DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 395 
highest point upon which the opercular retractors act, and from which it is there 
carried up free to the tentacular ring. 
This being the mechanism by which the retraction of the animal within its cell is 
effected, I proceed to explain the mode of its operation. 
The tentacula from being expanded in the form of an inverted cone are brought 
together into a straight line and immediately begin to descend. Their descent is 
effected by the contraction of the muscle which passes from the base of the cell to 
the tentacular ring, (fig. 3. a 9.) whilst at the same time the stomach is drawn down 
by its retractor. (3. a 8.) The whole body, however, does not descend in a mass, 
but must be folded up in a somewhat complicated manner, in order that the cell may 
completely inclose it. For this purpose the oesophagus, surmounted by the tentacula, 
descends first, whilst the integument of the upper part of the body begins to be in- 
verted at the point where it has its insertion around the tentacular ring. As the de- 
scent of the tentacula proceeds, the inversion of the integument continues, forming a 
close sheath around them, (Plate XXI. fig. 12. c) until the extremities of the arms have 
descended to a level with the top of the unyielding portion of the cell. The animal 
is now completely drawn in, the stomach brought close to the bottom of the cell, and 
the oesophagus bent in the form of a letter S ; the tentacula generally lying straight 
in the axis of the cell encased in their tegumentary sheath, and so separated from the 
fluid in the general visceral cavity ; the centre of which they have the appearance of 
occupying, whilst they are in effect external to it. The animal being thus retracted, 
the next step of the process is to draw in the upper part of the cell after it. This 
process, however, always commences before the retraction of the body is completed, 
and by the time that the end of the arms are on a level with the base of the setae. 
(fig. 11.) These latter bodies are then immediately brought together in a bundle, 
and begin to descend apparently by the action of the lower of the two sets of oper- 
cular retractors already described. Their descent, like that of the tentacles, takes 
place exactly in the axis of the upper part of the cell, and is accompanied by an in- 
version around them of its flexible portion, similar to that of the integument of the 
body around the tentacula during their descent (fig. 10.). Whilst the lower set of 
muscles are drawing down the setae, the upper set complete the retraction of the 
flexible part, and the whole operculum is thus packed closely in the upper part of the 
cell, the end of which now presents a triangular indentation, corresponding with the 
triangular arrangement of the opercular retractors (fig. 3. b, and fig. 9.). 
In this position of the animal it is impossible to define the whole course of the in- 
tegument, but when the tentacles are drawn unusually low, (fig. 9.) that portion of it 
which forms their sheath may be readily seen passing up to the base of the setae, 
around which it appears to have an attachment, and to be then continued up the 
sides of the inverted operculum to the angle at the top of the cell, whence it probably 
again descends to line the sides of the cell. 
Thus the whole process of retraction may be easily accounted for, and the office of 
3 F 
MDCCCXXXVII. 
