"The coming of life u 



tubes or branches to carry the more or less dissolved nutri- 

 ment to all parts of the body. Respiration and excretion are 

 maintained through the exposed surfaces of the cell-layers. 



The protozoa showed us what could be accomplished by the 

 single cell. Then cell-colonies appeared; and in volvox cells 

 set apart purely for reproduction, the first need and function 

 is to have cells devoted entirely and solely to its performance. 

 It insured the survival of the species, as digestion maintained 

 the survival of the individual. These are the two absolutely 

 essential functions and therefore the first to be provided with 

 special organs. The ccelenterates show us a stage where these 

 two organs have attained their simplest and most primitive 

 forms and structure. We might say that the plant almost at- 

 tained but never outgrew this stage. There is a good sug- 

 gestion in the name given by the older naturalists to this 

 stage of animal life, zoophytes or plant-animals. 



These are not the only contributions which the ccelenterates 

 made to the building of the body of higher animals. They 

 developed quite a variety of tissues, groups of cells whose 

 form and structure are adapted to the performance of a special 

 function, but these cell-groups are not yet arranged and united 

 in an organ having a special shape. An organ usually consists 

 of several tissues; so a coat, corresponding to an organ, is 

 made of several materials, cloth, thread, etc., arranged in 

 a shape fitted to the wearer. 



In the ccelenterates we find muscular and nervous tissues, 

 generally in a rudimentary form. It begins to look as if these 

 might some day take shape in exceedingly interesting organs. 

 We find sensory cells each with its fine hair responding to 

 delicate vibrations in the water or exposing a very large sur- 

 face to chemical stimuli of various kinds. They are the 

 promise of organs of hearing, smell, and taste. Little patches 

 of pigment are sensitive to light and feel it, in a crude way, 

 the direction of its source. They continually stimulate the 

 cells of the growing and developing nervous tissue. Even in 

 hydra this has formed a plexus beneath the skin, a sort of 



