1 68 Dwellers of the Sea and Shore 



that sponges are plants. This misconception no doubt 

 arises from the appearance of those which are in com- 

 mercial use. Such sponges, however, are only the 

 horny skeletons which in the living animals were com- 

 pletely covered with a soft tissue of cells. Even in life, 

 they are by no means typical of the group in general. 

 For sponges are not only extremely variable in size and 

 color; they are also found to grow in a vast variety of 

 forms. Some species are so small that they are scarcely 

 visible to the naked eye; others grow to such propor- 

 tions that their dimensions are only to be measured in 

 feet. Again, they range from masses so indefinite in 

 outline as to appear like monstrous aberrations, to sym- 

 metrical structures of exquisite shapeliness. Neverthe- 

 less, the fibrous framework (spongin) of the common 

 commercial sponge identifies it as belonging to the Non- 

 Calcarea, one of the three classes into which all of these 

 animals are divided. The skeleton of those sponges 

 belonging to the Calcarea differ in being composed of 

 mineral spicules (lime) or a combination of lime and 

 spongin. This flinty material gives these individuals 

 a hard and rigid character. It should be noted, how- 

 ever, that among the calcareous sponges some odd ex- 

 amples exist whose skeletons consist wholly or nearly 

 so of particles of sand. 



Although the sponges are undoubtedly animals, they 

 are very low ones. Indeed, they stand near the foot 

 of the ladder that symbolizes the evolution of the 

 many-celled organisms. There are many other animals 

 which are still lower, but they consist of a single cell. 

 The sponges, however, are only a step higher, for, 

 although their bodies are composed of an aggrega- 



