28 CCELENTERATES. 



is much more advanced than in the Protozoa. True ova are 

 found in the layer of tissue known as the mesoderm, or middle 

 layer. These ova go through a process of cell-division known 

 as segmentation, a process henceforth to be observed in all the 

 following groups of animals. The single cell, the ovum, at first 

 divides into two, but, unlike the protozoon, it does not separate ; 

 then by further division four cells are produced, then eight, then 

 sixteen, then thirty-two ! Eventually there is formed a free- 

 swimming body, a larva, which is composed of a number of 

 cells ciliated on the exterior. This larva is called an araplii- 

 hlastula, which, after leading a free aquatic life, eventually 

 settles down, and, fixing on to a stone on the floor of the 

 sea, becomes gradually metamorphosed into a sponge. Most 

 sponges are marine ; a few, however, are fresh-water — one 

 common form, Spongilla fluviatUis, being often abundant in 

 our streams. 



COILENTERATBS. 



Coelenterates include the Jellyfish, Sea-anemones, and Corals. 

 These marine animals have regular consistent tissues. The cells 

 of which they are built up have lost their original form, and 

 have become sorted out into different groups, each with their 

 special functions, the various groups forming the tissues of 

 which the animals are built up. In the outer layer of cells (the 

 skin or epithelium) there are found in all Coelenterates, more or 

 less highly developed, certain cells that are known as " thread- 

 cells " — cells that are modified as weapons of offence and defence, 

 being endowed with stinging propensities. Each of these cells, 

 or " cnidoblasts," is provided with an internal barbed thread. 

 When the cell is touched, this thread, like a flagellum of one 

 of the Protozoa, is darted out and enters the skin of the prey 

 or enemy, carrying with it a certain amount of poison, which 

 produces the curious stinging and even paralysing sensation we 

 experience when a jellyfish settles upon us when we are swim- 

 ming in the sea. The amoeboid cell-unit here loses its individ- 



