Ill 



4 321 



ties of the setse, it is seen to form a complete sheath around them. 

 The animal being thus retracted, the next part of the process is to 

 draw in the upper portion of the cell after it. The setse are now 

 brought together in a bundle (jig. 46, Q, ), and are gradually 

 drawn inwards, inverting around them the rest of the flexible por- 

 tion of the cell until they form a close fasciculus (fig. 46, 3 & 4, a), 

 occupying the axis of the opening of the tube, and forming a com- 

 plete protection against intrusion from without. 



(147.) The muscular system exhibits the earliest appearance of 

 muscular fibre. The filaments are unconnected by cellular tissue, 

 and have a watery transparency and smooth surface, neither do 

 they exhibit cross markings or a linear arrangement of globules, 

 even when examined under the highest powers of the microscope. 



The muscles may be divided into two sets, one serving for the 

 retraction of the alimentary apparatus, the other acting upon the 

 setse around the mouth of the cell, and serving for the inversion of 

 its flexible portion. The bundles of muscular fibre which act upon 

 the alimentary canal are two in number, and arise from near the 

 bottom of the cell : one of these is inserted into the stomach 

 (fig. 45, a, 8) ; the other passes upwards along the side of the 

 oesophagus (fig. 45, a, 9), to be attached in the vicinity of the 

 tentacula : the latter fasciculus is evidently the great agent in 

 drawing the animal into its retreat, and in doing so it throws the 

 alimentary canal into close sigmoid folds. 



The muscles which close the operculum are arranged in six 

 distinct fasciculi ; they arise from the inner surface of the upper 

 hard part of the cell, and act upon the upper flexible portion of the 

 tube and upon the setse (fig. 46, d, d). 



The mode in which the protrusion of the tentacula is effected is 



