102 



MARINE INVERTEBRATES 



Various forms of sponge-spicules. 



genera consists respectively of a 

 horny or silicious or calcareous 

 substance, or of the first two of 

 these substances combined. The 

 sponge of commerce has the 

 first kind and is composed en- 

 tirely of exceedingly fine flexible 

 fibers of a horny substance called 

 spongin. In other species the 

 spongin is intermixed with spic- 

 ules of silica, or of carbonate 

 of lime, in various shapes. In the sponges, so much valued as 

 curiosities, called "Venus's flower-basket" and "glass-rope 

 sponge," the framework is composed of silicious spicules alone. 

 The spicules have a great variety of shapes, being rod-like, 

 knobbed, three-pointed, six-pointed, anchor-like, etc., and are a 

 feature in the classification of sponges. 



The sponge is traversed throughout by a canal system, con- 

 sisting of a series of tubes through which water circulates, carry- 

 ing air and food to the ani- 

 mal. The exterior of the 

 sponge has numerous small 

 pores and a comparatively 

 few large openings. The fine 

 pores are inhalent, taking in 

 and straining the water of 

 its coarser floating material, 

 and then passing it through 

 perforations in their sides 

 into sacs lined with peculiar 

 cylindrical cells having flagel- 

 late hairs, each hair having a 

 collar at its base. These 

 cells, called choanocytes, re- 

 semble independent animals 



To illustrate choanocytes (Ch) : sectioi. o, a cal- of the ProtOZOCt, knOWn aS 

 careous sponge. Ect, ectoderm; Mes, mesoderm ; . _ 



tf, calcareous spicuie; EIZ, ovum.. flagellcete Infusoria or Ckoa- 



