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SYSTEMATIC ZOOLOGY TOR TEACHERS. 



production. I will now state, however, that from this point on through the 

 life of the sponge, the cells are not all alike and few remain spherical in 

 form. They become changed, differentiated, into other forms. As seen, some 

 assume forms suitable for producing muscle cell, some, nerve cells, some, cells 

 for producing the skeleton, spicules etc. 



Forms and Species of Sponges. 



Glancing back to the single-celled animals a moment, we will find that 

 among them the form of the cells differ somewhat. In the amoeba it was 

 spherical when the animal was at rest, but could be changed at will. In the 

 Paramecium or slipper infusoria, it had a definite shape in the mature animal. 

 This was also the case in the bell animalcule and in many other kinds. 

 That is, in dividing, an amoeba always produces two animals which are sim- 

 ilar to itself, and the same is true of other protozoans. I say produce an- 

 imals similar to themselves, for there is always, probably, individual variation ; 

 in other words, there are no two slipper infusoria which are exactly alike, 

 any more than there are any two men or women exactly alike, and slight 

 though that difference may be, there is a difference. This is individual 

 variation, but this variation is always limited. That is, limited if the slipper 

 infusoria always lives under the same condition of environment; conse- 

 quently the animals within these limits possess certain characteristics which 

 render them recognizable as slipper infusorias. What is true of slipper 

 infusorias is also true, not only of other protozoans, but of all animals, 

 through the ascending scale, up to man. Animals alike, within these limits, 

 and thus divided from other animals within other limits, constitute what we 

 call species. 



Fig. 88. 



A, ideal section of cilia cell; B, collar cell of sponge 



