THE FLEXIBLE CREEPERS. 481 
remains, unsolved as yet, an inviting fortress of research for 
daring young physiologists to scale and take. 
Let me say what further I have observed on the matter. 
The sac itself is frequently drawn with force to one side or 
other, or down in the longitudinal direction, or across towards 
the breast. These movements are effected by means of slender 
muscle-threads, which are attached to it and to various parts 
of the body. The white substance lies in irregular masses in 
the sac, sufficiently solid to push out the elastic sides in wart- 
like knobs, but loose enough to allow the light to pass through 
the interstices as they move over one another. The walls of 
the sac are contractile ; for we see its form change, and 
instantly some of the granules of opaque matter are forced 
higher up in the tube, rising and sinking by turns, like the 
mercury in a thermometer. Indeed, a thermometer, with its 
tube and its bulb, and their contents, affords a capital illustra- 
tion of the whole organ. By means of their motions we are 
able to trace the course of the tube much farther than when it 
is empty, and to see that it passes to the front of the head, 
between the two frontal eyes, and, bending over with an arch, 
is lost somewhere in the ciliated face. 
The transparent body is inclosed in an equally transparent 
skin, which is quite flexible, but apparently tough and strong, 
like wet parchment. It falls into transverse folds as the 
animal bends and wriggles from side to side, allowing much 
freedom of motion ; and these folds have the additional 
advantage of allowing the body to be lengthened or shortened, 
at the will of the animal, at both ends ; the skin becoming- 
inverted at two or three points, when either the head or the 
foot is drawn in. Both these movements are almost every 
moment being performed, to a greater or less extent ; and are 
quite distinct from the evolution and involution of the head 
itself, by which the rotating “ ears ” are suddenly turned out 
for swimming, or the contrary. 
In all this order the muscular system is displayed with a 
clearness far greater than in any of the forms that we have 
yet considered. As might have been presumed from the 
untiring, vigorous, precise, and most varied motions of every 
part of these minute animals — motions which excite the 
admiration of the observer who remembers their dimensions, 
— the muscles are exceedingly numerous and well developed. 
In every species in which the viscera present the requisite 
clearness (for some are so constantly opaque, either from the 
distension of the alimentary canal with food, or the successive 
growth of great turbid eggs in the ovary, that we can hardly 
discern any of the more delicate details of their internal 
structure), the great longitudinal and diagonal muscles are 
