i66 



THE BOOK OF NATURE STUDY 



waste the unselective waste of life in the first method of re- 

 production. The waste of life among the swimming-bells is 

 less obvious than among their larger allies the jelly-fish, which 

 are more nearly related to the sea-anemones. We may see every 

 summer what fearful havoc untoward currents and winds play 

 with these by the myriads which strew the beach. But if the 

 little swimming-bells for the most part perish unseen, we have no 



reason to doubt that 

 the slaughter among 

 them is as great, and 

 therefore it is not sur- 

 prising to find that 

 some of the sea-firs 

 have abandoned the 

 habit of producing 

 free-swimming bells. 

 The little Hyractinia, 

 so often found on the 

 shells of hermit crabs, 



FIG. 83. The common hermit-crab in a whelk-shell . - ,. 



covered by hydractinia. (After Allman.) 1S an example OI this. 



On the other hand, 



0^7XseeFig.82),which often covers bladder-wrackwith its delicate 

 tracery, is an example of a form which does produce swimming- 

 bells. These two may be added to the aquarium for the sake 

 of the story they tell. Another 

 form, which may be named is 

 Clava squamata, a pretty pink zoo- 

 phyte, found on seaweed, in which 

 one can see with the naked eye 

 the sessile buds which replace the 

 swimming-bells of Obelia. 



We have said nothing above of 

 the sponges, for under the ordinary 

 conditions not much is to be learnt 

 by keeping them in an aquarium, 

 and they are very liable to pollute the water for other more 

 interesting animals. 



In conclusion, we may note that a considerable number of 



FlG. 84. Clava squamata on sea- 

 weed, with sessile buds. (After 

 Allman.) 



