CHAPTER XIV. 



PHYLUM II: PORIFERA. 



181. General Survey. All sponges live in water. 

 All except one family live in the ocean. They are found 

 in all parts of the oceans, but those species whose skeleton 

 we use as a bath sponge are found most abundantly in 

 the Mediterranean and Red Seas and in the Indian Ocean. 

 This is the only respect in which sponges are of value 

 to mankind. Several species are found along our own 

 shores in relatively shallow water, and these with the fresh- 

 water sponge are the forms most usually studied in the 

 laboratories of the country. The group is not a very 

 large or important one. It is interesting to zoologists 

 chiefly because of the following facts : 



1 . It is the lowest group in which the animals are made 

 up of three layers of cells. In some respects they are 

 intermediate between the protozoa and the higher animals. 



2. They are attached like plants in their adult stage 

 (see Figs. 44 and 45), and they bud and branch in a non- 

 sexual way, so that the descendants of one individual 

 come to cover much space as a single colony. But it is 

 to be remembered that they start life as a single cell and 

 in the early stages the animal is ciliated and free-swim- 

 ming, much like the Protozoa. It is the lowest group of 

 animals to pass through the stages mentioned in 121. 



3 . Instead of having one mouth, as all the higher animals 

 do, the sponge has numerous small mouths ("pores"} 

 through which the currents of water carry minute parti- 



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