io8 PHYLUM MOLLUSCA 



Exercise 2. Make a large sketch of the mantle cavity of the animal 

 showing these organs, and label all. 



With fine scissors and forceps carefully dissect away the deli- 

 cate transparent body wall and expose the organs beneath, taking 

 care not to injure them. 



The Excretory System. The kidneys and their external open- 

 ings have already been observed. As in other mollusks, the kid- 

 neys also communicate with the pericardial space. 



The Circulatory and Respiratory Systems. Pushing aside the 

 organs which partly conceal it, observe again the systemic heart ; 

 note its shape and slightly asymmetrical position. Extending 

 from its forward end is the anterior aorta, which takes blood to 

 the forward part of the body ; its course cannot be followed at 

 present. The hinder part of the body is supplied with blood by 

 the posterior aorta. This vessel, as we have already seen, leaves 

 the hinder end of the systemic heart ; it sends off two pairs of 

 small arteries to the stomach and to other viscera, and then 

 branches into the three mantle arteries already mentioned. Find 

 them all and trace them as far as possible. Observe again the two 

 branchial hearts. Note the branchial artery, by which blood 

 passes from the branchial heart to the gill; also the branchial 

 vein, through which it passes from the gill to the systemic heart. 



Observe again the veins which bring the blood to the branchial 

 hearts. The precavals bring blood from the forward part of the 

 body. Trace them forward. They enter the kidneys near the for- 

 ward end of those organs and traverse their glandular walls back 

 to the branchial heart. Press aside the rectum and the forward 

 end of the kidneys and observe where the two precavals come 

 from beneath and enter the kidneys. With fine scissors cut the 

 connective tissue which binds the veins, and also the mesentery 

 which holds down the rectum and the ink bag, and turn these or- 

 gans back. Trace the two precavals forward ; they will be seen to 

 come from a delicate median vein which may be followed into the 

 head. Observe again the postcaval veins, which bring blood from 

 the hinder part of the body and join the branchial hearts near the 

 same place as the precavals. Their forward ends also traverse the 

 glandular walls of the kidneys and are here conspicuous ; back of 



