396 DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 
each set of muscles satisfactorily explained; but the 'protrusion of the animal is effected 
by a totally different mechanism. 
Of course the different stages of protrusion occur in the inverse order of those of 
retraction. The bundle of setee (fig. 10. a) first makes its appearance rising out of the 
apex of the cell, and followed by the flexible portion (fig. 11. b) on which it is set. 
The tentacula next pass up between the setae and thrust them asunder, while the in- 
tegument of the animal is seen gradually rolling outwards from around the tentacles, 
(fig. 12. c .) These latter continue to emerge and the integument to be everted from 
around them, until the base of the tentacles has risen above the top of the expanded 
setae, when the act of protrusion is completed, the tentacula separate and expand, 
and the cilia commence vibrating*. 
During repeated observations of the various steps of the process, I had in vain 
searched for a set of muscles having an antagonist power to the former and lifting 
the animal out of its cell. But I could discover no structure of this kind ; and in- 
deed it is not easy to imagine how such a mechanism could act, since the upper 
flexible portion of the sheath could afford no fixed point of attachment for elevating 
muscles, whilst from the want of rigidity in the bod}", and the manner in which it is 
folded up in its cell, no muscles arising from a lower point could effect its expan- 
sion. 
After examining several species for an explanation of this phenomenon, I at length 
obtained a clue to it from one more favourable for examination than the rest. (Plate 
XXIV. fig. 3. a.) 
In this species the body is capable of protruding for some distance beyond the 
mouth of the cell, in which state its delicate membranous walls may be readily traced 
downwards on one side to nearly the bottom of the ceil, (Plate XXIV. fig. 3. a 3.) 
from the inner surface of which they are capable of being separated in about one 
third of its circumference and from top to bottom of the cell, but remaining appa- 
rently in immediate connexion with the other two thirds. 
This separation of a portion of the parietes of the body from the inner surface of 
the cell, I found invariably to accompany the protrusion of the animal ; and on ex- 
amining further, I discovered upon this part of the body two rows of delicate, short, 
transverse filaments, arranged at a little distance from, and parallel to, each other. 
{a b and d 3.) These fibres were distinctly seen to contract whenever the protrusion 
of the animal took place, and to become relaxed again upon its retiring into its cell ; 
the walls of the latter being so pellucid that the minutest alteration in the form of 
these muscles was readily seen. When contracted to their utmost, each filament 
was reduced to just half its original length, at the same time that its thickness was 
doubled, and the little knot upon its centre appeared also somewhat thickened. 
(Plate XXIV. fig. 4 and 5.) During their contraction, the unattached part of the pa- 
* Figs. 9, 10, 11, 12, represent the different stages of protrusion and retraction in the order in which they 
occur. 
