HOMARUS 171 



careful not to injure the gills. Extending up into the gill 

 cavity are seven epipods belonging to the three maxillipeds 

 and the four anterior pereiopods. They separate the gills 

 into groups. Each group will be seen to correspond to a 

 segment. The gills show three sorts of attachments: (a) to 

 the appendages themselves (podobranchs) , {b) to the articu- 

 lar membranes between appendages and body wall {arthro- 

 branchs), and (c) to the body wall itself {pleurobranchs) . 

 There are two arthrobranchs in some segments, one behind 

 and above the other. How is the current of water forced 

 through the gill chamber? What is the function of the epi- 

 pods? What direction must the water take through the gill 

 chamber? Examine the structure of a gill. Move one of the 

 appendages to which a gill is attached and see the effect on 



the gill. 1 



Internal Anatomy. — Remove the carapace (beginning at 

 the middle of the posterior margin and cutting forward, hold- 

 ing the cartilage knife parallel with the surface) as far 

 laterally as the upper limits of the gill chambers and an- 

 teriorly to the base of the rostrum. What is the pigmented 

 membrane for? Dissect it off so underlying organs may be 

 seen. 



1. The chitinous stomach lies near the anterior end with 

 the ophthalmic artery running along its mid-dorsal line. Be- 

 side and behind the stomach are two masses of muscle which 

 you have severed from the carapace. These are the man- 

 dibular muscles, and each is divided into an anterior and a 

 posterior bundle. Lateral to these muscle masses are the 

 yellow-green digestive glands, commonly called liver. Be- 

 tween and in front of the posterior mandibular bundle note 

 the gonads, and follow one forward by pressing aside the 

 muscle mass. In the male the testis is a slender, white, con- 

 voluted cord, which ends blindly against the side of the 

 stomach. The extent and position of the far thicker yellow 

 ovary is much the same (unless the animal be mature, in 

 which case it will be found greatly enlarged and orange). 



1 The crayfish differs slightly in gill arrangement. 



