316 REPORT—1850. 
(7.) Vaginal Sphincter.—The vaginal sphincter is a circular band surround- 
ing the termination of the invaginated endocyst at the point where the latter 
passes into the tentacular sheath. Though a contraction of the endocyst at 
this spot, as if occasioned by the action of a powerful sphincter, may be 
always observed when the polypide is completely retracted, yet the demon- 
stration of an actual muscle is by no means easy. I have however convinced 
myself of the existence of a distinct structure at the place where the con- 
traction occurs, and, though I have not observed any evident fibres, I have 
no hesitation in viewing this structure as a sphincter muscle on which the 
contraction in question is dependent. The wse of the sphincter is to close 
the sheath after the recession of the viscera, and thus protect the latter from 
all annoyance from without. 
Besides the seven sets of muscles now described, a high magnifying power 
and properly adjusted illumination will enable us to detect in the walls of 
the endocyst, towards its anterior extremity, numerous delicate fibres which 
run transversely round the cell. They are doubtlessly muscular, and by 
their action constrict the endocyst in a transverse direction, and thus aid in 
the protrusion of the viscera. I have not succeeded in determining how far 
down the cell they extend, as the structure soon becomes concealed under 
the increasing opacity of the superjacent tissues. Circular muscular fibres 
are also very evident in the walls of the stomach; these have already been 
described in connection with the histology of the digestive system. 
2. Muscles of Paludicella—The muscular system of Paludicella differs in 
some important points from that of the species with bilateral lophophores. 
The muscles may here be divided into five sets :— 
(1.) The Retractor Muscle of the Polypide—This resembles in attachments 
and use the corresponding muscle in the other species, but is not divided like 
the latter into two distinct fasciculi. 
(2.) The Superior Parieto-vaginal Muscles.—These constitute four strong 
fasciculi, which, arising from the sides of the cell near the top, are inserted 
into the opposed surface of the invaginated endocyst. The fibres of each 
fasciculus are inserted one after another in a straight line, commencing near 
the line of invagination, and extending for some distance down the invagi- 
nated tunic. ‘These four lines of insertion are placed at nearly equal di- 
stances from one another, and thus cause the orifice and invaginated tube to 
assume a regular quadrilateral figure. The corneous ribs already described 
correspond to the centre of the intervals between the insertion of the muscles. 
Mr. Hancock* enumerates, under the name of Superior Tube Retractors, 
two small additional fasciculi, which he describes as originating below those 
just mentioned, and as inserted also below them into the invaginated tube, 
their insertion becoming of course superior to them when the tube is evagi- 
nated during the exserted state of the polypide. The marine Polyzoa cer- 
tainly afford an analogy for the existence of these muscles; but, though I have 
carefully sought for them in Paludicella, I have not succeeded in detecting 
them here as distinct fasciculi, and I prefer viewing them as some of the 
inferior fibres of the superior parieto-vaginal muscles. The use of the supe- 
rior parieto-vaginal muscles is to assist in the invagination of the tube, and 
dilate it when completely retracted, thus acting as antagonistic to the vaginal 
sphincter, while the inferior fibres will check the complete evagination during 
exsertion. : 
(3.) The Inferior Parieto-vaginal Muscles——These are about four strong 
fibres, first pointed out by Mr. Hancock ; they arise from the inner surface 
* Loc. cit. 
