IX 



BELOW the Ccelenterata there are two grades of animal 

 structure, both of importance in palaeontology the 

 Porifera or Sponges, and the Protozoa, simplest of all 

 animals. 



The Porifera are so named because they take in their 

 food, not by a single mouth as do all animals from 

 Ccelenterates upwards, but by innumerable pores scattered 

 over the surface of the body. As a consequence of this, 

 and of their fixed mode of life, the form of the body 

 is much more variable than in any phylum yet considered, 

 and is sometimes very indefinite indeed. The skeleton 

 in the more primitive forms is composed of great numbers 

 of separate s'picules usually of amorphous silica (opal), 

 but of calcite in one class. These spicules form a sup- 

 port to all parts of the body, but are only united by soft 

 tissues, so that on the death of the sponge they fall apart 

 and are scattered on the sea-floor. In many cases two 

 kinds of spicules may exist in the same sponge body 

 spicules, elongated and often branched, and dermal spicules, 

 flat or rounded. A simple spicule in the fresh state, when 

 seen under the microscope, has the appearance of a 

 fragment of therrnometer-tube, being glassy and with a 



