28 ZOOLOGY 



agreeable objects or to remove it farther from disagreeable ones 

 in a word, the function of the nervous system is not to 

 originate but to collect, add together, and distribute. 



When the nerve cells of such creatures as jelly-fish are studied 

 under the high power of the microscope, every intermediate 

 stage between an ordinary sensory epithelial cell and a nerve 

 cell can be seen. We find some cells with receptive and 

 transmitting ends whose bodies are still wedged between neigh- 

 bouring epithelial cells : and no doubt is left in our minds that 

 a nerve cell, like a muscular cell, a sense cell, and a gland cell, 

 must be regarded as nothing more than a modification of an 

 ordinary epithelial cell. We have now to consider two types 

 of cell which cannot be regarded as modifications of epithelial 



FlG. 6. Diagram showing gradual change of an epithelial 

 cell into a nerve cell: ax, axons; sh, sense-hairs of 

 incipient nerve cell. 



cells : these are (1) the cells of the blood and internal skeleton, 

 and (2) the reproductive cells. 



It must seem strange to the ordinary reader to associate 

 blood and skeleton. That they are closely associated is one 

 of the curious and unexpected discoveries of scientific zoology. 

 The simplest type of internal skeleton is a more or less firm 

 secretion separating two layers of epithelium ; such a skeleton 

 is found in many simple polyp-like creatures which adhere to 

 the seaweeds on our coast. But the character of this " base- 

 ment membrane," as it is termed, varies indefinitely from a 

 thin elastic membrane to a thick gelatinous semi-flnid mass 

 such as is found in jelly-fish. When the skeleton takes on 

 the latter form we find in it free cells, which are capable of 

 slow " amoeboid " movement. These cells have been budded 

 off from the epithelial cells, but they are of quite a different 



