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straightening of tlie oesophagus, which in the ciliobrachiates 

 of the sea is bent upon itself during tlie retracted state of 

 the polype. In none of the fresh water species, however, 

 which I have examined, with the exception of Paludicella 

 articulata, does this curvature of the oesophagus appear to 

 exist; we therefore cannot in these instances have recourse 

 to its agency in accounting for the phenomenon now under 

 consideration, and we are consequently driven to the parietal 

 muscles, or to the general contractibility of the internal tunic, 

 as the only provision by which this important act can be 

 eiFected. I mention the general contractibility of the internal 

 tunic as a probable agent in protrusion, for 1 do not think 

 the existence of the parietal muscles throughout the entire 

 order as yet sufficiently established. 



" We shall now suppose the polype withdrawn into the 

 recesses of its cell, and that hunger or some other stimulus 

 impresses on it a desire of protrusion. The parietal muscles, 

 which appear to me to be the direct agents in effecting the 

 protrusive act, now begin to contract, and thus exercise a 

 pressure on the fluid which surrounds the polype, and is in- 

 cluded between the latter and the internal membrane of the 

 cell. The compressed fluid in its turn acts upon the polype, 

 and by its upward pressure against that portion of the flexi- 

 ble tunic which is carried in by the animal during its re- 

 treat, will tend to produce an eversion of this membrane, 

 which, in the completely retracted state, constitutes a tube 

 of some length between the summit of the fasciculus of ap- 

 proximated tentacula and the orifice of the cell. The oper- 

 cular muscles at the same time coming into play will, by their 

 nicely adjusted action, keep the invaginated tunic exactly 

 in the axis of the orifice, and thus materially assist in effect- 

 ing the necessary protrusion, which, by the continued action 

 of the parietal muscles, will go on increasing till the com- 

 plete evagination of the reflected membrane has taken place. 

 This, then, I conceive to be the true account of the protru- 



