CHAPTER II. 
BRANCH II.—PORIFERA (Sponazs). 
General Characters of Sponges.—Although the sponges 
were formerly supposed to be compound or social Amcebe, 
and more recently monads, from the striking resemblance 
of their epithelial cells to certain monads, and have been 
generally regarded as Protozoans, later researches have 
shown that they are in reality many-celled animals, and that 
for a short period of their life they follow the same develop- 
mental path as the higher animals. It was also discovered 
that they reproduce by eggs, the latter undergoing segmen- 
tation and assuming the condition of .a. three-layered sac, 
the three layers being identical with those of the higher 
branches of the animal kingdom, so that the gap between 
the Protozoans and sponges is a wide one, and the latter are 
more nearly allied to the Hydra, for example, than to any 
one-celled animal. 
One of the simplest sponges, such as Ascetta primordialis 
Haeckel, is a spindle or vase-shaped cylinder, attached by its 
base, with the cellular soft portion supported by a basket- 
work of interlaced needles or spicules of silex or lime. The 
cells are arranged in three layers, the innermost (endoderm) 
being provided each with acilium. The spicules, and also 
the eggs, are developed in the middle layer (mesoderm), 
Moreover, the walls of the body are perforated by multitudes 
of small pores (whence the name of the branch, Porifera), 
through which the water percolates into the body-cavity, 
carrying minute forms of life or food-particles, which are 
individually thrown into each cell by the action of the single 
cilium thrust out of the collar of the cell, much as in an in- 
dividual monad such as Codosiga (Fig. 21). Each cell re- 
