CHAPTER II. 



SECOND BRANCH OF ANIMALS. 

 SPONGES (Port/era, pore-bearing). 



General Characteristics. The sponges were for many 

 years considered plants, but now they are known to be 

 many-celled animals. In the Ascetta (Fig. n), we have 

 a vase-shaped cylinder; 7, composed of cells arranged in 

 three layers. In the second or middle layer is developed 

 a network of deli- 

 cate objects of lime, 

 called spicules (Fig. 

 u), that form the 

 skeleton, and sup- 

 port the cellular, jel- 

 ly-like mass. The 

 walls of the vase are 

 everywhere perfo- 

 rated with pores, /, 

 through which water 

 passes, carrying food. 

 The cells of the in- 

 ner layer are pro- 

 vided with a cilium, FIG. 10. Spicules of flint-sponges, highly 

 or lash, ///, and, magnified, 



taken individually, 



resemble monads. As food floats by, each cilium throws 

 ''he minute bits against its cell ; the soft portion is absorbed, 

 the harder parts being rejected, and, wafted along by the 

 cilia, find egress at the single large opening, O. In this 



