A SOW-BUG AT 
copulatory organ which extends from the forward border of the 
abdomen backward. The female, besides lacking this organ, 
may have a brood-sac on the ventral surface of the thorax, which 
is composed of plates attached to the inner side of the first five 
pairs of walking legs and contains eggs or young. 
The appendages. First observe the seven pairs of walking legs ; 
they are the thoracic legs numbering from two to eight; ex- 
opodites and gills are wanting in them. The gills, instead of 
being thoracic structures, as in the decapods, are attached to 
the abdominal legs. With a fine needle separate the flattened 
appendages of the first five abdominal segments. The endopo- 
dite serves as the gill, while the exopodite is large and plate- 
like and covers the endopodite. The appendages of the head 
may be best studied from the hinder pair forward. They con- 
sist of one pair of maxillipeds, which belong to the first thoracic 
somite, two pairs of maxille, one pair of mandibles, and one pair 
of antenne, the second, the first pair of antenne being rudi- 
mentary. The maxillipeds are plate-like and cover the other 
mouth-parts. Carefully remove the maxillipeds and study the 
mouth-parts. 
Exercise 2. Construct a table showing the relation of the 
appendages and the somites similar to that made use of 
in the dissection of the crayfish or the lobster (page 35), 
leaving out of consideration, however, the protopodites, 
exopodites, and endopodites. 
