xii.] THE FRESH-WATER CRAYFISH. 145 



y. Cut away the gills, noting the two large chan- 

 nels in the stem of each, and observe the 

 cervical groove at the front of the gill- chamber 

 with the scaphognathite (21. d. a.) lying in it. 



[. In the lobster there are 20 gills on each side, 

 arranged as in the crayfish, except that there are 

 14 on the side of the body.] 



12. Circulatory organs. Immerse the animal in water 

 with its ventral surface downwards : cut away care- 

 fully with a pair of scissors the dorsal part of the 

 carapace which lies behind the cervical suture and 

 that part of the wall of the thorax from which the 

 gills have been removed. 



A chamber (the pericardial sinus) is thus laid bare 

 in which lies a polygonal sac, the heart. 



a. The six openings from the sinus into the heart ; 

 two superior, two inferior, and two lateral : pass 

 bristles into them. The arteries arising from the 

 heart; five anterior, one (ophthalmic) single in 

 the middle line, the others (antennary and hepatic) 

 in pairs; one, the sternal, the largest of all, given 

 off from the posterior end. 



b. Cut away the terga of the abdominal somites and 

 follow back the superior abdominal branch of the 

 sternal artery, removing carefully the muscles 

 which lie over it in the abdominal region. It 

 will be seen as a transparent tube lying in the 

 middle line on the intestine (14. b.), or in the 

 female lobster separated from it anteriorly by 

 the posterior ends of the two ovaries. It gives 

 off branches from its upper side to the muscles 

 over it, and also a pair of branches which run out 



M. 10 



