82 ZOOLOGY DIEECTIOXS 



Kemove the walking legs and study to secure data for com- 

 pleting the table but no drawings are required. 



19. The genital openings of the male occur on the basal 

 segment of the fifth pair of Avalking legs, while those of the fe- 

 male occur in a similar portion on the third pair of walking legs. 

 Note that the first two pairs of abdominal appendages are dif- 

 ferent in the two sexes. Examine both sexes but no drawings 

 are required. 



20. In the male the first two pairs of abdominal appendages 

 are highly modified for use in copulation and are not easily hom- 

 ologized. When at rest they lie in a groove directed forward be- 

 tween the bases of the walking legs. In the female, the first pair 

 of abdominal appendages is also modified. In the table asked for 

 under section 9, do not try to work out the homologies for 

 these modified structures of the first two or first abdominal ap- 

 pendages. 



IV. Internal Organs 



Circulatory System 



1. With scissors cut away the dorsal surface of the 

 carapace, by two cuts which extend from the hind margin for- 

 ward toward the eyes, and connect anteriorly by a transverse 

 cut. Do not injure the underlying parts when removing the cut 

 part of the carapace. 



2. Remove the delicate skin along the median line and 

 expose the pericardial sinus. This cavity receives the purified 

 blood from the gills. 



3. The heart, which lies within the pericardial sinus, is a 

 thick-walled muscular organ. It receives the blood from the 

 sinus by three pairs of valvular apertures, the ostia. 



4. What is the function of the circulatory system? Notes 

 required. 



5. Five vessels pass anteriorly from the heart. The 

 median one is the opthalmic artery. On either side of this is an 



