XXIV INTRODUCTION. 



is continued to the digestive sac, and the pylorus is 

 situated at the point of junction between the two. 

 Amongst the marine Polyzoa the portion of the inner wall 

 of the stomach which abuts upon the pylorus is always 

 clothed with cilia. 



The intestine varies much in form : in some cases it is 

 a long and slender tube (Woodcut, fig. vii.), with perhaps 

 a slight dilatation at one point ; in others it is short, wide 

 below, and narrowing towards the upper extremity; in 

 others, again, it expands into two distinct chambers one 

 placed a little above the point of origin, and the other 

 (rectum) a little below the anus. The excreta are retained 

 for some time in the intestine, and, as Allman has re- 

 marked, probably yield to the absorbent tissues their re- 

 maining nutritious elements ; they are subsequently ex- 

 pelled through the terminal orifice. 



Apart from the variations already referred to, there are 

 one or two important modifications of the digestive system 

 which must be noticed. In some species a globular organ 

 is interposed between the oesophagus and the stomach, 

 which exhibits a highly specialized structure, and discharges 

 the functions of a gizzard (Woodcut, fig. ix. g). It is 

 an enlargement of the cardiac portion of the stomach, 

 the walls of which are here much thickened, and furnished 

 with a powerful apparatus of muscles. The inner surface 

 of the cavity is lined in part by a number of pointed 

 processes (gastric teeth) ; and by means of these and the 

 vigorous contractions of the muscular wall the food is 

 crushed, as in a grinding mill, and then delivered to the 

 stomach to be treated with the biliary secretion and ex- 

 posed to the roll of its peristaltic movements. This 

 organ in its fully developed form is only met with in 



