328 



which is antagonized, first, by the parietal muscles, as will 

 be presently explained, and, secondly, by the true retractor 

 muscles; for these muscles, acting through the medium of the 

 polype, in most instances nearly centrally, or in the axis of 

 the tube, will not, in their ordinary action, possess any di- 

 lating power, but, on the contrary, will tend to close the 

 apertureby approximating the sides of the tubular reflection 

 of the cell. 



" Since we have thus seen that the opercular muscles are 

 incapable of producing the closure of the orifice, it becomes 

 an interesting subject of inquiry to determine by what means 

 the act in question is performed, and indeed a shght consi- 

 deration will render manifest the simple yet efi'ective mecha- 

 nism appropriated to this purpose. The great agents by 

 which the closure of the cell is eflTected are to be found in 

 the parietal muscles, for these fibres, by pressing the fluid of 

 the cell against the tube of invaginated membrane (fig. 4, c, c), 

 will approximate the sides of this tube to one another, at the 

 same time that the membrane will be thrown upwards against 

 the aperture of the cell, thus completely closing the orifice, 

 and enabling the little animal to rest secure from all intrusion 

 in the recesses of the polypidom. 



" After this account of the muscular system of the polypes, 

 the mechanism will now be easily understood by which the 

 animal is protruded from its cell when hunger calls it 

 forth to seek its food in the surrounding medium, or when 

 desirous of exposing its respiratory surface still more per- 

 fectly to the vivifying influence of the aerated water. 



" Previously to the discovei-y of the parietal muscles no 

 satisfactory explanation had been given of the protrusive act 

 of the polype, and even since the detection of these muscles 

 by Dr. Farre, their share in efiecting the protrusion of the 

 animal would appear to be underrated. Dr. Farre considers 

 their influence in this respect as of secondary importance, 

 and would seem to attribute the act in question mainly to the 



