102 MARINE INVERTEBRATES 
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, ete., 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 choanacytes, re- 
semble independent animals 
To illustrate choanocytes (Ch): sectio1. 0. a cal- of the Protozoa, known as 
careous sponge. ct, ectoderm; Mes, mesoderm; . 
Ny, calcareous spicule; Hiz, ovum. flagellete Infusoria or Choa- 
Various forms of sponge-spicules. 
