BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 381 
then passing up in front of the polype, the superior extremities 
are inserted around the base of the tentacular disc. These fibres 
draw the polype down into the cell, and like those of the same 
muscle in the other Bryozoa, when unemployed, lie in a somewhat 
cramped and disordered state, fig. 2 J, 1. 
The second and third sets of muscles are the tube-retractors ; 
the former or inferior, figs. 1 p & 2 m, m, is much the larger; it 
is composed of four compressed bundles of stout, linear fibres 
placed close together, but distinct from each other. These bundles 
are associated together in pairs, one on each side of the tube ; the 
inferior ends of these pairs of bundles arise wide apart from the 
posterior wall of the cell opposite the orifice. As they pass up 
the tube, the bundles converge,and reaching within a short distance 
of the lips of the orifice, they are inserted upon the inner surface 
of the tube-walls at four opposite points ; the fibres of each bundle 
being attached one above the other in the same longitudinal plane. 
This peculiar arrangement causes the margins of the orifice to fold 
into four portions on the retraction of the tube ; and its end, fig. 
3, consequently assumes a square form, the angles corresponding 
to the insertions of the muscular bundles. 
The third set of muscles, figs. 1 ¢ & 2 », n, the superior tube- 
retractors, are made up of only four fibres, two on each side of the 
cell, having their origin immediately below that of the set just 
described ; their other ends are attached to the inner surface of 
the tube above the insertion of the inferior set, and at the base of 
the membranous cup, fig. 1 Z, before alluded to, at the mouth of 
the cell. The inferior and superior tube-retractors are homologous 
to the double set of opercular muscles described by Dr. Farre, in 
the marine species, differing only from those in Bowerbankia densa 
by being divided into four bundles instead of into three, as they 
are in that species. The action of these muscles is obvious. The 
superior retractors, having their insertion at the base of the 
membranous cup at the mouth of the cell, draw it down base first 
in the axis of the tube, at the same time folding in around it the 
lips of the cell. The inferior set then taking up the work, com- 
plete the inversion of the tube. Dr. Farre, however, supposed 
that the opercular muscles were not merely for drawing the tube 
