SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



181 



sponge is forced through a fine sieve, the uninjured separated cells will 

 crawl together like amoebae and build many small new sponge bodies, 

 each cell taking on the form appropriate to its position in the developing 

 sponge. 



In a sponge, then, the cells have become slightly (and not irreversibly) 

 differentiated and show some division of labor and degree of cooperation. 

 However, they are not organized into definite tissues, and the sponge 



tentacle 



ectoderm turned back to 

 show nerve-net in mesoglea 



Fig. 13.5. Hydra, a simple metazoan animal. The inset shows a part of the nerve net at the 

 base of the ectodermal layer. (Inset based on Wolcott, Animal Biology.) 



has no organs nor any means of coordination other than a simple me- 

 chanical relationship between the cells. The body functions are carried 

 on by the cells acting as units, and the whole sponge functions at what may 

 be called the cellular level of construction. 



Tissue level. The simplest animals in which specialized cells are 

 organized into definite tissues are the coelenterates (Coelenterata), a 

 group to which belong Hydra, jelly fishes, corals, and sea anemones. In 

 them the grouping of cells into tissues brings increased efficiency. The 

 cells are enabled to act in a more coordinated fashion and with massed 



