PHYLUM II, PORIFERA (SPONGES) 



Sponges are organisms consisting of many cells, plant-like 

 in appearance and extremely variable in shape. Sponges grow- 

 ing side by side will frequently fuse into one mass, and almost 

 any fragment of a sponge will grow into a perfect individual. 

 They are fixed, aquatic, and usually colonial animals. Nearly 

 all sponges are marine and are most abundant in the shallow 

 parts of the ocean (Euspongia, Cliona) ; but some with a 

 siliceous skeleton (Euplectella) occur in the abyssal portions. 

 The only sponges inhabiting fresh water are the green sponges 

 (Spongilla, etc.) ; the natural color of these is a light gray, but 

 they are usually colored green through the presence of algae. 



Typically a simple sponge looks like a hollow, attached vase ; 

 it consists of a cylindric body pierced by many canals which 

 conduct water through the wall into a large central or paragas- 

 tric cavity ; this opens to the exterior by a large opening, the 

 osculum. Water is drawn in through these many small canals 

 and forced out through the large osculum. These canals are 

 of three kinds, {a) the incurrent, leading from the outside into 

 the wall of the sponge, (b) the flagellate or gastral, and (<:) the 

 excurrent, leading into the central cavity. 



The flagellate canals, — the gastral layer, consist in all 

 sponges of collar-cells ; they are the principal organs for the 

 capture and digestion of food. 



The paragastric or central cavity may be very deep, often is 

 very shallow, at times it may disappear so that the excurrent 

 canals are upon the surface. Forms with one central cavity 

 and one osculum are regarded as individuals, those with more 

 than one as colonies, but as it is difficult to distinguish between 



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