CHAPTEK IX 

 PHYLUM PORIFERA 



SPONGES 



The name of this phylum, Porifera (p6 rif'er a), means ''pore- 

 bearers," and this, these animals certainly are. This group is 

 thought to be sort of an aberrant type with peculiar relations, but 

 the group is often considered the simplest and lowest type of 

 Metazoa, notwithstanding the presence of a simple mesoderm which 

 is lacking in Coelenterata. For a long time sponges were thought 

 to be plants, and it was not until 1857, only a little over ninety 

 years ago, that they were fully acknowledged as animals. 



They are sessile in habit, being fastened to piers, pilings, shells, 

 rocks, etc., for life. There is entire lack of locomotion. Most 

 sponges, bath sponges included, live in the sea. There are only a 

 few small fresh-Avater forms. They have tissues but are without 

 organs. The body is in the form of a hollow sac with many canals 

 piercing the walls and making connection between the internal 

 cavity and the outside. The pores of these canals are essentially 

 mouths. There is only one general exit from the cavity. All 

 sponges have some type of skeletal structure; some possess hard, 

 calcareous, or siliceous spicules, and others have a flexible fiberlike 

 material as a skeleton. 



The organization of the sponges is a loose one, and the interde- 

 pendence of part upon part is not great. An animal with hundreds 

 of mouths cannot be very highly organized. Some authorities show 

 a rather close comparison between sponges and colonial Protozoa. 

 The sponges possess collar cells or choanocytes which are similar to 

 the cells of the colonial mastigophoran, Proterospongia. 



There are workers who hold that sponges may have arisen from 

 a common ancestor with the choanoflagellate type of colonial Pro- 

 tozoa. For a time sponges themselves were considered colonial 

 Protozoa. The sponges do not have a distinct enteron or digestive 

 cavity, but digestion is entirely intracellular (within cells). The 

 germ layers are not well-established ; the layer which seems to begin 

 like endoderm develops into the external layer. The so-called ecto- 



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