178 INTRODUCTION TO ZOOLOGY CHAP, xn 



overcrowd the tank, as these creatures will only thrive when there 

 is plenty of oxygen in the water. When a hermit crab is kept, 

 it must be given a selection of shells larger than the one it is 

 inhabiting, so that it may " move " when its increase in size makes 

 this desirable. 



2. Investigation of the Structure of Crustacea. Dead shrimps 

 or prawns and a small crab can easily be obtained from the fish- 

 monger, and their structure investigated ; sketches should be 

 made in illustration. 



3. Study of the smaller living Crustacea. Fresh-water shrimps, 

 water slaters, water fleas, cyclops, and cypris may usually be easily 

 obtained by drawing a fine net through the water of any pond 

 which has plenty of vegetation in it. Interesting experiments 

 may be carried out to illustrate the rapidity with which some of 

 these forms multiply. Single specimens of the water flea (Daphnia) 

 should l)e isolated, and the curious parthenogenesis which occurs 

 noted. 



When possible ail the more minute of these creatures should be 

 examined under th'e microscope, when their beauty and complexity 

 of structure will become more apparent. Enlarged sketches should 

 be made of them. 



4. Woodlice should be hunted for under stones or logs of wood. 

 They may be identified by reference to British Woodlice, by Webb 

 and Sillem. 



