46 ZOOLOGY. 
be, with the outer world. These pores may open and then 
be permanently closed, new ones opening elsewhere. The 
osculum bursts open by the accumulation of water between 
the two layers in the same manner as the pores. Finally, 
in certain sponges the horny fibres grow out from the outer 
cell-layer and extend inward, surrounding the spicules, the 
latter developing from the middle cell-layer. 
It appears, also, that all sponge embryos form a two and 
afterward three-layered sac (gastrula), in which in the sim- 
plest sponges there is a primitive body-cavity and a prim- 
itive mouth, while in the higher calcareous sponges and in 
the silicions forms the body - cavity is only temporarily 
open, being afterward filled up by the interior ciliated 
cells, and thus forming a compact mass. 
In the sponges, also, the larva or free-swimming young 
is a three-layered sac, which is either hollow or, more com- 
monly, solid, and may attach itself at the end of its free- 
swimming life by one end to some fixed object. The body- 
cavity may persist in the simpler forms through life, though 
in most sponges there is no genuine digestive cavity, but a 
large series of minute digestive sacs communicating by canals 
with the large ones leading to the oscula. The more or less 
regular spherical form of the young of most sponges becomes 
lost as they grow ; they become irregular in form, encrust- 
ing rocks, and their development retrogrades rather than 
advances. 
In the fresh-water Spongilla there is a special provision for 
the maintenance of the species. In autumn are formed the 
so-called ‘‘ seed,’’ being capsules in which are enclosed eggs 
which in the spring develop young sponges. This cyst or 
capsule may be compared to the buds or winter eggs of the 
Polyzoa or of the water-flea (Daphnia). 
From the members of the next branch, the sponges differ 
in the great irregularity of their form, the lack of a definite 
digestive cavity and of tentacles. 
Order 1. Calcispongie.—The sponges may conveniently 
be divided into two orders. Those belonging to the first 
secrete spicules of lime, and there are no digestive or ampul- 
laceous sacs, but the minute canals are lined with ciliated cells. 
