HOMARUS AMERICANUS. 131 



cles are strongest ? Find how the muscles are attached to the 

 "thumb." 



Find the openings of the sexual ducts on the basal joints of 

 the pereiopods; the last pair in the male, the next to the last 

 pair in the female. In the female there is an opening into a 

 seminal receptacle through a triangular elevation on the ventral 

 side of the thorax. 



4*. Gills. Remove the gill-cover of the left side, being care- 

 ful 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 them- 

 selves (podobranchs) , (6) to the articular membrane between 

 appendages and body-wall (arthrobranchs) , 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 epipods? 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. 



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 



