INTRODUCTION. 



XXXI 



The band which, when in a state of tension, seemed to 

 be solid, is found, when relaxed, to be made up of a num- 

 ber of separate fibres. Complex as the muscular system 

 of the Polyzoa is, the individual muscle is universally of 

 the simplest and most rudimentary kind a bundle of 

 distinct threads; it never presents the consolidated form 

 which exists in the higher animals. In the large muscles 

 the fibres appear (under favourable circumstances) trans- 

 versely striated (Woodcut, fig. xiii.). When the muscular 

 band is at rest, the threads which compose it Fig. xiii. 

 separate and lie crumpled up within the cell* 



Two sets of muscles (the parieto-vayinal] 

 are concerned in regulating the action of 

 the invaginated sheath, and pass from the 

 cell-wall to various points on its surface. A 

 powerful sphincter closes the sheath after 

 the retraction of the polypide, and keeps all 

 safe within. 



The parietal muscles, which contract the 

 inner layer of the cell- wall, as well as those 

 which control the tentacles and the peristal- 

 tic movements of the alimentary canal, have 

 been previously noticed. For further detail 

 I must refer to the writers already cited. 



The mode in which the retraction of the polypide is 

 effected will be sufficiently evident from the foregoing 

 hasty sketch of the muscular mechanism of the cell. But 

 the act of protrusion requires a more detailed explanation. 

 It will be borne in mind that thte polypide is suspended 

 from the lophophore, and hangs free within the cavity of 

 its cell, bathed on all sides by the perivisceral fluid. When 

 the want of food or oxygen compels it to issue from its 



Striated mus- 

 cular fibre. 



