ZOOLOGY: W. J. CROZIER 
381 
During progression on a surface, the periphery of the body is the part 
vitally concerned, for the animal may be transversely arched in such a way 
that, save at the very anterior end, the periphery is the only part actually 
in contact with the substratum. The attachment of the peripheral edge of 
the worm is produced by local suction. Although slime is secreted there, the 
manner in which the attachment is made to a small surface, e.g., a pencil 
point, and the way this attachment may be instantaneously released when 
the worm begins to creep, force one to believe that the edge of the body is 
not being held in place by sticky slime. This use of the edge of the 'foot' is 
similar to that found in Chromodoris, and perhaps in other large midibranchs 
(Crozier and Arey, 1918). 5 
At rest, the outline of L. 'tincta' is usually thrown into 'stationary waves.' 
These waves may appear truly opposite, not alternate, especially at the an- 
terior end of the worm. Occasionally, at the beginning of creeping, a 'mono- 
taxic' movement is executed, but this operation is characteristically 'ditaxic,' 
the lateral halves of the body contracting in alternate sequence. 
With one of the several species of Pseudoceros which are found inhabiting 
colonies of the tunicate Ecteinascidia turbinata Herd., I have been able to 
make observations which show that muscular creeping operations are probably 
general among polyclads. In this form also the locomotor waves are essen- 
tially alternate, 'ditaxic,' and retrograde, the axial region of the body being 
usually arched away from the substratum, thus separating the body into 
halves. The wave is a region lifted from the substratum. Two waves may 
be present on each side, simultaneously. Here, as in the Leptoplana, when 
the muscular waves are absent, no creeping progression can be detected. In the 
Pseudoceros the waves are especially well seen when the animal is creeping 
on the surface film. A rippling movement of the outline of the body usually 
accompanies the 'body- waves,' and when viewed from above this is the only 
contractile operation to be seen. The peripheral rippling may, however, be 
very slight or even absent altogether. 
Neither of these flat- worms has been observed to creep in any but an 
anterior direction. 
These observations tend to show that in turbellarians generally, muscular 
operations analagous to those executed by the foot of chitons and of gastro- 
pods are essentially concerned in creeping locomotion. 
1 Contributions from the Bermuda Biological Station for Research. No. 89. 
2 Stringer, C. E., 1917, These Proceedings, 3, 691-692. 
3 Parker, G. H., 1911, /. Morph., Wistar Inst., Phila., 22, (155-170). 
4 Gamble, F. W., 1901, The Cambridge Nat. Hist., 2, p. 1-96, 47 fig., London 
6 Crozier, W. J., and Arey, L. B. The sensory behavior of Chromodoris zebra. (In press). 
