Unicellular and Multicellular Animals 



its wall is pierced in many places, so as to permit of a freer cir- 

 culation, through the digestive cavity, of the water which con- 

 tains the food material. The water passes in through these 

 numerous perforations, and out through the main central opening 

 or ' mouth.' The sponges do not appear to represent a stage 

 in the main line of Evolution, but lead us almost immediately into 

 a cul-de-sac. We therefore cannot pause to describe fully the 

 many peculiar and interesting developments which occurred in 

 the group. An ordinary ' sponge,' by the way, bears the same 

 relation to the creature which produces it as does a ' coral ' to 

 the coral animal. It represents, that is to 

 say, the skeletons of a large colony of indi- 

 viduals. The structure of a sponge is shown 

 in Fig. 32. 



The other great group of primitive multi- 

 cellular animals is that of the Ccelenterata. 

 and as an example of the most primitive of 

 these we may take the common freshwater 

 Hydra. The Hydra reaches a length of FIG. 32. Diagrammatic 

 nearly half an inch, and is to be found at- section of lower sponge ' 



^, inner cell layer. 



tached to water-weed and the like in streams. 

 It consists of a hollow tube-shaped body 

 which is fixed by the so-called ' foot.' Two 

 layers of cells form the wall of this tube, these 

 being separated by a thin membrane of gelatinous material. At the 

 upper end is the mouth, which leads immediately into the internal 

 cavity or stomach. The mouth is surrounded by a ring of from six 

 to eight tentacles, which are outgrowths of both cell layers. The 

 cells of the inner layer are large, and bear cilia that protrude into 

 the internal cavity. Their functions are those of digestion and 

 absorption. Part of the protoplasm of the outer cells is modified 

 into a fibrous, contractile substance, which represents the begin- 

 nings of muscle tissue. The outer layer also forms a protective 

 skin-like covering. In the outer layer also occur a large number 

 of stinging cells, each of which has a complex mechanism for 

 injecting a fluid poison into any creature which should happen 



43 



;, middle jelly-like layer. 

 z, outer cell layer. 

 a, digestive cavity. 

 /", perforations in the 

 wall. 



