CHAPTER II. 



BRANCH II. PORIFERA (SPONGES). 



General Characters of Sponges. Although the sponges 

 were formerly supposed to be compound or social Amoeba?, 

 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 A^etta 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 orspicules of silex or lime. The 

 cells are arranged in three layers, the innermost (endoderm) 

 being provided each with a cilium. 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- 



