THE ARTHROPODS 



359 



Notice that the animal consists of body and appendages. Are 

 the ends of the body similar? Observe carefully the head end 

 (anterior] and the hinder end (posterior}. Notice that the body is 

 divisible into an unjointed head portion (cephalothorax) , which 

 means head-thorax, and a jointed flexible hinder portion (abdomen). 

 Are there any indications of 

 joints in the cephalothorax? 

 Is it flexible ? Notice that the 

 animal has right and left sides. 

 Examine several individuals 

 and determine whether the 

 sides are exactly alike. Ex- 

 amine the lower (ventral) and 

 upper (dorsal) surfaces. To 

 what surfaces are most of the 

 appendages attached ? In how 

 many planes could a knife be 

 passed so as to cut the animal 

 into two similar halves ? Use 

 terms anterior, posterior, dor- 

 sal, ventral, longitudinal and 

 transverse in describing the 

 position of planes. Is the cray- 

 fish bilaterally symmetrical ? 



Estimate length of the ani- 

 mal and also of the cephalo- 

 thorax and the abdomen 

 separately. Notice that the 

 body is covered by a hard, 

 outer skeleton or case (exo- 

 skeleton). Does the same sub- 

 stance cover the appendages ? 

 Notice color of living animal. FIG. 115. Blind crayfish from Mam- 

 Abdomen. At the extreme moth Cave. (From Packard.) 

 posterior end is the tail-fin. 



Notice that it is composed of a central flattened structure (telson) 

 and on either side a double fan-like plate. Spread the tail-fin and 

 make an outline drawing. 



How many segments in the abdomen, excluding the telson ? 

 Number the segments of the abdomen, beginning with the anterior 

 segment, and later mark numbers on drawings. Are the segments 

 similar in shape? Sketch abdomen as seen from the left side. 



