52 The Ottawa Naturalist. [June-July 



suggest also that the Dental Apparatus of the Eehinoidea had 

 a similar origin. The still greater specialization here lead to 

 an early loss of the little used epineurals of the outer portions 

 of the ambulacra. Covering plates or epineurals in Eehinoidea 

 were undoubtedly once present and if not already found we may 

 with every reason still expect to find them in older members of 

 this group. 



The absence of the usual members of an aborad skeleton 

 and the presence of the shifted interradial may lead others to 

 consider that we are viewing the aborad face of the oral skeleton. 

 This would make Stelleroid ambulacra of the plates here desig- 

 nated as epineurals. I may say that I have myself entertained 

 this idea only to reject it and I am prepared to defend my 

 position. 



THE EVOLUTION OF THE WORLDS. 



By J. S. Plaskett, B.A., F.R.S.C, The Observatory, Ottawa. 



(Continued from page 34). 



And now what happens when the particles begin to move 

 towards the centre of gravity. Work is done by them and, as 

 the form that work done eventually takes is heat, it is evident 

 that, as the nebula condenses under its own attraction, the 

 temperature rises, it grows hotter. A very striking example 

 of condensation accompanied by heat must have been often 

 noticed by those who use automobiles or bicycles. When air 

 is pumped into the tires, the pump becomes quite hot. Possibly 

 some of you have put this down to friction but you would find 

 it impossible to generate much heat by running the plunger 

 up and down in the open. You push the air particles closer 

 together, do work on them, which is converted into heat and the 

 temperature rises. On these two laws, that of gravitation, and 

 that of the transference of work into heat is based the whole 

 scheme of stellar evolution. Gravitation is the force that 

 impels the particles to do the work that is transferred with 

 heat. As condensation of the nebula proceeds it grows smaller, 

 approximates in form to a sphere, gets hotter and hotter and 

 becomes star or sun-like in its form and temperature. 



It may be as well to digress for a moment and try to get a 

 clear conception into our minds as to the physical condition of 

 the stars. The great majority of the stars or suns are entirely 

 gaseous, composed of incandescent vapors at enormously high 

 temperatures, our sun about 11,000F., while the white and 



