38 INVERTEBRATE ZOOLOGY 
The circulatory system. The heart lies within an enclosed space 
called the pericardial sac, the walls of which, the pericardium, will 
have been partially destroyed by the removal of the under-skin. 
The heart, the abdominal artery with its lateral branches, and 
the five anterior arteries have been studied and drawn. Care- 
fully press aside the heart and note the median dorso-ventral artery 
which leaves the abdominal artery near the heart and passes 
ventrally. This artery supplies with blood a ventral longitudinal 
artery, which lies in the mid-ventral line in the thorax and 
abdomen. 
Remove the dorsal abdominal artery and the heart from the 
body and float them in clean water. Note the six valvular 
openings of the heart, two being on the dorsal side, two on the 
ventral, and one on each of the lateral sides. These can be 
seen by blowing on the heart through a blow-pipe. 
Exercise 20. Draw a dorsal view of the heart showing the 
valves there present. 
The course of the circulation of the blood is the following: 
by the contraction of the heart the blood is sent through the 
arteries to all parts of the body; after bathing the different 
tissues it collects in a ventral blood-sinus, a passage in the 
ventral portion of the body-cavity in which lie the ventral 
nerve-chain and the ventral abdominal artery, and passes 
towards the gills; from the ventral sinus it passes to the gills 
through afferent veins, one of which runs to each gill and 
along the outer edge of it; it then runs through the delicate 
gill-filaments, where it is aérated, and passes by efferent veins 
on the inner edges of the gills back to their base; here six 
larger branchial veins collect the blood and carry it to the 
pericardial sac, whence it is taken through the valvular open- 
ings into the heart. 
Exercise 21. Draw a diagram representing the entire circulatory 
system. 
