HOMARUS AMERICANTTS. 139 



posed of two segments, coxopod and basipod; the endopod of five, 

 segments, ischipod, meropod, carpopod, propod, and dactylopod. 

 The exopod is composed of one long and many short segments. 

 How is the appendage modified to serve in feeding? 

 Make a drawing of the third maxilliped. 



(c) Remove the remaining five mouth appendages and com- 

 pare each with the third maxilliped. These are, beginning pos- 

 teriorly, the second maxilliped, first maxilliped, second maxilla 

 (with a broad paddle, the scaphognathite, the use of which 

 should be understood), first maxilla, and the mandible. Just 

 back of the mandibles are two small flaps, the paragnatha, 

 which are not true appendages. Do you understand the use 

 of each of these appendages? Most of the appendages have 

 parts that may be compared with the typical biramous appen- 

 dage, but they are much modified to serve special functions, and 

 the exact homologies are not important. Between the man- 

 dibles note the mouth, bounded in front by the labrum. 



Drawings of these appendages may be made if time permits. 



(d) The antennce are biramous. Notice on the ventral side of 

 the basal joint of an antenna the opening of the green gland or 

 nephridium. 



(e) The antennules, though branched, are not considered 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 otocyst. 

 Do you know the probable function of the antennules and of 

 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? What reason is there for 

 having these appendages different? Do you think the arrange- 



