XII.J THE FRESH-WATER CRAYFISH. 139 



clubbed ends and penetrate a short way into the 

 exoskeleton. 



11. The respiratory organs. Remove now the branclrio- 

 stegite on one side and examine the gills : they are 

 18 in number, arranged in two sets. 



a. Six are attached to the epipodites of some of 

 the appendages (2nd and 3rd maxillipedes, 

 chelae, 1st, 2nd, and 3rd pair of ambulatory 

 limbs). 



ft. The . remaining 12 are fixed to the sides of 

 the body, and each consists of a central stem 

 giving off a number of delicate filaments. 



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. 



[8. 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) 



