A CRAYFISH OR A LOBSTER 29 



Exercise 19. Draw an outline of the animal's body, showing the 

 segmentation and the organs mentioned above, and label them all. 



The Circulatory System. The heart lies within an inclosed 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 dorsoventral 

 artery which leaves the abdominal artery near the heart and 

 passes ventrally. This artery supplies with blood a ventral 

 longitudinal artery, which lies in a midventral position 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 open- 

 ings 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 blowpipe. 



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 col- 

 lects in a space in the ventral portion of the body cavity in which 

 lie the ventral nerve chain and the ventral abdominal artery, and 

 passes toward the gills ; from this space it then 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 fila- 

 ments, where it is aerated, and passes by efferent veins on the 

 inner edges of the gills back to their base ; here six larger bran- 

 chial veins collect the blood and carry it to the pericardial sac, 

 whence it is taken through the valvular openings into the heart. 



Exercise 21. Draw a diagram representing the entire circulatory 

 system. 



The Reproductive System : the Female Genital Organs. The 

 position of the ovaries has already been observed. In the cray- 



