164 ARTHROPOD A. 



place them in a dish of sea-water together. Do they seem 

 disturbed? Compare their actions with those in shells. 



4. Place an empty shell in the dish and see what happens. 



5. Put more empty shells in the dish, but be sure they are 

 not quite large enough for the crabs. Then add some larger 

 shells and watch the crabs test them to determine which will 

 serve best. 



A drawing is desirable. 



Thompson: The Metamorphoses of the Hermit Crab. Proc. Bost. Soc. 

 Nat. Hist., 31, 1903. 



HIPP A. (Sand Mole.) 



On sand beaches, between low- and high-water mark, there 

 may frequently be seen the shallow depressions that mark the 

 places where these animals have burrowed. They may be dug 

 out with a shovel, but they quickly disappear again. 



1. Notice their shape and the ease and rapidity with which 

 they burrow. 



2. Place specimens in a dish containing sand and a little sea- 

 water and try to determine just how the burrowing is done. This 

 may frequently be done by holding a specimen so it just touches 

 the sand. Which end goes into the sand first? Notice the posi- 

 tions in which the appendages are held. Does this have any- 

 thing to do with the direction in which it burrows? Does the 

 animal jump or crawl ? In what direction and how can it swim ? 



3. Examine the body and see if it is divided into head, thorax, 

 and abdomen. Why has the telson such a peculiar shape ? 



4. Examine the appendages. 



(a) The stalked eyes. 



(b) The biramous antennules and the exceedingly long, feath- 

 ery antennae. What is the usual position of the antennae ? 



(c) The mouth appendages. Are strong, hard mandibles 

 present ? What must the character of the food be ? 



(d) The thoracic appendages. How many are there? Are 

 they similar? Are there any chelae? 



