A SQUID 129 
The excretory system. The kidneys and their external openings 
have already been observed. As in other mollusks, the kidneys 
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 into the systemic heart. 
Observe again the veins which bring the blood to the bran- 
chial hearts. The precavals bring blood from the forward part 
of the body. Trace them forward. They enter the kidneys 
near the forward end of those organs and traverse their glan- 
dular 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 organs 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 glan- 
dular walls of the kidneys and are here conspicuous; back of 
these they are much wider, but are very thin-walled and not 
