126 DIGESTION AND RESPIRATION 



some other spot to which it may attach itself, The hairs^ 

 after accomplishing their purpose, fall off, leaving the germ 

 to gradually develop into the sponge. 



This sponge is a natural production, and we have already 

 hinted, has been known from the times of highest antiquity. 

 As is well known, all naturalists are now satisfied of the 

 animal nature of this species of creation, although they were 

 once thought to represent the lowest and most obscure 

 grades of animal existence, and that so close to the confines 

 of the vegetable world, that it was considered difficult in 

 some species to determine whether they were on one side 

 or the other. 



According to a generally accepted view, the channels of 

 the sponge perform the two functions of digestion and respi- 

 ration. The rapid currents of aerated water which traverse 

 them lead into them the substances necessary to the nourish- 

 ment of these strange creatures, and at the same time carry 

 off all excremental matter. At the same time, the walls of 

 these animals present a large absorbing surface which sep- 

 arates the oxygen with which the water is charged, and dis- 

 engages the carbonic acid which results from respiration. 

 But science is far from being settled in its views as to the 

 organization and development of these obscure and complex 

 creatures ; nor is it more advanced in its knowledge of the 

 duration of life and the quickness of growth in sponges. 

 Nor can it be denied, also, that these beings constitute, in 

 spite of the investigations of modern naturalists, a group 

 still somewhat problematical as to their position in the scale 

 of animal life, and that they still are very imperfectly known 

 as regards their internal organization. 



The demand for sponges is increasing annually, and it is 

 only a question of time when the trade must cease. The 

 submarine fields are constantly being cleared, and the de- 

 struction is such that the reproduction will cease to b© 

 adequate. 



