32 



CHAPTER III. 



SPONGES, CCELENTERATES, AND ECHINODEEMS. 



Spongid^e or Porifera. 



A SPOXGB is a compound structure of true animal nature. It 

 is composed of contractile tissue, which is supported by a skele- 

 ton of hard spicules or fibres. In past ages sponges were thought 

 to be plants, but their true animal nature has long since been 

 demonstrated. The simplest form of sponge is represented by 

 a fixed cylindrical tube, with an exhalant opening, called the 

 osculum, at the free end. The contractile wall of the cylinder is 

 supported by rayed spicules, whicli may be calcareous or siliceous 

 and of very variable form : it is perforated by small pores, known 

 as inhalant pores, which lead into ciliated internal chambers. 

 In these ciliated chambers are found cells lining the cavities 

 peculiar to the Sponge. Such cells are called " collar cells," 

 each being provided with a long cilium and a distinct nucleus 

 in the lower part of the cell. The reproduction of sponges 

 is much more advanced than in the Protozoa. True ova are 

 found in the layer of tissue known as the mesoderm, or middle 

 layer. These ova go through a process of cell-division known 

 as segmentation, a process henceforth to be observed in all the 

 following groups of animals. The single cell, the ovum, at first 

 divides into two, but, unlike the protozoan, it does not separate ; 

 then by further division four cells are produced, then eight, then 

 sixteen, then thirty-two ! E\entually there is formed a free- 



