298 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
of the animal. It often swims along with its spiral path tangent to a surface, 
almost every turn bringing the animal against the surface. Such a tangent surface 
may be represented by the line x-y in text figure 2. But, as the figure shows, of 
course it is always the dorso-dextral angle which comes in contact with the surface, 
and the tooth or teeth protect the soft head from injury. No teeth are present on 
the left side, because they would serve no purpose in that position. 
Finally, the twisting of the foot and toes, so that they can turn only to the 
right, finds its explanation along the same line. The entire animal is constructed 
on the plan of turning to the right, and the arrangement of the toes is merely 
another adaptation to this. If the toes were so arranged as to bend downward, a 
sudden stroke with them would turn the organism toward the ventral side, quite in 
opposition to the other tendencies of the animal. But with the toes turning to the 
right, their action is brought into harmony with the rest of the behavior of the 
animal. On getting to a place where it can go forward no further, or as a result of 
other strong stimulation, the animal turns its toe or toes suddenly and strongly 
to the right and forward. By this the usual swerving of the animal to the right is 
strongly accentuated; the path of the animal is thus suddenly changed. 
I have attempted to give an explanation of 1 lie decrease in size of the right toe 
in the general account of the toes (p. 284), which may be referred to here to com- 
plete the account of the factors which result in the production of asymmetry in 
this group. 
A few other points should be mentioned in regard to the movements of these 
animals. There are a few of the smaller species of Battulidce, with short thick 
bodies, such as Diurella porcellus Gosse and D. brachyura Gosse, which do not 
invariably swim in a spiral, though they do usually. In some cases one of these 
animals will be seen to swim for a short distance in the following manner: With 
ventral side down (or up), the body swings on the long axis from side to side, giving 
it a peculiar rocking motion, but without revolving completely. After swimming 
for a short distance in this manner the animal may suddenly begin to revolve and 
continue its course in a spiral path like the other Battulidce. 
The Battulidce . not infrequently creep along the substratum with the coronal 
face against the surface. Under these circumstances the animal of course does not 
revolve. But the unsymmetrical structure produces its effect even in this case. 
The animal very rarely moves in a straight line, but usually follows the curve indi- 
cated by the form of the body, thus circling continually to its right. That this 
might perhaps be expected will be seen by examining the figures of Bait ulus 
lophoessus Gosse (pi. xi, fig. 99) and Diurella ticjris Muller (fig. 1), as seen from 
above. 
The habit which these animals have of affixing themselves to foreign objects 
by a string of mucus has already been described (p. 293). 
The above account of the movements of the Battulidce has been drawn from a 
study of a considerable number of species. Indeed, all through the work on the 
group special attention was paid to this matter. I have studied especially in this 
connection Diurella hgris Muller, Diurella porcellus Gosse, Diurella brachyura 
Gosse, Diurella stylata Eyfertli, Balt ulus raft us Muller, Battulus carinatus Lamarck, 
Battulus bicristatus Gosse, Battulus mucosus Stokes, Battulus bicuspes Pell, and 
Battulus elongatus Gosse. In all these the behavior is essentially as set forth above. 
