36 PHYLUM ARTHROPODA 



Class : Crustacea. Division : Malacostraca. Order : Isopoda 

 A SOW BUG {PORCELLIO, ONISCUS, OR ARMADILLIDIUM) 



This animal is one of the few terrestrial crustaceans. It may 

 be found at any time of the year under stones, logs, etc., and 

 in other moist, dark places, where it lives on decaying organic 

 matter. 



The animal must be studied with the aid of a hand lens or a 

 dissecting microscope. Compare the animal with the crustaceans 

 already studied. Notice the flattened body. It is composed of 

 twenty somites, of which five are cephaHc, eight are thoracic, 

 and seven are abdominal, and much less fusion has taken place 

 among them than is the case in the decapods. The head and the 

 thorax are not covered by a carapace and thus are not joined to- 

 gether to form a cephalo thorax. The apparent head is composed 

 of six fused somites, five of which are cephalic and one thoracic. 

 The remaining seven thoracic somites are free and movable. 

 Count them. Count the abdominal segments. Six will be found, 

 the last two abdominal somites being fused together. 



Find the eyes : they are not on stalks, but are sessile. Only 

 one pair of antennae appears, the first pair being rudimentary. 

 Notice the pair of anal feelers which extend back from the hinder 

 end of the body. These are homologous to the last pair of ap- 

 pendages, like the cerci of orthopterous insects, and have a similar 

 function, that of orienting the animal as to what is behind it. 



Study the ventral side of the animal. Notice if it be a male or a 

 female. The male has a long, dark-colored, tube-shaped 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 medial side of the first five pairs of 

 walking legs and contains eggs or young. Has your specimen a 

 brood sac? 



The Appendages. First observe the seven pairs of walking legs ; 

 they are the thoracic legs numbering from two to eight ; gills are 

 wanting in them. The gills, instead of being thoracic structures, 



