256 
G. E. Coghill 
THE SWIMMING MOVEMENT. 
The movements of Diemyctylus embryos are of two main t}^pes: 
(1) the flexure, which is a bending of the body in one direction only; 
(2) the ‘‘S” movement or reaction, which is a bending of the more 
cephalic and the more caudal parts of the body in opposite direc- 
tions, giving the form of the letter S. 
The flexure may occur in several varieties. It may be a ‘"head : 
flexure,” which effects a movement of the head only; a “pectoral i 
flexure,” which affects slightly more of the trunk than the head I 
flexure does; a “mid-trunk flexure,” which is effected by the mus- i 
cles of the middle portion of the trunk only; a “general flexure,” ' 
which involves the bending of the whole trunk. In the mid-trunk I 
or pectoral flexure the parts cephalad and caudad of the flexed i; 
part may assume positions parallel to each other, in the form of 1 
the letter U. This may be designated as the ‘‘U” reaction. The • 
general flexure may be extended till the body assumes more or less ^ 
a coiled condition. This movement may be termed the “coiled 1 
reaction.” j 
The various forms of the flexure are not to be considered as 
essentially distinct, for, with possibly the exception of the U reac- 
tion, they develop gradually one into the other in the order men- 
tioned. Nevertheless, the distinctions are useful for descriptive ; 
purposes. 1 
The first member of this series to appear in the course of devel- 1 
opment of the embryo is the head flexure; the next is the pectoral ij 
flexure, and, as the embryo advances in age, the flexure extends 
farther caudad until it involves the entire trunk in a general flexure, 
and, finally, in a coiled reaction. In ontogeny, then, the flexure 
develops cephalo-caudad. This is true for responses to stimula- 
tion on the tail bud as well as for responses to stimulation on the 1 
head. 
In the development of any particular flexure, pectoral, general or 
coiled, the same progression cephalo-caudad is observed. If the n. 
trigeminus or n. vagus is stimulated by a touch, the normal reac- i 
tion is a head flexure, and, if the embryo is sufficiently advanced in f 
age, this flexure progresses caudad until the whole trunk is involved. , » 
In like manner, if the touch is upon the tail bud, the response i 
begins in the head region and progresses caudad. The physio- x 
logical development of a flexure, then, is corielated with its onto- t 
genetic development. 
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