114 THE SPONGES— PHYLUM PORIFERA 



front half can regenerate a new posterior half, but the posterior half 

 cannot regenerate the anterior half. A little more highly specialized is 

 an animal like the lobster which will be killed if cut in two, but which 

 can regenerate a lost leg. 



If this line of reasoning is correct (that differentiation of cells is re- 

 sponsible for the loss of the power of regeneration), we would expect 

 the power to be greater in the embryos of higher animals before the cells 

 become differentiated. Embryological observations show this to be true ; 

 an early embryo, even of a complex animal like man, has about the same 

 degree of regenerative power as does the sponge. If broken into two 

 parts each part will regenerate the lost portion and two complete indi- 

 viduals are produced instead of the one that would have been formed. 

 The same may occur to give four individuals if broken into four parts, 

 with the record number now standing at five. Higher numbers are lack- 

 ing, probably not through any lack of regenerative powers of the embryo, 

 but through a lack of sufficient room for development within the mother's 

 body. With this in mind, it is easy to see how any of us might have 

 been twins through a simple cleavage of our bodies in our early develop- 

 ment. Thus, we can see that the distinction between the more complex 

 and the simpler animals is merely one of degree of development. All 

 start life at about the same level ; all are one-celled animals in the begin- 

 ning; some never get beyond one cell; some form an aggregation of 

 similar cells such as scypha ; some go on developing and the cells become 

 highly differentiated to form a complex organism such as man. 



The Metazoa 



Since scypha has a body composed of many cells it is one of the 

 metazoa, or many-celled animals, as compared to the protozoa, or one- 

 celled animals. This term, metazoa, is not a phylum name, but is called 

 a subkingdom and is used for convenience in referring to animals with 

 many cells in their body in contrast to those with only one. 



We might say that the protozoan cells are individualists — each cell 

 does everything for itself and there is no limitation of action that must 

 necessarily accompany a group organization. After any cell division, 

 the two daughter cells break apart and form separate individuals. In 

 the metazoa the cells remain together after cell division and in time 

 will form a large group of cells where there can be a division of tasks 

 and a specialization of the different cells for the tasks to be performed. 

 All cells function in a way that benefits the entire organism. The 

 metazoa, by living as a combined group of cells, usually attain 

 a much larger size than the protozoans and have many other advan- 



