STYELA. 99 



of the mouth. It is widest about the middle, and 

 narrows considerably at the ends. The branchial 

 tubercle is a large, oval, smooth boss with the con- 

 voluted ends turned downwards, and, meeting below, 

 very little involuted. 



The tentacular filaments are numerous, long, and 

 slender, and are placed not much below the margin 

 of the branchial sac. 



The heart is a long, narrow tube, lying between the 

 mantle and the lining membrane on the left side, at 

 some little distance from the lower dorsal margin of 

 the branchial sac. It is placed, as usual, in a cavity of 

 its own a sort of pericardium. The dorsal extremity 

 reaches the endostyle much higher up than usual ; and 

 the ventral extremity abuts against the lower margin 

 of the cardiac end of the stomach, and gives a large 

 branch to each side of that organ. The vessels supply- 

 ing the test are small, and in S. mamillaris enter it 

 farther forward than is usually the case. 



The ganglion of the nervous system is situated 

 immediately below the branchial tubercle; but 011 

 account of the opacity of the inner tunic it is difficult 

 to trace the nerves. 



The alimentary tube of S. tuber osa is short and 

 very simple in its arrangement. It protrudes from 

 the inner wall of the right side of the pallial cavity, 

 towards the ventral margin. The mouth is a circular 

 orifice and is bordered with a widish, puckered, lip- 

 like, expansion ; it opens into the branchial sac some 

 way up the ventral margin, the posterior extremity of 

 the sac passing backwards considerably beyond it. 

 The oesophagus is short and constricted; it extends 

 backwards or downwards to the stomach, which is well 

 marked though not large, and is regularly elliptical. 

 The intestine passes backwards and downwards from 

 the lower end of the stomach, and then, turning 

 upwards, forms a widish vertical loop next the ventral 

 margin ; it then slopes gradually towards that margin 

 in a gentle sigmoid curve, and, on reaching the cloaca, 



