; 
ON FRESHWATER POLYZOA. 315 
Shall first consider it in those species with bilateral lophophores, and after- 
wards attend to its disposition in Paludicella. 
1, Species with bilateral Lophophore—tIn all these the disposition of the 
muscles is exceedingly similar; seven distinct sets may be considered as de- 
monstrated. 
(1.) Retractor Muscles of the Polypide.—These, which are the largest and 
most powerful muscles of the animal, consist of two fasciculi which arise far 
down from the inner surface of the endocyst, and thence pass upwards, one 
along each side of the alimentary tract, to be inserted into the upper part 
and sides of the esophagus. A few accessory fasciculi may also be generally 
seen arising near the origin of the former, and inserted into the sides of the 
stomach. The use of the retractor muscles is very obvious ; acting towards 
the bottom of the comparatively fixed tube, they retract the whole alimentary 
canal with the tentacular crown, so as to place them in a state of security in 
the interior of the cceneecium. 
_ (2.) The Rotatory Muscles of the Crown.—These also consist of two fasci- 
euli, which arise along with the set just described, and passing up in company 
with these, separate from them at some distance below the crown, and thence 
pass outwards to the right and left to be inserted each into its own side of 
the lophophore. ἔζβε : to rotate the tentacular crown and depress the lobes. 
(3.) The Tentacular Museles—The muscular apparatus of the tentacula 
consists of a set of delicate parallel bands, which may be observed running 
from below upwards upon the margin of the lophophore; these bands are con- 
tinuous with one another below, and when they arrive at the intervals between 
the roots of the tentacula, each divides into two others, which run along the 
opposite sides of two neighbouring tentacula, The margin of the lophophore 
in the interval of the bands presents an oval transparent space, which looks 
almost exactly like an aperture, and it would seem to be these spaces which 
M. Van Beneden has taken for aquiferous mouths; after very careful ex- 
amination, however, I have convinced myself that no aperture exists here, ~ 
the apparent mouths being merely transparent spaces in the lophophore. 
(4.) The Elevator Muscle of the Valve.—This is a small, but very evident 
fasciculus, occupying the interior of the oral appendage, and visible through 
its transparent posterior walls; it arises from the lophophore near the base 
of the valve, and-passing forwards and upwards, is inserted into the posterior 
surface of the anterior wall of the valve. Use: to elevate the valve and draw 
it backwards from the mouth. 
_ (5.) Superior Parieto-vaginal Muscles.—These consist of numerous short 
bands, which arise all round froni the inner surface of the endocyst, com- 
mencing close to the line of invagination, and extending for some distance 
downwards., From this origin they pass transversely inwards, and are inserted 
into the opposed surface of the invaginated endocyst ard tentacular sheath. 
Use: to dilate the invaginated endocyst and sheath, and assist in keeping 
the peieineted endocyst and upper portion of the sheath permanently in- 
verted. 
(6.) Inferior Parieto-vaginal Muscles.—These consist of several radiating 
bands longer and stronger than the last, below which they arise from the 
inner surface of the endocyst in a single plane perpendicular to the axis of 
the cell, and thence passing upwards and inwards, are inserted into the sheath 
in a plane parallel to that of their origin, and just below the termination of 
the superior parieto-vaginal muscles. Use: to steady the sheath and regulate 
its position during the protrusion of the polypide, and to form a fixed plane 
on which it may roll outwards with the polypide in the act of protrusion. 
