1 8 Laboratory Guide in Zoology 



THE CLASSIFICATION OF THE PORIFEHA 



The animal kingdom as a whole is broadly divided into 

 two great groups, — the Protozoa, or one-celled animals, and 

 the Metazoa, or many-celled animals. The examples 

 treated of in Chapter I. all belong to the Protozoa, while 

 the sponge is the first example of the Metazoa. 



The Porifera (formerly called sometimes Spongida) is 

 the second phylum or branch of the animal kingdom, and 

 the lowest of the Metazoa. 



For a long time in the early history of science the real 

 nature of sponges was not understood. Naturalists could 

 not agree as to whether they were plants or animals, or 

 part plant and part animal. It has, however, been 

 acknowledged for many years that they belong to the 

 animal kingdom. After this point was reached there was 

 much discussion as to their place in the kingdom. Some 

 zoologists believed that they were simply masses of single- 

 celled individuals living together, each for and by itself : 

 consequently they placed the sponges in the Protozoa. It 

 is now known, however, that a single sponge individual is 

 made up of a great number of cells, all working for a com- 

 mon end — the existence of the sponge individual. To do 

 this well, some of the cells perform the function of protec- 

 tion, some that of nutrition, while others serve the purpose 

 of reproduction. In other words, the cells of the Sponge 

 organism are differentiated into tissues, those doing the 

 same kind of work forming the same tissue. 



Sponges are aquatic animals of wide distribution : 

 although most abundant in salt water, a few small chloro- 

 phyl-bearing species Hve in fresh-water brooks and other 

 streams. Their lives, except when they are very young, 

 are spent attached to some submerged object where micro- 



