is not true. The dorsal antenna in this species lies to the left of the striated 
area, in a notch in the outer side of the left ridge. 
A similar position is found in almost all species in which the striated area is 
developed as a single ridge. The antenna lies to the left of the ridge, usually at 
about the left edge of the striated area. (See pi. xi, figs. 95 and 100.) 
Lateral antennce.- — In most free-swimming rotifers (as in the Notommatadce , 
from which the Rattulidce are without doubt derived) the two lateral antenna! are 
situated one on each side, in the posterior third of the body, symmetrically with 
relation to one another. Many species of the Rattulidce have preserved nearly this 
primitive position, though usually with slight variations. There maybe a tendency 
for the right antenna to be a little farther forward (as in Rattulus scipio Gosse, pi. 
v, tig. 52, and Rattulus car-hiatus Lamarck, pi. xi, fig. 95), or to be a little nearer 
the dorsal side, as in Rattulus elongatus Gosse (pi. xil, figs. 102, 103, 105), or the 
opposite tendencies may be shown. But in some cases there is a very remarkable 
asymmetry in the positions of the two antennae. In Rattulus cylindricus Imhof, 
for example, the left antenna is at about the middle of the length of the body (pi. vn, 
fig. 63), while the right antenna is very far back, at the place where the loriea is 
joined by the foot. In Diurella stylata Eyferth the left antenna is still farther 
forward (figs. 28, 29, pi. hi), while the right one is on the posterior part of the body. 
There are probably no species of Rattulidce in which the lateral antennae can 
not be found by careful search. 
INTERNAL ORGANS. 
The internal organs partake, to a considerable degree, of the asymmetry so 
characteristic of the external anatomy of the Rattulidce. Otherwise the internal 
structure in this group does not present a great deal that is different from what is 
found in most of the related Rotifera, so that I shall treat of it only briefly. The 
Rattulidce are not a favorable group for a stud}’ of the characteristic internal 
structure of the Rotifera. 
(1) Alimentary caned. — The alimentary canal shows the following parts: The 
mouth opens into a muscular pharynx known as the mastax, containing ehitinous 
jaws or trophi. From the mastax a short, narrow tube, the oesophagus, passes 
backward to widen into the large, thick-walled stomach. The stomach narrows to 
form the intestine, which passes straight back to the anus. The entire course of 
the alimentary canal is well shown in fig. 63, pi. vii ( Rattulus cylindricus Imhof), and 
fig. 77, pi. ix ( Rattulus bicristatus Gosse). 
Mouth. — The mouth opens on the truncate anterior end, or corona, near its 
ventral side (pi. in, fig. 31, m). At its sides are two curves of cilia (b) which serve 
the purpose of carrying small particles of food to the mouth. 
