OPEKCULAK APPAKATUS. 



467 



begins to be inverted at tbe point where it has its insertion around the 

 base of the tentacles (c). As the descent of the tentacula proceeds, the 

 inversion of this membrane continues ; and when the extremities of the 

 arms have reached the level of the extremities of the setae, 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 setae are now brought together in a bundle (fig. 239, 2, a) 



Fig. 239. 



432 



Bowerbankia, showing the opercular apparatus. 



and are gradually drawn inwards, inverting around them the rest of the 

 flexible portion of the cell, until they form a close fasciculus (fig. 239, 

 3 & 4, a) occupying the axis of the opening of the tube, constituting a 

 complete protection against intrusion from without. 



(1208.) 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. 



(1209.) The muscles may be divided into two sets: one for the 

 retraction of the alimentary apparatus ; the other acting upon the setae 

 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. 238, a, 8) ; 

 the other passes upwards along the side of the oesophagus (fig. 238, 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. 



(1210.) The muscles that 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 setae (fig. 239, d d). 



2n2 



