ALIMENTARY CANAL. 



13 



In most Tunicata the digestive organs are placed behind the 

 pharynx and atrial cavity (Fig. 3), and the part of the body con- 

 taining them is frequently called the abdomen, as opposed to 

 the thorax or pharyngeal region. In monascidians, however, the 

 pharynx extends the whole length of the body and the digestive 



B. 



rt 



FIG. 12. Diagram showing the anatomy of Atcidia (after Herdman, fromPerrier). A mouth ; 

 a anus ; B pharynx ; C heart ; cd duct of hyponeural gland ; cl atrial cavity ; E exhalent aper- 

 ture ; e stomach ; / dorsal tubercle (ciliated pit) ; gph subneural gland; i intestine ; m mantle ; 

 N ganglion ; od genital duct ; oe oesophagus ; pb atrial cavity ; rt terminal ampullae of the 

 test-vessels ; T tunic or test ; tp tentacles ; vd endostylar vessel ; vm blood-vessels of mantle ; 

 vp vascular strands crossing the atrial cavity ; vt blood-vessels of the test ; r rectum. 



viscera are placed on one side of it, usually the left (Fig. 12). 

 They are embedded in the mantle and usually cause a projection 

 into the atrial cavity, dona forms an exception to this rule. 

 The digestive canal consists of oesophagus, stomach, intestine 

 and rectum. It is twisted upon itself in various ways and ends 

 by opening into the atrial cavity in the middle line either at 



