SPONGES. 159 



its walls the result will be the formation of numbers of 

 chambers or ampullce around a central cavity. With this 

 modification the ampullae become the seat of digestion, 

 while the central cavity has no longer that function but 

 becomes a cloaca, a part of the passage leading the waste- 

 water to the exterior. This is the Sycon type. Further 

 complications are introduced by the formation of branch- 

 ing incurrent canals leading from the pores to the ampullae 

 and similar excurrent canals connecting the ampullae with 

 the cloaca. 



There are considerable difficulties in homologizing the 

 layers of the sponges with those of other Metazoa. The 

 digestive cavities (gastral cavity, ampullae) are lined with 

 an entoderm of collared flagellate cells, while the rest of the 

 body is made up of connective tissue covered by an epi- 

 thelium of flattened cells. In this connective tissue the 

 eggs and sperm are formed (fig. 10), and usually the em- 

 bryos remain here until well advanced in development. 



A few sponges have no skeleton, but most species have a 

 firm support for the soft parts, arising in the connective 

 tissue. This skeleton may consist of small particles 

 (spicules) of carbonate of lime or of silica, often much like 

 crystals in form; or of fibres of a horny substance; or 

 again, both spicules and fibres may occur together. In 

 the sponges of the stores we have nothing but the horny 

 fibres, all of the flesh having been washed away; but in 

 this skeleton we can trace roughly the systems of canals, 

 the cloaca, and the osculum. 



Sponges reproduce by budding and by eggs. In budding 

 small outgrowths occur, and these gradually become 

 larger, and finally an osculum is formed. From the eggs 

 are formed little free-swimming embryos, which later 

 settle down and grow into the adult. 



