FRESH-WATER SPONGE AND SOME ALLIES 275 



The fresh-water sponge has no special digestive organs, 

 no circulatory system, respiratory system, muscular system, 

 nervous system, nor sense-organs. Food and oxygen are 

 carried in together, and oxidation takes place freely because 

 oxygen can go anywhere within the sponge. The oscula 

 sometimes contract when the sponge is disturbed, but this 

 action takes place because of the power of contracting which 

 is possessed by certain cells that are not true muscle-cells. 

 There are, however, two important systems of organs pos- 

 sessed by the fresh- 

 water sponge: a set 

 of structures which 

 constitute the skele- 

 ton, and special 

 cells for reproduc- 

 tion. The substance 



'V 1NX ^^^^yJ^^ 



which serves as a ^ --c 



skeleton supporting FIG. 137. Section of Fresh- Water Sponge (Spon- 

 the soft, protoplas- 

 mic cells, is made 

 up of microscopic 

 spicules (compare Fig. 139) of various forms, and of very hard 

 material called silica. These spicules, interlacing a-nd cross- 

 ing one another, give a very firm texture to organisms that 

 would otherwise be formless and slimy. 



Certain cells along the canals develop eggs and sperma- 

 tozoa. Each sponge-mass is capable of producing both kinds 

 of cells, but not at the same time. While some masses are 

 producing eggs, others in the same pond or brook are pro- 

 ducing spermatozoa. The eggs are retained in position on the 

 canals, while from other specimens the spermatozoa escape and 

 swim out into the water, to be drawn later, quite by accident, 

 into the incurrent canals of sponges with eggs. The eggs are 

 fertilized and immediately begin the process of development. 



a, surface; 



Much enlarged (After Huxley) 



, dermal pores ; c, ampullae, lined with 

 collar-cells ; d, osculum 



