PORIFERA. 



T6l 



crete the spicules which form the skeleton, and still others are 

 reproductive. The spicules of which the skeleton is made are 

 different in different classes of sponges. They may be cal- 

 careous, siliceous, or horny. The sponge of commerce illus- 

 trates the last class. The spicnles may be isolated and inde- 

 pendent, or become fused into a continuous skeleton. It is 

 the skeletal part which prevents the otherwise soft animal 

 from becoming collapsed into a shapeless mass, and thus en- 

 ables the cavities, by means of which nutrition is effected, to 

 be kept open. It is the variety in the skeletons, too, which 

 gives the diversity of form seen in the individuals of different 

 species. 



205. Nutrition. The food of sponges is essentially similar 

 to that of the single-celled Protozoa. It is carried in by the 

 water currents, which enter the pores, pass along the canals 

 lined with flagellate entoderm into the cloaca, and from there 

 reach the exterior by way of the osculum. The food particles 

 are taken up principally by the entoderm cells lining the radial 

 chambers and by the amoeboid cells which belong to the mesen- 

 chyma. In these cells digestion takes place as in Amoeba. The 

 indigestible parts of the food are returned to the current and 

 are eliminated through the osculum. There is no circulation. 

 The digested food diffuses from cell to cell or is carried by the 

 amoeboid cells. Respiration occurs through all the cells which 

 are in contact with the water. 



206. Sensation and Motion. Sponges are fixed and vege- 

 tative in their adult life, and show very little of the more 

 active functions. In addition to the ciliated and amoeboid cells 

 already described, the pores may be closed in response to 

 stimulus. Both nervous and muscular elements have been 

 described as occurring in these regions, but there is some ques- 

 tion as to the degree of their structural differentiation. 



207. Reproduction by outgrowth or budding is common. 

 In this way large colonies arise from a single individual. New 

 colonies may arise, especially in the fresh-water sponges, by 



