32 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 arteiy 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 aerated, 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. 



