32G 



able deviation from the general arrangement of these muscles 

 in the salt water species. In plumatella repens, however, 

 and perhaps in most other species of fresh-water ciliobra- 

 chiates, their arrangement is very peculiar. In this zoo- 

 phyte, they consist of a series of about twenty-five distinct 

 delicate fasciculi, which arise from the internal surface of the 

 cell at regular intervals, and in a plane perpendicular to its 

 axis, and thence radiating inwards are inserted into the op- 

 posed surface of the reflected tunic. 



" In assigning their proper office to the muscles which have 

 been already described as the true reti'actor apparatus of the 

 polype, no difficulty whatever is met with ; neither can we 

 be at a loss in discovering the true function of the parietal 

 muscles, for these acting upon the flexible internal tunic of 

 the polype cell, must necessarily, by their contraction, dimi- 

 nish transversely the space included between this tunic and 

 the body of the polype, a function the great importance of 

 which, in the economy of the little animal, will presently be 

 apparent. When, however, we attempt to explain the action 

 of the opercular muscles, the task will perhaps be found not 

 quite so easy. It has been stated that Dr. Farre assigns to 

 these muscles the office of drawing in the flexible portion of 

 the polype tube after the retreating polype, and by their con- 

 tinued action closing the orifice of the cell. I cannot help 

 thinking, however, that in ascribing this office to the oper- 

 cular muscles. Dr. Farre is correct but to a very limited 

 extent, and that their chief use is directly opposite to that 

 assigned to them by this excellent observer. The use which 

 I would assign to the opercular muscles is, first, that of 

 assisting the polype in its protrusion, an office which they 

 accomplish by fixing and preserving in the axis of the polype 

 tube that portion of the reflected tunic (fig. 4, c, c) which 

 is included, during the retracted state of the animal, between 

 the summit of the fiisciculus of approximated tentacula, and 

 the orifice of the cell ; and, secondly, what is a still more 



