CHAPTER TWENTY-SEVEN 



SPONGES 



I. THE Sponges or Porifera (pore-bearing) constitute 

 a very distinct phylum of animals, little related to any 



characters 

 of sponges 



Drawing by W. P. Hay 



FIG. 45. A simple sponge attached to a seaweed. On the right the same animal 



is shown in vertical section. 



others. They are all aquatic, the great majority being General 

 marine. They have existed in great abundance for 

 many millions of years, as is proved by their fossil re- 

 mains. The genera and species are very numerous, and 

 of very diverse structure and appearance; yet all are 

 sponges, and there has apparently been no tendency to 

 evolve into anything higher. There is a similar lack of Mode of life 

 progressive tendencies in the life of the individual 

 sponge. The ovum or egg cell is fertilized by a minute 

 flagellated sperm-cell, as in the higher animals. The 

 fertilized cell becomes a ciliated larva, which swims 

 about for a time and then becomes fixed to some object 

 and develops into a sponge. There is a veritable meta- 

 morphosis, the creature becoming entirely changed, and 



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