166 



OBGANS OF DIGESTION. 



the internal organs, is reflected over the aperture of the cell, 

 and becomes continuous with the tentacular circle. In con- 

 sequence of 

 this union be- 

 tween the po- 

 lype and its 

 cell, it follows, 

 that when the 

 animal retires 

 therein, that 

 portion of the 

 tunic (c) 

 pushed out- 

 wards by the 

 exit of the po- 

 lype, is drawn 

 inwards on its 

 retreat by a 

 process of in- 



Fig. 175. Pluraatella repens. n, natural size; b, the valuation, so 

 same magnified. ^ the flex . 



ible extremity of the cell is at the same time a sheath for 

 the body, a support to the tentacula, and a door for closing 

 it. In fig. 175, muscular bands are seen passing from the 

 inner membrane of the cell to the body of the polype, by 

 which the retraction of the animal and the invagination of 

 the superior part of the cell is effected. At a, we see the 

 natural size of the polypedom of Plumatella; at b and c, the 

 cells and polyps magnified and protruded in search of prey ; 

 at d, the polype withdrawn into its cell, and the orifice closed 

 by the retraction (c) of the integument. 



[ 318. In the TUNICATED MOLLUSCA the digestive organs 

 are very simple. At the bottom of the cavity formed by the 

 muscular mantle is found the mouth, a simple absorbent 

 tube, opening into the stomach ; that organ is surrounded 

 by the follicles of the liver, the ducts from which enter its 

 cavity; the short intestine terminates near the ventral aperture 

 of the muscular sac. 



[319. In the CONCHIFERA, as in the oyster (Ostrea eduHs, 

 fig. 176), the mouth, surrounded by four labial plates (r\ 

 opens into an oval stomach (a) ; the intestine (d,f) makes 



