A LOBSTER OR A CRAYFISH. 



Exercise 2. Draw, in their order, the right antennule, antenna, mandible, 

 first maxilla, second maxilla, first maxilliped, second maxilliped, 

 third maxilliped, and the fourth and sixth swimmerets, removing 

 each, in the reverse order, if this has not already been done. 



Technical Note. Cut away the gill cover from the left gill chamber and 

 place the specimen under water. 



j) The gills. Extending up into the gill cavity are seven epipods, 

 large blade-like flaps which separate the gills into groups. They 

 are attached one each to the three maxillipeds and the four ante- 

 rior walking legs. The gills themselves show three methods of 

 attachment to the bases of the appendages just named (podo- 

 branchs), to the articular membrane between appendage and 

 body wall (arthrobranchs) and to the body wall itself (pleuro- 

 branchs). There are two arthrobranchs in some segments. 

 What purpose is served by having some of the gills attached to the 

 basal joints of the appendages ? 



Exercise 3. Sketch the gill groups in position, side view, showing also out- 

 line of the cephalothorax and bases of the appendages. 



Exercise 4. Make a diagram in your note book showing the presence or 

 absence of the three sorts of gills and the epipod in each of the eight 

 segments of the thorax. The following will serve for a plan; write the 

 name of the appendage in the first column and indicate by figures, in 

 the appropriate space, the number of each kind of gill: 



