SOW-BUG (Oniscus or Porcellio). 



Can you make out three regions in the body: head, 

 thorax, and abdomen? Where would you draw the lines 

 between the regions? 



Examine a thoracic segment. Does it resemble in any 

 way an abdominal segment of a crayfish? Study the legs. 

 Can you find exopod and endopod? How many legs 

 do you find? Are any of them terminated with pincers? 

 Look beneath the thorax for thin overlapping membranes 

 attached to the bases of the legs. They will be found only 

 in females. Between them and the lower surface of the 

 body is a chamber or brood-pouch to contain the eggs or 

 young. Do you find anything in this cavity? 



How many segments do you find in the abdomen? 

 Notice the last pair of abdominal appendages extending 

 behind the body. Turn the animal on its back, and with 

 the needle pull apart the flattened plates on the lower sur- 

 face of the abdomen. These are the gills. Do they pre- 

 sent any of the characteristics of appendages? How 

 many of these gills do you find? Examine them all and 

 see which ones bear white spots (air-chambers). Draw a 

 pair of these gills. 



Examine the head. Where are the eyes? Are they 

 on stalks? What are the peculiarities of the antennae? 

 Can you, with the lens, find another pair of minute an- 

 tennae? The mouth-parts form a short, thick projection 



73 



