28 MUSCULAR SYSTEM. 



or anterior to them when the tube is evaginated during the exserted state of the polypide. 

 The marine Polyzoa certainly afford an analogy for the existence of these muscles ; but, 

 though I have carefully sought for them in Paliidicella, I have not succeeded in detecting 

 them here as distinct fasciculi, and I prefer viewing the " superior tube-retractors " of 

 Hancock as some of the posterior fibres of the anterior parieto-vaginal muscles. The action 

 of the anterior parieto-vaginal muscles assists in the invagination of the tube, and dilates 

 it when completely retracted ; they thus act as antagonistic to the vaginal sphincter, while 

 the posterior fibres will check the complete evagination during exsertion. 



(3.) Posterior Parieto-vaginal Muscles. 



These are about four thin fasciculi (PI. X, figs. 3, 4, ;"), first pointed out by Mr. Hancock; 

 they arise from the inner surface of the endocyst near the top of the cell, two upon the haemal, 

 and two upon the neural side, and are inserted into the opposed surface of the tentacular 

 sheath. Their action checks the complete evagination of the sheath in the way we shall 

 presently see. 



(4.) Var/inal Sphincter. 



This was also pointed out for the first time by Mr. Hancock. It consists of a set of 

 fibres (PI. X, fig. 3, u) which run transversely round the invaginated tunic. I have not suc- 

 ceeded in dividing it into an inferior and superior set, as described by Mr. Hancock. Its 

 action closes the invaginated endocyst after the retraction of the polypide. 



(5.) Parietal Muscles. 



These are numerous, short but strong, and very evident fibres (PI. X, figs. 3, 4, v), which 

 nm transversely in the endocyst in small groups of two or three fibres each, embracing about a 

 third or fourth of the circumference of the cell. Their action compresses the endocyst, and by 

 thus diminishing the cavity of the cell, effects the exsertion of the polypide. 



The description now given of the muscular system in the fresh-water Polyzoa, will 

 enable us to understand the mechanism by which the protrusion and retraction of the 

 polypide are effected. 



The grand agency to which we must assign the protrusive act, is without doubt the 

 contraction of the endocyst effected by the parietal muscles, or by the general contractility 

 of the tunic itself ; and, indeed, it does not seem possible to refer the act of protrusion to any 

 other cause than the consequent pressure of the perigastric fluid against the body of the 

 polypide, and the necessary compulsion of the latter to move in the direction of least resist- 

 ance, or through the orifice of the cell : for the mere straightening of the oesophagus, to 



