METRIDIUM MARGINATUM MILNE-EDWARDS 53 



peristome, gullet with longitudinal ridges and siphonoglyphs, 

 both complete mesenteries with their muscles and inner and 

 outer septostomata, mesenteric filaments, acontia, gonads (if 

 present), bases of incomplete mesenteries converging to the 

 centre of the foot, and the sphincter columnae in the wall under 

 the capitulum. 



8. Carefully cut out both primary mesenteries beginning 

 at the centre of the foot and passing one blade of the scissors 

 under the mesentery. When the cut has reached the wall, 

 continue it in the same manner all the way to the capitu- 

 lum. Then cut in the same manner the gullet, beginning at 

 its lower end, until you reach the mouth. Lift the mesentery 

 carefully with a forceps and cut across the peristome. Place 

 both mesenteries for future examination in a dish with water. 

 The removal of the primary mesenteries exposes the incomplete 

 mesenteries in the exocceles on both sides of the gullet. Make a 

 half-page drawing showing all structures including the mesen- 

 teric filaments, gonads, and acontia. 



9. Place one of the removed primary mesenteries on a slide 

 and examine against a window. Make a full-size drawing show- 

 ing the entire mesentery with the strips of the walls to which it 

 is attached. Label foot, column, capitulum, peristome, gullet, 

 mesenteric filament, acontium, gonad (if present), sphincter 

 columnse, inner septostoma, outer septostoma, longitudinal 

 muscle (banner), oblique muscle, transverse muscles. The two 

 basal muscles are not always discernible. Owing to the trans- 

 parency of the mesentery, the muscles of both sides are equally 

 well visible. In reality, the longitudinal muscle is on one side, 

 while the transverse and oblique muscles are on the other 

 side. 



10. Put a small piece of an acontium on a slide in a drop of 

 glycerine, cover with a cover glass and press the latter until 

 the acontium is completely mashed. Examine under the micro- 

 scope at 400 diameters. Make a drawing of a long, spindle- 

 shaped nematocyst with the filament in it. 



