MULTIPLICATION 



11 



is lined with a single layer of peculiarly shaped columnar cells, 

 each possessing a flagellum. 



The skeleton is developed in the middle layer and may 

 consist of silicious or of calcareous spicules of a great variety of 

 form, sometimes they are anchor shaped,, 

 and again others are club shaped, spciar 

 shaped, or cruciform. The so-called glass 

 sponges sometimes have beautiful silici- 

 ous skeletons. In other cases the skele- 

 ton consists simply of fine, flexible, inter- 

 woven fibers of tough, horny spongin. 

 It is the skeleton, denuded of the flesh, 

 or sarcode, that covers it in life, which 

 forms the commercial sponge. A few 

 sponges have no skeletons. 



Nutrition. — There are no organs of 

 digestion, circulation, or respiration in 

 the sponge. The food consists of micro- 

 scopic plants or animals, or of minute 

 particles of organic matter floating in 

 the water. The food-laden water enters 

 through the inhalant pores and is carried 

 by the movement of the flagella through 

 the canals or paragastric cavities. The 

 food as well as oxygen is taken up by 

 the cells lining the canals and by the 

 ameboid cells. The waste is carried out 

 by the outgoing currents of water, which 

 empty through the osculum, or, if the 

 sponge is complex, the oscula. 



Locomotion.— At first the larval sponge 

 is free swimming, by means of cilia. 

 It soon becomes "^xled to some stone 



or other object or animal, and assumes the fixed ways of its 

 ancestors. 



Multiplication. — (1) Asexual, by external budding and the 

 consequent formation of a united colony, or by internal gem- 

 mules; (2) sexual, thus insuring the perpetuation of the species. 

 Sponges are hermaphroditic, that is, both the male elements 



Fig. 4. — A simple 

 sponge {Calcolynthus 

 primigenius) with part 

 of outer wall cut away. 

 (After Hiickel.) 



