538 The American Naturalist. [June, 
sometimes there are developed at the top or bottom, cross-bars, 
or there is a cluster, curving upward or downward ; some are 
Ae 
Axe HE 
Fig. 2. Various forms of sponge spicules. (After Sollas). 
shaped like harpoons, with spines along the sides, all pointing 
in one direction. Some are curved; some have an umbrella- 
like top; some are oval, star-shaped, or are developed in the 
form of a rosette (Fig. 2). Sometimes these spicules are simply 
scattered promiscuously through the fibrous net-work; but in 
other instances they become united during growth at their 
free ends, and a network is formed from which results such 
“species as the beautiful Venus’s Flower Basket sponge. 
Each one of the spicules of a sponge originates in a single 
cell, within which it remains until fully grown. “ During its 
growth the spicule slowly passes from the interior to the ex- 
terior of the sponge, and is finally (at least in some species) 
cast out as an effete product. The sponge is thus constantly 
producing and disengaging spicules; and in this way we may 
account for the extraordinary profusion of these structures in 
some modern marine deposits and in the ancient stratified 
rocks.” (W. J. Sollas). 
While the sponge as a mass does not show any differentia- 
tion into special parts or organs, there are frequently scatter 
throughout its tissues certain wandering amceboid cells. 
These seem to perform special purposes. Some act as scav- 
engers; others as carries of nourishment; while some become — 
