184 Mr. A. Hancock on the Anatomy of the 
The muscular apparatus of Paludicella differs in some respects 
from that of Plumatella and Fredericella. In the former there 
are six sets of muscles—three in connexion with retraction, two 
with protrusion, and one for closing the orifice on the retreat of 
the polype. Of the retractors one set acts directly upon the ani- 
mal, the other two upon the tubular orifice of the cell. The 
former set, Pl. IV. fig. 1 0, the most powerful in the animal, is 
similar to the tentacular retractors of Dr. Farre: it differs only 
from the polype-retractors in Plumatella and the other genera 
already spoken of in not being divided into two bundles. It is 
composed of numerous, stout, long, linear fibres originating from 
the inner surface of the anterior wall of the cell more than half- 
way down; 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, L. 
The second and third sets of muscles are the tube-retractors ; 
the former or inferior, figs. 1 p & 2m, 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 di- 
stance 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 lon- 
gitudinal 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. 8, consequently assumes a square form, the 
angles corresponding to the insertions of the muscular bundles. 
The third set of muscles, figs. 1 g & 2 n, 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 /, before alluded to, at the mouth 
of the cell. The inferior and superior tube-retractors are ho- 
mologous 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 
