170 ARTHROPODA 



typical biramous appendage, but they are much modified to 

 serve special functions, and the exact homologies are not im- 

 portant. Between the mandibles note the mouth, bounded in 

 front by the labrum. 



Drawings of these appendages may be made if time per- 

 mits. 



(d) The second antennae are biramous. Notice on the 

 ventral side of the basal joint of an antenna the opening of 

 the green gland or nephridium. 



(e) The first antennae, though branched, are not con- 

 sidered to be of the biramous type. Do you know why? 

 Remove one and note on the dorsal surface of the basal joint 

 a groove at whose median extremity is a small hole, the 

 opening into the statocyst. Do you know the probable func- 

 tion of the antennules and of the statocyst? What reason 

 is there for having both first and second 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? What reason 

 is there for having these appendages different? Do you 

 think the arrangement of the appendages would aid the lob- 

 ster 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 muscles are strongest? Find how the muscles are at- 

 tached to the "thumb." 



Find the openings of the sexual ducts on the basal joints 

 of the pereipods; the fifth pair in the male, the third pair 

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

 seminal receptacle through a triangular elevation on the ven- 

 tral side of the thorax. 



4. Gills. — Remove the gill cover of the left side, being 



