294 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
anterior part of the body seems to be twisted over to the right, the posterior part 
over to the left. This will perhaps best be appreciated by examining fig. 1 (. Diurella 
tigris Miiller). The anterior pai-t of the ridge is far to the right of the middle line. 
The single tooth is also to the right of the middle; in those species where there are 
two teeth (as Rattulus longiseta Schrank, Diurella porcellus Gosse, etc.) the left 
tooth is nearly in the middle line, the right tooth much to the right of that line. 
At the posterior end, on the other hand, the indications are that what was 
primitively dorsal has passed to the left, while 1 lie right-hand one of the paired 
structures has taken a dorsal position. The dorsal projection of the lorica over the 
foot has become shifted to the left (shown particularly well, for example, in the 
figure of Rattulus lophoessus Gosse, pi. xi, fig. 99). The right toe has come to lie 
nearly on the dorsal side of the left one, so that the concavity of the toes (originally 
ventral) has become directed to the right (fig. 1). Thus the foot and toes can bend 
only to the right, not to the left. 
The body has become not merely twisted on its primitive straight axis, however, 
but is often bent at the same time so as to form a segment of a spiral (seen espe- 
cially well in fig. 1, of Diurella tigris Muller). As a result of this the left side has 
become convex, the right side concave. (Compare the following dorsal views in which 
this is evident: Figs. 1, 16, 29, 46, 52, 75, 78, 95, 99, 103.) These features are, of 
course, much more marked in some species than in others. 
These general changes have induced certain secondary ones. The originally 
right toe, which has become dorsal, gradually degenerates until it has become in 
many species a mere rudiment. The right mucus reservoir is likewise involved in 
this change, becoming smaller than the left. Owing, perhaps, to the enlargement of 
the left side as a result of its convexity, and the diminution of the right side owing 
to the concavity falling here, there is a tendency for the internal organs to be better 
developed on the left side than on the right. This is most strikingly brought out iu 
the structure of the trophi. The right half of the trophi, as shown in the account 
of these organs, is almost invariably smaller than the left, and in many cases is 
quite rudimentary. 
Altogether, we may say that the body in the Rattulidce tends to take the form 
of a segment of a spiral, and that this change from the primitive bilateral symmetry 
has induced also a considerable number of subsidiary changes. 
What is the significance of this peculiar condition in the Rattulidce? 
The key to the asymmetry of this group is to be found in a study of the move- 
ments and behavior of the animals. The unsymmetrical structure is, of course, not 
a primitive condition, but these animals were originally bilaterally symmetrical. 
The fundamental plan of structure is still that of bilateral symmetry; certain parts 
have been reduced or changed in position so that asymmetry has resulted, but the 
bilateral ground plan is easily traceable. The nearest relatives of the Rattulidce are 
still bilaterally symmetrical. Probably no one familiar with the Rotatoria will be 
inclined to question the view that the Rattulidce, are derived from the Notommatadce. 
The Notommatadce are typically creeping forms. They live among the weeds, on 
the surfaces of which they creep about by means of their cilia, keeping the mouth, 
as a rule, against the surface. 
The differentiations shown by bilaterally symmetrical organisms are usually 
brought into relation theoretically with their methods of movement, and doubtless 
