160 ARTHROPOD A. 



Move one of the appendages to which a gill is attached and see 

 the effect on the gill.^ 



Internal Anatomy. — Remove the carapace (beginning at the 

 middle of the posterior margin and cutting forward, holding the 

 cartilage knife parallel with the surface) as far laterally as the 

 upper limits of the gill chambers and anteriorly 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. Beside and 

 behind the stomach are two masses of muscle which you have 

 severed from the carapace. These are the mandibular 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. Between 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, convoluted 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). 



2. The heart extends through the posterior third of the tho- 

 rax. Remove the upper part of the delicate ^pericardium sur- 

 rounding it, cut its arterial and other connections, and place it 

 in water. Note the shape, the origin of the arteries, and the 

 three pairs of ostia. Do you understand how it receives blood? 



3. Trace the gonads as far as the abdomen, noting the single 

 anastomosis between those of opposite sides just in front of the 

 heart. Beneath the heart the sexual ducts are given off — vasa 

 deferentia in the male, oviducts in the female. Trace one out- 

 ward and downward to its opening by removing a portion of the 

 body-wall and of the basal joint of the proper leg. 



4. Remove the posterior lateral body-wall forward to a posi- 

 tion opposite the anterior third of the stomach. Pull the an- 



^ The craj'fish differs slightly in gill arrangement. 



