140 THE BIOLOGY OF THE ANIMAL 



return again to this point and see if you can dis- 

 cover any constant characters of shape, size, etc., 

 by which you can tell the sex. What is the aver- 

 age size of your specimens? E"otice the hard- 

 ness of the exoslieleton or "sheU.'' Do you find 

 all parts of the body covered by it ? Do you find 

 the exoskeleton especially modified in places? 

 "Where ? To what extent ? Notice that the ani- 

 mal consists of the body proper and the ap- 

 pendages. Note the position of the various 

 parts of the body and of the appendages when 

 the animal is at rest. 



B. The body proper. — Note that this consists of an ante- 

 rior (ceph.alothorax) and a posterior (abdomen) 

 region. What proportion of the body does each 

 occupy? As regards general structure, how do 

 you distinguish them? Are both composed of 

 joints or segments ? Do both bear append- 



Examine first, 



I. The abdomen. — Of how many joints — i. e., seg- 

 ments or metameres — does it consist ? Is this 

 number constant? How are the segments con- 

 nected? Note the baU-and-socket joint at the 

 side. On which segment is the baU ? On Avhich 

 the socket ? Are all the segments connected in 

 this way ? Are the abdominal segments aU alike ? 

 Do all bear appendages? How many append- 

 ages on each segment ? Can you trace any sim- 

 ilarity of structure among the appendages ? With 

 a sharp scalpel carefully cut away from its fel- 

 lows the third abdominal segment, which may be 



