A SOW-BUG 41 



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 maxillae, one pair of mandibles, and one pair 

 of antennae, the second, the first pair of antennae 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 similar to that made use of in the dissection 

 of the crayfish or the lobster. (See page 29.) 



