HOMARUS AMERICANUS. 155 



the otocysts? What reason is there for having both antennules 

 and antennae? 



(/) Compare the pereiopods with the third maxilliped. 

 Which is lacking, endopod or exopod? Examine each of the 

 joints of one of these appendages and see in what directions 

 the appendage may be moved. Are there any ball-and-socket 

 joints? Compare the chelae with the other pereiopods and see 

 how they differ. To what part of a chela does the last segment 

 of the last pereiopod correspond. W T hat reason is there for 

 having these appendages different? Do you think the arrange- 

 ment of the appendages would aid the lobster in climbing over 

 rough bottom? 



Open one of the large chelae and determine how the muscles 

 are arranged to control its opening and closing. Which mus- 

 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 fifth pair in the male, the fourth pair in 

 the female. 1 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 .tfce articular membranes 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? W T hat direction must the water 

 take through the gill chamber? Examine the structure of a gill. 



1 In the crayfish the female opening is on the third pair. 



