OTHER CRUSTACEANS 155 



of appendages and are called nauplii. How do the males differ from 

 the females aside from the egg-sacs? Internally the digestive tract 

 will be seen. There is no heart. How may circulation of the body 

 fluid be effected? 



Exercise 2. — The Larger Crustacea or Malacostraca. 



(e) Such crustaceans as the amphipods of fresh water, the isopods 

 represented by the pill-bugs which are found under logs and stones 

 on land, the edible crabs, lobsters, and shrimps are representative 

 Malacostraca. 



(f) Fresh-water amphipods may be examined alive in a small dish 

 of water and their structure and activities noted. Terrestrial isopods 

 may be similarly examined in a dish without water; do they "hide" 

 under the litter when bits of damp paper are placed in the container? 

 Do they "play possum"? A careful study would show that each of 

 these types has the same fixed number of somites, and so appendages, 

 as the crayfish. 



(g) Examine a specimen of a crab and compare with the crayfish. 

 The abdomen is greatly reduced and folded close against the ventral 

 surface of the thorax. Understand the functions of the abdomen and 

 its appendages in the two sexes. The cephalothorax with its carapace 

 is extended laterally. Pereiopods and chelipeds will be recognized. 

 There are the same appendages about the mouth as in the crayfish, 

 but these are covered by the single large pair next anterior to the 

 chelipeds. To which pair of appendages in the crayfish do these cor- 

 respond? Note the characteristic positions of eyes, antennules, and 

 antennaB. Dissected specimens may be demonstrated to show the 

 gills, the digestive tract, and the nervous system, all of which are 

 modified in conformity with the shortening of the body antero-pos- 

 teriorly and its lateral extension. In life the crab scuttles about on the 

 bottom or swims with paddle-like fifth pereiopods, going sidewise 

 through the water. The soft-shelled crabs served as food are merely 

 individuals that have recently molted. Larval crabs have a well- 

 developed abdomen, but as development proceeds this becomes reduced 

 and folded under the thorax. Examine any other representatives of 

 the Malacostraca that may be available. 



