46 INVERTEBRATE ZOOLOGY 
CRUSTACEA 
A LAND ISOPOD. A SOW-BUG (Porce/lio, 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 vegetable 
matter. 
The animal must be studied with the aid of a hand lens or a 
dissecting microscope. Compare the animal with the crusta- 
ceans already studied. Notice the flattened body. It is com- 
posed of twenty somites, of which five are cephalic, 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 together to form a cephalothorax. The apparent 
head is composed of six fused somites, five of which are ce- 
phalic 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 antenne 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 
appendages, like the cerci of orthopterous insects, and have a 
similar function. 
Bxercise 1. Draw a dorsal view of the animal on a scale of 10. 
Number the thoracic and the abdominal segments. 
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 
