DIGESTIVE SYSTEM 113 



III. DISSECTION OP THE DIGESTIVE SYSTEM. 



Cut away the part of the collar to the left of the pul- 

 monary aperture. Detach the right side of the collar, with 

 the anal a/pertwe, from the body-wall. Separate4he rectum 

 from the body by cutting along the right side of the mantle, 

 just below the rectum, as far back as the hinder end of the 

 mantle-cavity. Turn the rectum back, and pin it down. 



Make a median longitudinal incision through the floor of 

 the mantle-cavity, and continue it forwards along the head to 

 its anterior end. Dissect the flaps away from the underlying 

 parts, and pin them out right and left. 



Carefully dissect off the thin' skin covering the spiral 

 visceral mass ; and unravel the reproductive organs, which 

 a/re white in colour and very bulky, and the alimentary canal, 

 the coils of which are very intimately connected with the liver. 



Arrange the reproductive organs on the right, the digestive 

 organs on the left side of the dissection. 



The removal of the integument from the visceral mass 

 requires care, but presents no great difficulty. Special care is 

 needed at the posterior end of the mantle-cavity, where the 

 parts are rather firmly bound together. 



The dissection is greatly facilitated by frequent washing 

 under the tap. 



A. The Alimentary Canal. 



The alimentary canal is a convoluted tube, of nearly uni- 

 form size along the greater part of its length, but presenting 

 special features in its anterior portion. 



1. The buccal mass or pharynx is the enlarged anterior end 

 of the canal, into which the mouth opens. It forms- 

 , a prominent rounded mass in the head, with stout 

 muscular walls, and will be more fully examined 

 at a later stage of the dissection. It contains the 

 odontophore (p. 122). 



Insert a seeker into the mouth, and note that it passes 

 upwards and backivards into the buccal mass. 



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