380 
ZOOLOGY: W. J. CROZIER 
been supposed to cause creeping owing to their beatings. But the muscular 
operations involved in the creeping are of so obvious a kind as to be quite 
unmistakable. These movements are the same whether the worm is in water 
or out of it, whether it is creeping upon the surface film or on submerged 
objects. 
The essential feature of the progression is illustrated in Figures A, B, C. 
Creeping is rapid, as quick as 5 mm. per second. One side of the body at the 
anterior end is lifted from the substratum, thrust 4 to 5 mm. forward, and firmly- 
attached to the substratum; then the opposite side is thrust forward in a similar 
way. On each side of the body a wave is thus initiated, which may travel 
the whole length of the animal or may go only one-half to one-third the dis- 
tance toward the tail. The edge of the body is thrown into wavy wrinkles; 
these progress toward the posterior end, which is reached in 3 to 4 seconds. 
The succession of these waves is rapid, so that 4 to 5, or more, may be present 
at one time on each side of the body. This rapidity makes it difficult, ex- 
A B C 
FIG. 1 ILLUSTRATING THE METHOD OF CREEPING IN A SPECIES OF LEPTOPLANA. 
DIAGRAMMATIC. 
A, at rest; B, C, the initation of alternate waves of body contraction which are responsi- 
ble for creeping. (Natural size.) 
cept at the anterior end itself, to be sure in all cases of the truly alternate 
relation of the waves on the two sides; but in many instances they undoubtedly 
remain alternate, especially in slow creeping. The shuffling movement of 
the worm under these circumstances recalls vividly that of a retrograde alter- 
nate ditaxic gastropod foot. 3 
When Leptoplana 'tincta' is induced to creep with special rapidity, the 
body-waves become more pronounced, and although they remain distinctly 
alternate and 'ditaxic' in their origin at the anterior end of the animal, they 
may fuse to form a single retrograde, 'monotaxic' wave after about a third of 
the worm's length has been traversed. The locomotor wave thus becomes 
more allied to that developed in swimming, for although some other lepto- 
planas swim through the water by means of winglike flappings of the lateral 
extensions of the body, — a process analagous to the parapodial swimming of 
Aplysia — in L. 'tincta' the swimming movement involves a sinuous longi- 
tudinal ' gallop ' of the body as a whole. 
