XXX 



INTRODUCTION. 



Into the details of the muscular apparatus, upon which 

 these and the other movements of Fig. xii. 



the polypide depend, it is not my 

 purpose to enter here*; but one or 

 two of its leading elements may be 

 noticed. The largest and most 

 powerful of the muscular bands in the 

 cell of the Polyzoon are the great 

 retractors (Woodcut, fig. xii. gr.). 

 These consist of two broad fascicles, 

 which are attached to the lower part 

 of the cell-wall and extend upwards to 

 the base of the tentacles, where they 

 are inserted into the sides of the 

 oesophagus. When the polypide is 

 expanded, they extend as two (appa- 

 rently) solid bands throughout the 

 greater part of the length of the 

 cell; suddenly contracting, they 

 drag down the corona with the speed 

 of light, inverting the tentacular 

 sheath (doubling up the alimentary 

 canal at the same time on itself), 

 bringing all the parts into the shel- 

 ter of the cell, and disposing the 

 whole structure in orderly fashion 

 within its cavity. In these large 

 bands we can study to the greatest Bowerbankia. 



advantage the structure of the muscular tissue. 



* Full accounts of the different sets of muscles and of their respective 

 action maj be found in the works of Farre (Phil. Trans.), Van Beneden (on 

 Laguncula), Allman (Freshwater Polyzoa), Hyatt {op. cit.), Nitsche (on 

 Mcmbravipora vionhranacea), &c. 



