PRACTICAL ZOOLOGY 35 



Sea Urchin 



Aquarium Study. Note its mode of locomotion, its 

 sensitiveness, its movements in righting itself after hav- 

 ing been turned bottom side up. 



Structure. Trace all the resemblances you can be- 

 tween its external structure and that of the starfish. 



> 



Note that in spite of their difference in shape their 

 likeness in structure is very marked. Study the diges- 

 tive system, the teeth, and the long intestine. Note the 

 radial symmetry (Figs. 48, 226, 237, 293, 294). 



Exhibit the larval stages. 



The cake urchin (Eckinarachnius) and the holothurian 

 (Holothuria> Thyone, or Synaptd) may be used for com- 

 parison (Fig. 49). No other group of animals shows so 

 well as the echinoderms that the same plan of structure 

 may be associated with the greatest diversity of external 

 form. 



Annulata 

 Earthworm 



Field or Vivarium Study. Study living specimens 

 out of doors, note their castings along paths, the amount 

 of earth brought up ; the diameter of the burrows ; trace 

 the latter down into the soil. Place a live worm on the 

 surface of the soil and note its mode of locomotion ; its 

 method of burrowing ; the protection from enemies 

 afforded by its color; draw one out of its burrow and 

 note the resistance ; touch a worm with a bristle and 

 note its sensitiveness ; note the effect of plugging the 

 mouth of the burrow with bits of straw, leaves, etc. 



Watch the pulsation of the dorsal blood vessel. 



Structure. Note the shape of the body, the rings 

 composing it, the girdle, the mouth, the anus, the open- 

 ings of the reproductive glands, the bristles. 



Cut an alcoholic or formalin specimen open along the 



