66 



THE ORGANIZED ANIMAL 



the complex many-celled animal with all 

 of its intricate machinery to do the jobs 

 that were done so simply by the one-celled 

 animals. The price has been a long and 

 bitter struggle; whether or not it is worth 

 while only man has sufficient intelligence to 

 say. 



Assuming that during the millennia re- 

 quired for the unfolding of animal life, 

 some found satisfactory niches all along the 

 way and remained essentially unchanged 

 up to the present, it should be possible to 

 find such representatives and to arrange 

 them in order of their complexity, forming 

 a continuous series from single cells to the 

 most complex animals alive today. To be 

 sure, we would expect to find gaps between 

 groups and we would also expect that the 

 animals we do find would not be exact 

 duplicates of the originals. They too would 

 undoubtedly have undergone some minor 

 changes during this long period of time, 

 even though they remained in a relatively 

 unchanging environment. By erecting such 

 a series the story of evolving animal life 

 might become a little more clear. 



Most biologists agree that the Metazoa 

 took their origin from some single-celled 

 form. Surveying the thousands of species 

 of single-celled animals ( Protozoa ) , it seems 

 likely that the starting point could have 

 been among those that bear flagella (Fig. 

 4-1), some of which also contain the plant 

 pigment chlorophyll and are, therefore, 

 closely related to the plant world. Among 

 this large and varied assemblage of Pro- 

 tozoa are some that resemble one another 

 very closely, except that they exist in 

 groups of individuals which vary in number 

 from one to several thousand. Starting with 

 the single-celled Chlamydomonas, one can 

 arrange a graded series where individuals 

 differ only in the number of cells that cling 

 together. Pandorina is composed of eight 

 cells embedded in a spherical matrix of 

 jelly-like material. Each of the cells is not 

 greatly different from Chlamydomonas. 

 The combined beating of their flagella 



causes the entire colony to roll along in 

 a graceful manner. There is another form, 

 Pleodorino, which is composed of many 

 more cells, clustered in the shape of a hol- 

 low sphere; aside from the increased num- 

 ber of cells there is little difference between 

 this one and Pandorina. We do note one 

 rather interesting dissimilarity that will be 

 discussed later but should be mentioned 

 briefly now. Not all of the cells are the 

 same size; some are smaller than others, and 

 during reproduction the smaller ones are 

 unable to produce new colonies, in other 

 words, they are sterile. Therefore, in this 

 form there seems to be two kinds of cells, 

 reproductive cells and sterile or soma cells. 



A much larger aggregate is illustrated by 

 Volvox, SL beautiful hollow spherical col- 

 ony consisting of several thousand cells. 

 Again these cells resemble Chlamydomonas 

 in most respects, although there are tiny 

 bridges between individuals which tend to 

 lock them together more securely than the 

 loose jelly of other forms. Most all of the 

 cells are alike although there are some here 

 and there that are larger and have a dif- 

 ferent appearance. These are the reproduc- 

 tive cells; all others are soma cells. A more 

 careful observation will reveal that the re- 

 productive cells are of several kinds. Some 

 are bundles of tiny bodies, the sperm or 

 male cells, whfle others are large ovoid egg 

 cells. These special sex cells reproduce the 

 colony by a sexual process, that is, the 

 sperms are released into the water where 

 they swim to and unite with the egg. This 

 subsequently becomes a zygote, which 

 overwinters in a heavy-walled case (Fig. 

 4-1). Other reproductive cefls merely 

 divide and move into the hollow of the 

 sphere where they become small colonies, 

 known as daughter colonies. These eventu- 

 ally burst out, destroying the mother and 

 becoming adult colonies themselves. 



Two striking events occurred in this 

 gradual association of cells. First, simflar 

 cells aggregated into a mass which appar- 

 ently succeeded better, that is, there was 



