BRANCH PORIFERA 



All animals except the Protozoans are multicellular and are 

 classed as Metazoa. 



Differentiation. — In all we find, to a greater or less degree, 

 division of labor among the cells, or the differentiation into tissues 

 and organs for special functions. 



Reproduction. — True sexual reproduction is the characteristic 

 method among Metazoans. 



Porifera. — These aquatic, many-celled animals were formerly 

 considered as plants. Indeed, they look like seaweeds among 

 the rocks at the bottom of the sea. Most of the sponges are 

 marine, but there are a numlier of fre^li-water forms. 



Fresh-water si)onges are widely distributed, and are attached 

 to weeds [or suljmerged objects along the margins of clear 

 springs or ponds. 



Sponges vary in color from a greenish hue to red, brown, or 

 flesh color. All of the soft parts, as well as the skin or covering, 

 is gone from the commercial sjioiiges. 



Their shape, as is seen in the sponges of commerce, is irregu- 

 lar even in the same species ; it \'aries with t lie en^•ironment, in 

 order that the sponges may adairt themselves to the surface 

 to which they are attached or the deirth and currents of the 

 water. Their size varies from a fraction of an inch to two or 

 three feet in diameter. 



Structure. — The body of the Porifera consists of many cells 

 arranged in two layers, an inner, or emlodcnu, and an outer, or 

 ectoderm. There is a middle undifferentiated layer {nicKoglca). 

 The simplest sponge is cylindric or vase- shaped (Fig. 4), 

 while others, more complicated, consist of a system of branch- 

 ing tubes. At the free end of each is a small opening, the 

 osculum, or exhalant orifice, while the walls of th(> cylinder are 

 perforated by exceedingly minute inhnlanl pores. The ecto- 

 derm consists of flattened c(>lls, which are also found to extend 

 for a short distance inside the osculum, while the rest of the tube 



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