DR. A. FARRE ON THE STRUCTURE OF THE C1LIOBRACHIATE POLYPI. 411 
dozen to twenty cilia are concerned in the production of each apparent wave, the 
highest point of which is formed by a cilium extended to its full length, and the lowest 
point between every two waves by one folded down completely upon itself, the inter- 
vening space being completed by others in every degree of extension, so as to present 
something of the outline of a cone. (And it is remarkable that one of these corre- 
sponds very nearly in breadth with one of the supposed muscular lobes.) As, how- 
ever, the persistence of each cilium in any one of these positions is only of the shortest 
possible duration, and each takes up in regular succession the action of the adjoining 
one, so that cilium, which by being completely folded up formed the lowest point be- 
tween any two waves, now in its turn by its complete extension forms the highest 
point of a wave; and thus while the cilia are alternately bending and unbending them- 
selves, each in regular succession after the other, th e waves only travel onward, Avhilst 
the cilia never change their position in this direction, having in fact no lateral motion. 
When the waves travel very rapidly they appear smooth on one side and fringed on 
the other (fig. 23.). The whole of the ciliary motions are so evidently under the 
entire control of the animal as to leave not the slightest doubt in the mind of the 
observer as to this point. The whole fringe of cilia may be instantly set in motion, 
and as instantaneously stopped, and their action regulated to every degree of rapidity. 
Sometimes one or two only of the waves are seen continuing their action whilst the 
remainder are at rest, or isolated cilia may be observed slowly bending and unbending 
themselves, or projecting entirely at rest (fig. 21.). The body is generally somewhat 
pointed towards one extremity of the oval, and at this part may be observed a bundle 
of cilia, longer than the rest, and moving very rapidly (fig. 21. a.). Their vibrations 
were in several instances counted very evenly at 230 times in the minute, continuing in 
action whilst all the others were folded up. These may be respiratory, whilst the others 
are chiefly locomotive. There can be little doubt that this explanation of the action 
of the cilia in the gemmules is applicable also to those of the tentacula of the adult 
animal, and not only in this species but throughout the class generally ; for I have 
already observed that the tentacular cilia are infinitely more numerous when at rest 
than they appear to be in action : and I have also noticed, when their motions become 
languid, that here also they vibrate, not in the direction of the plane of the arms, but 
at right angles to it, and with the same hook-like form as in the gemmules. In this 
way the apparent travelling of the cilia up one side of the arm and down the other, 
as the eye is seduced to follow the waves which they seem to produce, is at once ex- 
plained. 
It would be impossible to explain the variety of motions which the gemmules are 
capable of executing, were it not obvious how complete is their control over the ac- 
tion of the cilia, which are their sole locomotive organs. They generally swim with 
the convex part forwards, and with the greatest rapidity. Sometimes they simply 
rotate upon their axis, or they tumble over and over ; or selecting a fixed point they 
whirl round it in rapid circles, carrying every loose particle after them. Others creep 
mdcccxxxvii. 3 H 
