327 



important function, that of alFording to the respiratory sur- 

 face, when the polype is retracted within the recesses of its 

 cell, a constant supply of fresh water, of which the little 

 animal would be deprived, were it not that some means 

 existed of dilating the tubular reflection of the tunic, an 

 office to the performance of which these muscles are fully 

 adequate, acting then in a state of antagonism to the parietal 

 muscles, which tend to keep the orifice of the tube closely 

 shut. 



" My objections to Dr. Farre's view of the function per- 

 formed by the muscles in question are referrible to three 

 heads: first, want of necessity in ascribing to them the office 

 for which this anatomist believes them destined ; secondly, 

 their inability to perform the function which he ascribes to 

 them ; and thirdly, the possibihty of assigning to them another 

 office in full accordance with the necessities of the animal. 



" That we are not obliged to seek for opercular muscles, 

 in order to account for the closing of the orifice when the 

 polype has retired into the recesses of its cell, is evident, if 

 we give the slightest consideration to the action of the true 

 retractor muscles. It is quite plain that the retraction of the 

 polype itself, which is efi'ected by the muscles which act di- 

 rectly ftpon it, is amply sufficient to produce the complete 

 invagination of the flexible termination of the cell; and, ac- 

 cordingly, observation will convince us that this invagination 

 follows exactly the retraction of the polype, and is evidently 

 related to the latter action as an effect to a cause. That the 

 opercular muscles cannot, except in a very partial manner, 

 produce the effect ascribed to them by Dr. Farre, is also 

 evident, when we reflect upon their course and attachments. 

 Arising from the circumferential portion of the cell, and 

 thence passing inwards, to be inserted into a point nearer to 

 the axis, they must, after the invagination of the tunic has 

 proceeded beyond the plane of their insertion, possess upon 

 the reflected tunic a decidedly dilatable action, — an action 



