46 INVERTEBRATE ZOOLOGY 



CRUSTACEA 



A LAND ISOPOD. 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 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 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 

 appendages, like the cerci of orthopterous insects, and have a 

 similar function. 



Exercise l. 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 



